Tesla invented his "Tesla Coil" around 1891 while he was repeating and then expanding on [[Heinrich Hertz]]' experiments that had discovered [[electromagnetic radiation]] three years earlier.<ref>W. Bernard Carlson, Tesla: Inventor of the Electrical Age, Princeton University Press - 2013, page 120</ref> Tesla decided to power his setup with the high speed [[alternator]] he had been developing as part of an improved [[Arc lamp|arc lighting]] system but found that the high frequency current overheated the iron core and melted the insulation between the primary and secondary windings in the [[Ruhmkorff coil]] originally used in Hertz setup. To fix this problem Tesla changed the design so that there was an air gap instead of insulating material between the primary and secondary windings and made it so that the iron core could be moved to different positions in or out of the coil.<ref>W. Bernard Carlson, Tesla: Inventor of the Electrical Age, Princeton University Press - 2013, page 122</ref> Tesla also found he needed to put the capacitor normally used in such setups between his alternator and the coil's primary winding to avoid burning out the coil. By adjusting the coil and the capacitor Tesla found he could take advantage of the [[resonance]] set up between the two to achieve even higher frequencies.<ref>W. Bernard Carlson, Tesla: Inventor of the Electrical Age, Princeton University Press - 2013, page 124</ref>
| align = right
| direction = horizontal
| header =
| image1 = Rowland resonant transformer.jpg
| caption1 = [[Henry Augustus Rowland|Henry Rowland's]] 1889 spark-excited resonant transformer,<ref name="Thomson" /> a predecessor to the Tesla coil.<ref name="Strong" />
| width1 = 218
| image2 = Tesla drawing 1899 - Evolution of Tesla transformer.png
| caption2 = Steps in Tesla's development of the Tesla transformer around 1891.<ref name="Tesla1899" /> (1) Closed-core transformers used at low frequencies, (2-7) rearranging windings for lower losses, (8) removed iron core, (9) partial core, (10-11) final conical Tesla transformer, (12-13) Tesla coil circuits
| width2 = 260
| footer =
}}
[[Nikola Tesla]] patented the Tesla coil circuit April 25, 1891.<ref name="Denicolai1">[http://www.saunalahti.fi/dncmrc1/lthesis.pdf Denicolai, 2001, ''Tesla Transformer for Experimentation and Research'', Ch.1, p. 1-6]</ref><ref name="Patent454622">US Patent No. 454622, Nikola Tesla ''[http://www.google.com/patents/US454622 System of electric lighting]'', filed April 25, 1891; granted June 23, 1891</ref> and first publicly demonstrated it May 20, 1891 in his lecture "''Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination''" before the [[American Institute of Electrical Engineers]] at [[Columbia University|Columbia College]], New York.<ref name="Martin1">The lecture "''Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination''" is reprinted in {{cite book
In Tesla's coil transformer the capacitor, upon break-down of a short [[spark gap]], became connected to a coil of a few turns (the primary winding set), forming a resonant circuit with the frequency of oscillation, usually {{nowrap|20–100 kHz}}, determined by the capacitance of the capacitor and the inductance of the coil. The capacitor was charged to the voltage necessary to rupture the air of the gap during the input line cycle, about {{nowrap|10 kV}} by a line-powered transformer connected across the gap. The line transformer was designed to have higher than normal [[leakage inductance]] to tolerate the short circuit occurring while the gap remained ionized, or for the few milliseconds until the high frequency current had died away.
| last1 = Martin
| first1 = Thomas Cummerford
| title = The Inventions, Researches and Writings of Nikola Tesla: With Special Reference to His Work in Polyphase Currents and High Potential Lighting, 2nd Ed.
}}, [http://www.ee-techs.com/techbook/wireless-3.pdf archive]</ref> Although Tesla patented many similar circuits during this period, this was the first that contained all the elements of the Tesla coil: high voltage primary transformer, capacitor, spark gap, and air core "oscillation transformer".
{{breakafterimages}}
===Invention===
The spark gap is set up so that its breakdown occurs at a voltage somewhat less than the peak output voltage of the transformer in order to maximize the voltage across the capacitor. The sudden current through the spark gap causes the primary resonant circuit to ring at its resonant frequency. The ringing primary winding magnetically couples energy into the secondary over several RF cycles, until all of the energy that was originally in the primary has been transferred to the secondary. Ideally, the gap would then stop conducting (quench), trapping all of the energy into the oscillating secondary circuit. Usually the gap reignites, and energy in the secondary transfers back to the primary circuit over several more RF cycles. Cycling of energy may repeat for several times until the spark gap finally quenches. Once the gap stops conducting, the transformer begins recharging the capacitor. Depending on the breakdown voltage of the spark gap, it may fire many times during a mains AC cycle.
{{multiple image
| align = right
| direction = vertical
| header =
| image2 = System of electric lighting - Nikola Tesla US patent 454622 fig1.png
| caption2 = First drawing of Tesla coil circuit from Tesla's April 25, 1891 patent.<ref name="Patent454622" />
| width2 = 160
| image3 = Early Tesla coil drawing 1891.png
| caption3 = Drawing of Tesla coil circuit from Tesla's May 20, 1891 lecture at [[Columbia University|Columbia College]], New York.<ref name="Martin1" />
| width3 = 160
| image4 = Elihu Thomson's Tesla coil circuit Feb 1892 rotated.png
| caption4 = Elihu Thomson's Tesla coil, published February 1892, identical to Tesla's except for a compressed air spark blowout ''(J)''.<ref name="Thomson1892" />
| width4 = 160
| footer =
}}
During the [[Industrial Revolution]] the electrical industry exploited [[direct current]] (DC) and low [[frequency]] [[alternating current]] (AC), but not much was known about frequencies above 20 kHz, what are now called [[radio frequency|radio frequencies]]. In 1887, four years previously, [[Heinrich Hertz]] had discovered [[Hertzian wave]]s ([[radio wave]]s), electromagnetic waves which oscillated at very high frequencies.<ref name="Aitken">{{cite book
A more prominent secondary winding, with vastly more turns of thinner wire than the primary, was positioned to intercept some of the magnetic field of the primary. The secondary was designed to have the same frequency of resonance as the primary using only the stray capacitance of the winding itself to ground and that of any "top hat" terminal placed at the top of the secondary. The lower end of the long secondary coil must be grounded to the surroundings.
}}</ref> This attracted much attention, and a number of researchers began experimenting with high frequency currents.
Tesla's background was in the new field of [[alternating current]] power systems, so he understood transformers and resonance.<ref name="Carlson" /><ref name="Sarkar1" /> In 1888 he decided that high frequencies were the most promising field for research, and set up a laboratory at 33 South Fifth Avenue, New York, where he researched them, initially repeating Hertz's experiments.
The later and higher-power coil design has a single-layer primary and secondary. These Tesla coils are often used by hobbyists and at venues such as science museums to produce long [[electrical breakdown|sparks]]. The ''American Electrician''<ref>This is an early electronics magazine.</ref> gives a description of an early Tesla coil wherein a glass battery jar, 15 × 20 cm (6 × 8 in) is wound with 60 to 80 turns of [[American wire gauge|AWG]] No. 18 B & S [[magnet wire]] (0.823 mm²). Into this is slipped a primary consisting of eight to ten turns of AWG No. 6 B & S wire (13.3 mm<sup>2</sup>) and the whole combination is immersed in a vessel containing [[linseed oil|linseed]] or [[mineral oil]].<ref>(Norrie, pg. 34–35)</ref>
Tesla first developed [[alternator]]s as sources of high frequency current, but by 1890 found they were limited to frequencies of about 20 kHz.<ref name="Sarkar1" /> In search of higher frequencies he turned to spark-excited resonant circuits.<ref name="Carlson" /> Tesla's innovation was in applying resonance to transformers.<ref name="Wheeler">"''Tesla is entitled to either distinct priority or independent discovery of''" three concepts in wireless theory: "''(1) the idea of inductive coupling between the driving and the working circuits (2) the importance of tuning both circuits, i.e. the idea of an 'oscillation transformer' (3) the idea of a capacitance loaded open secondary circuit''" {{cite journal
=== Magnifying transmitter ===
| last = Wheeler
Tesla built a laboratory in [[Colorado Springs]] and between 1899-1900 performed experiments on wireless power transmission there. The Colorado Springs laboratory possessed one of the largest Tesla coils ever built, which Tesla called a "[[magnifying transmitter]]" as it was intended to transmit power to a distant receiver. With an input power of 300 kilowatts it could produce potentials in the 12 to 20 megavolt range at a frequency of 150 kHz, creating huge 140 feet (42 m) "lightning" bolts. The magnifying transmitter design is somewhat different from the classic two-coil Tesla coil circuit. In addition to the primary and secondary coils it had a third "resonator" coil, not magnetically coupled to the others, attached to the top terminal of the secondary. When driven by the secondary it produced additional high voltage by [[resonance]], being adjusted to resonate with its own [[parasitic capacitance]] at the frequency of the other coils.
| first = L. P.
| title = Tesla's contribution to high frequency
| journal = Electrical Engineering
| volume = 62
| issue = 8
| pages = 355–357
| publisher = IEEE
| date = August 1943
| issn = 0095-9197
| doi = 10.1109/EE.1943.6435874
}}</ref> [[Transformer]]s functioned differently at high frequencies than at the low frequencies used in power systems; the [[magnetic core|iron core]] in the [[Ruhmkorff coil]] transformers he had been using caused energy losses due to [[eddy current]]s and [[hysteresis]].<ref name="Carlson" /> Tesla<ref name="Tesla1899">{{cite journal
| last1 = Tesla
| first1 = Nikola
| title = Some experiments in Tesla's laboratory with currents of high frequencies and pressures
}}</ref> independently developed a new type of transformer without an iron core, the "[[oscillation transformer]]".
This occurred in 1891 while Tesla was developing a "wireless" lighting system, with a [[gas discharge lamp|gas discharge light bulb]] that would glow in an electrostatic field from a high voltage, high frequency power source.<ref name="Carlson" /><ref name="Sarkar1" /> Tesla powered his [[Ruhmkorff coil]] with his high frequency [[alternator]] but found that the high frequency current overheated the iron core and melted the insulation between the primary and secondary windings. To fix this problem Tesla changed the design so that there was an air gap instead of insulating material between the windings, and made the iron core adjustable so it could be moved in or out of the coil.<ref>W. Bernard Carlson, Tesla: Inventor of the Electrical Age, Princeton University Press - 2013, page 122</ref> Tesla also found he needed to put the [[capacitor]] normally used in the Ruhmkorff circuit between his alternator and the coil's primary winding to avoid burning out the coil. By adjusting the coil and capacitor Tesla found he could take advantage of the [[resonance]] set up between the two to achieve even higher frequencies.<ref>W. Bernard Carlson, Tesla: Inventor of the Electrical Age, Princeton University Press - 2013, page 124</ref> He found that the highest voltages were generated when the primary winding in the "closed" tuned circuit with the capacitor and spark gap, was in [[resonance]] with the "open" secondary winding.<ref name="Wheeler" /><ref name="Sarkar1" />
The Colorado Springs apparatus consisted of a 53-feet (16 m) diameter Tesla coil around the periphery of the lab, with a single-turn primary buried in the ground and a secondary of 50 turns of heavy wire on a 9 feet (2.7 m) high circular "fence". The primary was connected to a bank of oil [[capacitor]]s to make a [[tuned circuit]], excited by a rotary spark gap at 20 - 40 kilovolts from a powerful utility transformer. The top of the secondary was connected to a 20 ft (6 m) diameter "resonator" coil in the center of the room, attached to a telescoping 143 feet (43.6 m) "antenna" with a 30-inch (76 cm) metal ball on top which could project through the roof of the lab.
File:Tesla lab coil1.jpg|One of Tesla's early coils at his New York lab in 1892, with a conical secondary.
Tesla's 1902 design for his [[Wardenclyffe Tower]] magnifying transmitter used a top terminal consisting of a metal frame in the shape of a [[toroid]], covered with hemispherical plates (constituting a very large conducting surface). The top terminal has relatively small capacitance, charged to as high a voltage as practicable.<ref>N. Tesla, US patent No. 1, 119, 732. "I employ a terminal of ''relatively'' small capacity, which I charge to as high a pressure as practicable." (emphasis added) Tesla's lightning rod, {{US patent|1266175}}, goes more into this subject. The reader is also referred to the {{US patent|645576}}, {{US patent|649621}}, {{US patent|787412}}, and {{US patent|1119732}}.</ref> The outer surface of the elevated [[Electrical conductor|conductor]] is where the electrical voltage chiefly occurs. It had a large [[radius]] of curvature, or was composed of separate elements which, irrespective of their own radii of curvature, were arranged close to each other so that the outside ideal surface enveloping them has a large radius.<ref>Patent 1119732, lines 53 to 69; In order to develop the greatest energy in the circuit without flashover to the coil, Tesla elevated the conductor with a large radius of curvature or was composed of separate elements which in conglomeration had a large radius.</ref> This design allowed the terminal to support very high voltages without generating corona or sparks. Tesla, during his patent application process, described a variety of resonator terminals at the top of this later coil.<ref>In ''Selected Patent Wrappers from the National Archives'', by John Ratzlaff (1981; ISBN 0-9603536-2-3), a variety of terminals was described by Tesla. Besides the [[torus]]-shaped terminal, he applied for hemi[[spherical]] and [[Oblate spheroid|oblate]] terminals. A total of five different terminals were applied for, but four were rejected.</ref>{{Primary source inline|date=January 2017}}
File:Tesla prototype magnifying transmitter.jpg|Prototype "magnifying transmitter"<ref name="ElectricalRev11-1898">{{cite journal | title = Tesla's system of electric power transmission through natural media | journal = The Electrical Review| volume = 43 | issue = 1094 | pages = 709 | publisher = H. Alabaster, Gatehouse and Co. | location = New York | date = November 11, 1898 | url = https://books.google.com/books?id=IelQAAAAYAAJ&pg=PA709 | accessdate = August 7, 2015}}</ref><ref>Tesla stated in Nikola Tesla [http://www.americanradiohistory.com/Archive-Electrical-Experimenter/EE-1919-06.pdf My Inventions - Ch. 5: The Magnifying Transmitter], Electrical Experimenter, Vol. 7, No. 2, June 1919, p. 112, that this picture showed a prototype of his magnifying transmitter, a smaller version of the apparatus installed in his Colorado Springs lab.</ref> in Tesla's New York lab around 1898 producing 2.5 million volts. The round "spiderweb" secondary coil is visible in background
File:Tesla electrical oscillator.gif|Compact coil designed by Tesla for use as an [[ozone]] generator for water treatment<ref name="TeslaJuly1919">{{cite journal | last1 = Tesla | first1 = Nikola | title = Electrical Oscillators | journal = Electrical Experimenter | volume = 7 | issue = 3 | pages = 228-229, 259-260 | publisher = Experimenter Publishing Co. | location = New York | date = July 1919 | url = http://www.americanradiohistory.com/Archive-Electrical-Experimenter/EE-1919-07.pdf | accessdate = August 20, 2015}}</ref>
</gallery>
Tesla was not the first to invent this circuit.<ref name="Britannica1903">{{cite encyclopedia
| accessdate = May 1, 2015}}</ref><ref name="Fleming" /> [[Henry Augustus Rowland|Henry Rowland]] built a spark-excited resonant transformer circuit ''(above)'' in 1889<ref name="Strong">{{cite book
}}</ref> and [[Elihu Thomson]] had experimented with similar circuits in 1890, including one which could produce 64 inch (1.6 m) sparks,<ref name="Thomson1892">{{cite journal
| accessdate = August 20, 2015}} In this letter Thomson lists papers he published in technical journals which support his claim to priority in inventing the "Tesla coil" resonant transformer circuit</ref>
<ref name="Thomson">{{cite journal
| last1 = Thomson
| first1 = Elihu
| title = Apparatus for obtaining high frequencies and pressures
| accessdate = May 2, 2015}}</ref><ref name="Fleming" /> However he was the first to see practical applications for it and patent it. Tesla did not perform detailed mathematical analyses of the circuit, relying instead on trial and error and his intuitive understanding of resonance.<ref name="Sarkar1" /> He even realized that the secondary coil functioned as a quarter-wave [[resonator]]; he specified the length of the wire in the secondary coil must be a quarter [[wavelength]] at the resonant frequency.<ref name="QuarterWave">"''The length of the...coil in each transformer should be approximately one quarter of the wave length of the electric disturbance in the circuit, this estimate being based on the velocity of propagation of the disturbaiice through the coil itself...''" US Patent No. 645576, Nikola Tesla, ''[http://www.google.com/patents/US645576 System of transmission of electrical energy]'', filed September 2, 1897; granted March 20, 1900</ref><ref name="Sarkar1" /> The first mathematical analyses of the circuit were done by [[Anton Oberbeck]] (1895)<ref name="Oberbeck">{{cite journal
| last1 = Oberbeck
| first1 = A.
| title = Ueber den Verlauf der electrischen Schwingungen bei den Tesla'schen Versuchen (On the electrical oscillations in Tesla's experiments)
| accessdate = May 1, 2015}}</ref><ref name="Fleming" /> and [[Paul Drude]] (1904).<ref name="Drude">{{cite journal
| last1 = Drude
| first1 = P.
| title = Über induktive Erregung zweier elektrischer Schwingungskreise mit Anwendung auf Periodenund Dämpfungsmessung, Teslatransformatoren und drahtlose Telegraphie (Of inductive excitation of two electric resonant circuits with application to measure-ment of oscillation periods and damping, Tesla coils, and wireless telegraphy)
| accessdate = July 25, 2015}}, [http://arxiv.org/pdf/1303.1588.pdf English translation]</ref><ref name="Denicolai1" />
{{breakafterimages}}
===Tesla's demonstrations===
[[File:TeslaWirelessPower1891.png|thumb|upright=1.4|Tesla demonstrating wireless lighting at his 1891 lecture at [[Columbia College, Columbia University|Columbia College]].<ref name="Carlson2" />.<ref name="Barrett">A description of a similar demonstration which Tesla organized at the Westinghouse exhibit at the 1893 Columbian Exposition in St. Louis is found in {{cite book |title=Electricity at the Columbian Exposition; Including an Account of the Exhibits in the Electricity Building, the Power Plant in Machinery Hall |last=Barrett |first=John Patrick |year=1894 |isbn= |pages=168-169 |url=https://books.google.com/?id=CLtIAAAAMAAJ&pg=PA169 |accessdate=29 November 2010}}</ref> The two metal sheets are connected to a Tesla coil oscillator, which applies a high [[radio frequency]] oscillating voltage. The oscillating electric field between the sheets [[ionize]]s the low pressure gas in the two long [[Geissler tube]]s he is holding, causing them to glow by [[fluorescence]], similar to [[neon light]]s, without wires. ]]
A charismatic showman and self-promoter, in 1891-1893 Tesla used the Tesla coil in dramatic public lectures demonstrating the new science of high voltage, high frequency electricity.<ref name="Carlson2">{{cite book
}}</ref> The [[radio frequency]] AC electric currents produced by a Tesla coil did not behave like the DC or low frequency AC current scientists of the time were familiar with. In lectures at [[Columbia University|Columbia College]] May 20, 1891,<ref name="Martin1" /> scientific societies in Britain and France during a 1892 European speaking tour,<ref name="Martin2">Thomas Cummerford Martin 1894 ''[https://books.google.com/books?id=7YPbAAAAMAAJ&pg=PA198 The Inventions, Researches and Writings of Nikola Tesla, 2nd Ed.]'', p. 198-293</ref> the [[Franklin Institute]], Philadelphia in February 1893, and the [[National Electric Light Association]], St. Louis in March 1893,<ref name="Martin3">"''On light and other high frequency phenomena''", Thomas Cummerford Martin 1894 ''[https://books.google.com/books?id=7YPbAAAAMAAJ&pg=PA294 The Inventions, Researches and Writings of Nikola Tesla, 2nd Ed.]'', p. 294-373</ref> he impressed audiences with spectacular [[brush discharge]]s and [[streamer discharge|streamers]], heated iron by [[induction heating]], showed RF current could pass through insulators and be conducted by a single wire without a return path, and powered light bulbs and motors without wires.<ref name="Carlson2">[https://books.google.com/books?id=LOhQAAAAYAAJ&pg=PA133 W. Bernard Carlson 2013 ''Tesla: Inventor of the Electrical Age'', p. 133-137]</ref> He demonstrated that high frequency currents often did not cause the sensation of [[electric shock]], applying hundreds of thousands of volts to his own body,<ref name="Goldman" /><ref name="Carlson2" /> causing his body to light up with a glowing [[corona discharge]] in the darkened room. These lectures brought the Tesla "oscillator" to the attention of the scientific community and made Tesla internationally famous.<ref name="Sterling" /><ref name="Uth" />
{{breakafterimages}}
===Wireless power experiments===
{{main|World Wireless System}}
{{multiple image
| align = right
| direction = horizontal
| header =
| image1 = TeslaWirelessIllustration.png
| caption1 = Light bulb ''(bottom)'' powered wirelessly by "receiver" coil tuned to resonance with the huge "magnifying transmitter" coil at Tesla's Colorado Springs lab, 1899.<ref name="LongestTransmission" />
| width1 = 120
| image2 = US Patent 645576 Nikola Tesla 1897 System of transmission of electrical energy.png
| caption2 = Tesla's proposed wireless power system, from his 1897 patent.<ref name="Patent645576" /> The transmitter ''(left)'' consists of a Tesla coil ''(A,C)'' driving an elevated capacitive terminal ''(B)'' suspended by a balloon ''(D)''. The receiver ''(right)'' is a similar terminal and resonant transformer.
| width2 = 175
| footer =
}}
Tesla employed the Tesla coil in his efforts to achieve [[wireless power|wireless power transmission]],<ref name="Lee" /> his lifelong dream. In the period 1891 to 1900 he used it to perform some of the first experiments in wireless power,<ref name="Curty" /><ref name="Shinohara">{{cite book
| accessdate = November 4, 2014}}</ref> transmitting radio frequency power across short distances by [[inductive coupling]] between coils of wire.<ref name="Shinohara" /><ref name="LeeZhongHui" /><ref name="Tesla1891" >Tesla, Nikola (May 20, 1891) [http://www.tfcbooks.com/tesla/1891-05-20.htm ''Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination''], lecture before the American Inst. of Electrical Engineers, Columbia College, New York. Reprinted as a {{cite book
}}</ref> In his early 1890s demonstrations such as those before the American Institute of Electrical Engineers<ref name="Tesla1891" /> and at the 1893 Columbian Exposition in Chicago he lit light bulbs from across a room.<ref name="LeeZhongHui" /> He found he could increase the distance by using a receiving [[LC circuit]] tuned to [[resonance]] with the Tesla coil's LC circuit,<ref name="Wheeler" /> transferring energy by [[resonant inductive coupling]].<ref name="LeeZhongHui" /> At his Colorado Springs laboratory during 1899-1900, by using voltages of the order of 10 million volts generated by his enormous [[magnifying transmitter]] coil (described below), he was able to light three incandescent lamps at a distance of about {{convert|100|feet|m}}.<ref name="LongestTransmission">The longest wireless power transmission for which there is credible evidence is probably Tesla's picture of a receiving coil with a 10 watt light bulb lit by power transmitted from his 300,000 watt magnifying transmitter, which he claimed was at a distance of 1938 feet (591 m). {{cite book|last1=Tesla|first1=Nikola|last2=Marincic|first2=Aleksandar|last3=Popovic|first3=Vojin|last4=Ciric|first4=Milan|title=From Colorado Springs to Long Island : research notes : Colorado Springs 1899-1900, New York 1900-1901|date=2008|publisher=Nikola Tesla Museum|location=Belgrade|isbn=9788681243442|page=169|url=https://teslauniverse.com/nikola-tesla/books/nikola-tesla-colorado-long-island-research-notes-colorado-springs-1899-1900-new}} This represents a throughput efficiency of only 0.0033%, which would seem to be adequately explained by conventional power transfer by radio waves.</ref><ref name="Cheney" /><ref name="CenturyMagazineCaption">Tesla was notoriously secretive about the distance he could transmit power. One of his few disclosures of details was in the caption of fig. 7 of his noted magazine article: ''[http://www.tfcbooks.com/tesla/1900-06-00.htm The Problem of Increasing Human Energy]'', Century magazine, June 1900. The caption reads: "''EXPERIMENT TO ILLUSTRATE AN INDUCTIVE EFFECT OF AN ELECTRICAL OSCILLATOR OF GREAT POWER - The photograph shows three ordinary incandescent lamps lighted to full candle-power by currents induced in a local loop consisting of a single wire forming a square of fifty feet each side, which includes the lamps, and which is at a distance of one hundred feet from the primary circuit energized by the oscillator. The loop likewise includes an electrical condenser, and is exactly attuned to the vibrations of the oscillator, which is worked at less than five percent of its total capacity.''"</ref> Today the resonant inductive coupling discovered by Tesla is a familiar concept in electronics, widely used in [[transformer|IF transformers]] and short range wireless power transmission systems<ref name="LeeZhongHui" /><ref name="Leyh">{{cite conference
| first1 = G. E.
| last1 = Leyh
| first2 = M. D.
| last2 = Kennan
| title = Efficient wireless transmission of power using resonators with coupled electric fields
| conference = NAPS 2008 40th North American Power Symposium, Calgary, September 28–30, 2008
| pages = 1–4
| publisher = Inst. of Electrical and Electronic Engineers
| accessdate = November 20, 2014}}</ref> such as cellphone charging pads.
It is now understood that [[inductive coupling|inductive]] and [[capacitive coupling]] are "[[Near and far field|near-field]]" effects,<ref name="LeeZhongHui" /> so they cannot be used for long-distance transmission.<ref name="LongestTransmission" /><ref name="Sazonov">{{cite book | last1 = Sazonov | first1 = Edward | last2 = Neuman | first2 = Michael R | title = Wearable Sensors: Fundamentals, Implementation and Applications | publisher = Elsevier | date = 2014 | pages = 253–255 | url = http://books.google.com/books?id=wGJzAwAAQBAJ&pg=PA253#v=onepage&q&f=false | isbn = 0124186661}}</ref><ref name="Agbinya2">{{cite journal
| last1 = Agbinya
| first1 = Johnson I.
| title = Investigation of near field inductive communication system models, channels, and experiments
| journal = Progress In Electromagnetics Research B
}}</ref> However, Tesla was obsessed with developing a long range wireless power transmission system which could transmit power from power plants directly into homes and factories without wires, described in a visionary June 1900 article in ''[[Century Magazine]]''; "The Problem of Increasing Human Energy".<ref name="Tesla1900">{{cite journal
| accessdate = November 20, 2014}}</ref> He claimed to be able to transmit power on a ''worldwide'' scale, using a method that involved conduction through the Earth and atmosphere.<ref name="Patent645576">US Patent No. 645576, Nikola Tesla, ''[http://www.google.com/patents/US645576 System of transmission of electrical energy]'', filed September 2, 1897; granted March 20, 1900</ref><ref name="Tesla1904">{{cite journal
| last = Tesla
| first = Nikola
| title = The Transmission of Electric Energy Without Wires
| accessdate = November 19, 2014}}, reprinted in ''[http://books.google.com/books?id=qPI5AQAAMAAJ&pg=PA30 Scientific American Supplement, Munn and Co., Vol. 57, No. 1483, June 4, 1904, p. 23760-23761]</ref><ref name="Broad">{{Cite news
| last = Broad
| first = William J.
| title = A Battle to Preserve a Visionary’s Bold Failure
}}</ref><ref name="Carlson3" >[https://books.google.com/books?id=5I5c9j8BEn4C&pg=PA209 Carlson 2013 ''Tesla: Inventor of the Electrical Age, p. 209-210]</ref> Tesla believed that the entire Earth could act as an electrical resonator, and that by driving current pulses into the Earth at its [[resonant frequency]] from a grounded Tesla coil with an elevated capacitance, the potential of the Earth could be made to oscillate, creating global [[standing waves]], and this alternating current could be received with a capacitive antenna tuned to resonance with it at any point on Earth.<ref name="Cheney1" /><ref name="Sewall">{{cite book
| last1 = Sewall
| first1 = Charles Henry
| title = Wireless telegraphy: its origins, development, inventions, and apparatus
}}</ref><ref name="Tesla1904" /> Another of his ideas was that transmitting and receiving terminals could be suspended in the air by balloons at {{convert|30,000|feet}} altitude, where the air pressure is lower.<ref name="Sewall" /><ref name="ElectricalRev11-1898" /><ref name="Patent645576" /><ref name="Lee" /> At this altitude, he thought, a layer of electrically conductive rarefied air would allow electricity to be sent at high voltages (hundreds of millions of volts) over long distances. Tesla envisioned building a global network of wireless power stations, which he called his "[[World Wireless System]]", which would transmit both information and electric power to everyone on Earth.<ref name="Carlson7" >[https://books.google.com/books?id=5I5c9j8BEn4C&pg=PA337 Carlson 2013 ''Tesla: Inventor of the Electrical Age, p. 337-346]</ref>
{{breakafterimages}}
===Magnifying transmitter===
[[Image:Tesla magnifying transmitter circuit.svg|thumb|upright=1.2|Circuit of magnifying transmitter at Tesla's Colorado Springs laboratory.<ref name="Denicolai2" /><ref name="Sarkar2" /> ''C2'' represents the [[parasitic capacitance]] between the windings of coil ''L3''.]]
Tesla's wireless research required increasingly high voltages, and he had reached the limit of the voltages he could generate within the space of his New York lab. Between 1899-1900 he built a laboratory in [[Colorado Springs]] and performed experiments on wireless transmission there.<ref name="Sarkar2" /> The Colorado Springs laboratory had one of the largest Tesla coils ever built, which Tesla called a "[[magnifying transmitter]]" as it was intended to transmit power to a distant receiver.<ref name="Tesla1919">{{cite journal
| last1 = Tesla
| first1 = Nikola
| title = My Inventions V. - The Magnifying Transmitter
}}</ref> With an input power of 300 kilowatts it could produce potentials of the order of 10 million volts,<ref name="Sarkar2"/><ref name="Cheney1">[https://books.google.com/books?id=HIuK7iLO9zgC&pg=PA188 Cheney, Margaret (2011) ''Tesla: Man Out of Time'', p. 187-189]</ref> at frequencies of 50-150 kHz, creating huge "lightning bolts" reportedly up to 135 feet long.<ref name="Sprott" /> During experiments, it caused an overload which set fire to the alternator of the Colorado Springs power company, destroying it, and Tesla had to rebuild the alternator.<ref name="Sprott" />
In the magnifying transmitter, Tesla used a modified design ''(see circuit)'' which he had been experimenting with since before 1898 and patented in 1902,<ref name="Patent1119732" /><ref name="Gerekos3" /> different from his previous double-tuned circuits. In addition to the primary ''(L1)'' and secondary ''(L2)'' coils, it had a third coil ''(L3)'' which he called the "extra" coil, not magnetically coupled to the others, attached to the top terminal of the secondary.<ref name="Sarkar2" /> When driven by the secondary it produced high voltage by [[resonance]], being adjusted to resonate with its own [[parasitic capacitance]] ''(C2)''<ref name="Sarkar2" /> The use of a series-fed resonator coil to generate high voltages was independently discovered by [[Paul Marie Oudin]] in 1893 and employed in his [[Oudin coil]].<ref name="Martin" />
The Colorado Springs apparatus consisted of a 51 foot (15.5 m) diameter Tesla transformer composed of a secondary winding ''(L2)'' of 50 turns of heavy wire wound on an 8 foot (2.4 m) high circular "fence" around the periphery of the lab, and a single-turn primary ''(L1)'' either mounted on the fence or buried in the ground under it.<ref name="Shunamen">{{cite journal
| accessdate = September 4, 2015}}</ref><ref name="Carlson4" >[https://books.google.com/books?id=5I5c9j8BEn4C&pg=PA267 Carlson 2013 ''Tesla: Inventor of the Electrical Age'', p. 267-268]</ref> The primary was connected to a bank of oil [[capacitor]]s ''(C1)'' to make a [[tuned circuit]], with a rotary spark gap ''(SG)'', powered by 20 to 40 kilovolts from a powerful utility step-up transformer ''(T)''. The top of the secondary was connected to the 100-turn 8 ft (2.4 m) diameter "extra" or "resonator" coil ''(L3)'' in the center of the room. It's high voltage end was connected to a telescoping 143 foot (43.6 m) "antenna" rod with a 30 inch (1 m) metal ball on top which could project through the roof of the lab.
{{multiple image
{{multiple image
| align = center
| align = center
| direction = horizontal
| direction = horizontal
| header =
| header =
| image2 = Nikola Tesla, with his equipment Wellcome M0014782.jpg
| image1 = Magnifier-Tesla-Electrostatic.png
| caption2 = Famous image of magnifying transmitter in operation with Tesla sitting next to it. This is a "trick" photo, a double exposure; Tesla was not in the room when the coil was operating.
| width1 = 66
| width2 = 183
| image2 = Tesla-Magnifier-Electrostatic.svg
| width2 = 70
| image3 = Tesla Colorado Springs lab-12 million volts.jpg
| image3 = Tesla Colorado Springs lab-12 million volts.jpg
| caption3 = Coil in operation, at -12 million volts. The 10 ft diameter "extra" coil is shown. The 51 ft. diameter secondary coil is visible dimly in background, and in the previous photo.
| width3 = 355
| width3 = 189
| image4 = Tesla Colorado Springs lab-magnifying transmitter with metal ball.jpg
| image4 = Tesla Colorado Springs lab-magnifying transmitter with metal ball.jpg
| caption4 = Discharge of same coil with a metal sphere capacitive terminal.
| width4 = 200
| image5 =
| width4 = 105
| image5 = Tesla Colorado Springs lab-primary circuit of Magnifying Transmitter.jpg
| width5 =
| caption5 = Primary circuit, showing oil capacitor bank ''(boxes, foreground)'', 40 kV supply transformer and rotary spark gap ''(rear)'', and part of secondary winding ''(wall, left)''
| footer = The huge "magnifying transmitter" coil at Tesla's Colorado Springs laboratory, 1899-1900. From left:<br/>
| width5 = 190
1. Circuit of basic Tesla magnifying transmitter from his February 19, 1900 patent.<ref name="Patent649621">[https://www.google.com/patents/US649621 US Patent No. 649621, Nikola Tesla, ''Apparatus for transmission of electrical energy'', filed February 19, 1900, granted May 15, 1900]</ref> The generator symbol at bottom represents any source of RF current; in Tesla's coils this was a [[resonant circuit]] composed of the primary excited by a [[spark gap]]<br/>
| footer = The huge "magnifying transmitter" coil at Tesla's Colorado Springs laboratory, 1899-1900, photos by photographer Dickenson Alley December 1899. The long arcs shown above were not a feature of the normal operation of the transmitter because they wasted energy; for these photos Tesla forced the machine to produce arcs by switching the power rapidly on and off.
2. Circuit of bipolar magnifying transmitter design Tesla used in his [[Wardenclyffe tower]] plant.<br/>
3. Colorado Springs coil in operation at 12 million volts. The 20 ft (6 m) diameter 30 ft (9 m). High "resonator" coil is shown. The streamer discharge is 65 feet (19.8 m) across.<br/>
4. Discharge of same coil with a metal sphere capacitive terminal.
}}
}}
{{breakafterimages}}
===Wardenclyffe tower===
{{main|Wardenclyffe tower}}
{{multiple image
| align = right
| direction = horizontal
| header =
| image1 = Tesla Broadcast Tower 1904.jpeg
| caption1 = [[Wardenclyffe Tower]] wireless station, one of the largest Tesla coils ever built, intended as a prototype [[radiotelegraphy]] and wireless power transmitter, built by Tesla at Shoreham, NY, 1901-1902. It was never completed.
| width1 = 272
| image2 = Tesla-Magnifier-Electrostatic.svg
| caption2 = Circuit reportedly used in Wardenclyffe plant
| width2 = 79
| footer =
}}
In 1901, convinced his wireless theories were correct, Tesla with financing from banker [[J. P. Morgan]] began construction of a high-voltage wireless station, now called the [[Wardenclyffe Tower]], at [[Shoreham, New York]].<ref name="Tesla1904" /><ref name="Sarkar4">{{cite book
}}, [http://www.ee-techs.com/techbook/wireless-3.pdf archive]</ref> Although it was built as a transatlantic [[radiotelegraphy]] station, Tesla also intended it to transmit electric power without wires as a prototype transmitter for his proposed "[[World Wireless System]]".<ref name="Tesla1919" /><ref name="Carlson7" /> Essentially an enormous Tesla coil, it consisted of a powerhouse with a 400 horsepower generator and a 187 foot (57 m) tower topped by a 68 foot (21 m) diameter metal dome capacitive electrode.<ref name="Tesla1919" /><ref name="Carlson6" >[https://books.google.com/books?id=5I5c9j8BEn4C&pg=PA318 Carlson 2013 ''Tesla: Inventor of the Electrical Age, p. 318-327]</ref> Underneath the surface Tesla installed an elaborate [[ground (electricity)|ground system]] that he said was needed to "grip the earth" to create the oscillating earth currents which he believed would transmit the power.
By 1904 his investors had pulled out<ref name="Carlson7" /> and the facility was never completed; it was torn down in 1916.<ref name="Broad" /><ref name="Tesla1919" /> Although Tesla seems to have believed his wireless power ideas were proven,<ref name="Coe" /> he had a history of making claims that he had not confirmed by experiment,<ref name="Hawkins">{{cite journal
| last = Hawkins
| first = Lawrence A.
| title = Nikola Tesla: His Work and Unfulfilled Promises
}}</ref> and there seems to be no evidence that he ever transmitted significant power beyond the short-range demonstrations mentioned above.<ref name="LongestTransmission" /><ref name="Cheney3">{{cite book
}}</ref> The few reports of long-distance power transmission by Tesla are not from reliable sources. For example, a widely repeated myth is that in 1899 he wirelessly lit 200 light bulbs at a distance of {{convert|26|miles|km}}.<ref name="Cheney3" /><ref name="Coe" /> There is no independent confirmation of this supposed demonstration;<ref name="Cheney3" /><ref name="Coe" /> Tesla did not mention it,<ref name="Coe" /> and it does not appear in his laboratory notes.<ref name="Cheney1" /><ref name="Marinčić">{{cite book
}}</ref> It originated in 1944 from Tesla's first biographer, John J. O'Neill,<ref name="Cheney" /> who said he pieced it together from "fragmentary material... in a number of publications".<ref name="O'Neill">{{cite book
| last1 = O'Neill
| first1 = John J.
| title = Prodigal Genius: The life of Nikola Tesla
In the 100 years since, others such as Robert Golka<ref name="Shunamen" /><ref name="Golka" /><ref name="Lawren" /> have built equipment similar to Tesla's, but long distance power transmission has not been demonstrated,<ref name="Leyh1">For example, using Tesla coils Leyh and Kennan only achieved 1.5% power throughput at a distance of 30 meters, only 5 times the transmitter diameter. {{cite conference
| first1 = G. E.
| last1 = Leyh
| first2 = M. D.
| last2 = Kennan
| title = Efficient wireless transmission of power using resonators with coupled electric fields
| conference = NAPS 2008 40th North American Power Symposium, Calgary, September 28–30, 2008
| pages = 1–4
| publisher = Inst. of Electrical and Electronic Engineers
| accessdate = November 20, 2014}}</ref><ref name="LeeZhongHui" /><ref name="Cheney" /><ref name="Coe" /> and the scientific consensus is his World Wireless system would not have worked.<ref name="Belohlavek">{{cite book
| work = Tesla: Master of Lightning - companion site for 2000 PBS television documentary
| publisher = PBS.org, Public Broadcasting Service website
| date = 2000
| url = http://www.pbs.org/tesla/dis/papad.html
| accessdate = November 19, 2014}}</ref><ref name="Cooper" /> Contemporary scientists point out that while Tesla's coils (with appropriate antennas) can function as radio transmitters, transmitting energy in the form of [[radio wave]]s, the frequency he used, around 150 kHz, is far too low for practical long range power transmission.<ref name="Shinohara" /><ref name="Coe" /><ref name="Brown" /> At these [[wavelength]]s the radio waves spread out in all directions and cannot be focused on a distant receiver.<ref name="Curty" /><ref name="Shinohara" /><ref name="Coe" /><ref name="Tomar" /><ref name="Papadopoulos" /> Tesla's world power transmission scheme remains today what it was in Tesla's time: a bold, fascinating dream.<ref name="Broad" /><ref name="Tomar" />
{{breakafterimages}}
===Use in radio===
{{main|Spark-gap transmitter}}
{{multiple image
| align = right
| direction = horizontal
| header =
| image1 = Powerful spark gap transmitter.png
| caption1 = Powerful spark-gap transmitter, showing series spark gaps ''(horizontal cylindrical objects)'', Leyden jar capacitors ''(vertical cylinders, rear)'', and resonant transformer ''(top)''
| width1 = 180
| image2 = Spark gap transmitter-Marconi patent 763772 fig 1.png
| caption2 = Spark transmitter circuit from Marconi's 1900 patent.<ref name="Patent763772" /> It's similarity to a Tesla coil can be seen; the only difference is the addition of a variable inductor ''(g)'' to tune the antenna ''(f)'' to resonance.<ref name="Rockman" /><ref name="Sterling" />
| width2 = 148
| footer =
}}
:"[The Tesla coil] ''was invented not for wireless but for making vacuum lamps glow without external electrodes, and it later played a principal part in other hands in the operation of big spark stations.''" --William H. Eccles, 1933<ref name="Eccles">{{cite book
}} quoted in [https://books.google.com/books?id=NBLEAA6QKYkC&pg=PA353 Sarkar, Mailloux, Oliner (2006) ''History of Wireless'', p. 268]. Eccles was a contemporary of Tesla</ref>
One of the largest applications of the Tesla coil circuit was in early [[radio transmitter]]s called [[spark gap transmitter|spark gap transmitters]]. The first radio wave generators, invented by Heinrich Hertz in 1887, were spark gaps connected directly to antennas, powered by [[induction coil]]s.<ref name="Sarkar3">[https://books.google.com/books?id=NBLEAA6QKYkC&pg=PA353 Sarkar et al (2006) ''History of Wireless'', p. 352-353, 355-357], [http://www.ee-techs.com/techbook/wireless-3.pdf archive]</ref><ref name="Aitken2">[https://books.google.com/books?id=Mez_AwAAQBAJ&pg=PA71&dq=tesla Aitken, Hugh 2014 ''Syntony and Spark: The origins of radio, p. 70-73]</ref> <ref name="Uth" /> Because they lacked a [[resonant circuit]], these [[transmitter]]s produced highly [[damped wave|damped]] radio waves. As a result their transmissions occupied an extremely wide [[bandwidth (signal processing)|bandwidth]] of frequencies. When multiple transmitters were operating in the same area their frequencies overlapped and they interfered with one another, causing garbled reception. There was no way for a receiver to select one signal over another.<ref name="Aitken2" /><ref name="Sarkar3" />
In 1892 [[William Crookes]], a friend of Tesla, had given a lecture<ref name="Crookes">{{cite journal
| accessdate = August 19, 2015}}</ref> on the uses of radio waves in which he suggested using [[resonance]] to reduce the bandwidth in transmitters and receivers. By using [[resonant circuit]]s, different transmitters could be "tuned" to transmit on different frequencies. With narrower bandwidth, separate transmitter frequencies would no longer overlap, so a receiver could receive a particular transmission by "tuning" its resonant circuit to the same frequency as the transmitter.<ref name="Sarkar3" /><ref name="Uth" /><ref name="Rockman">{{cite book
| last1 = Rockman
| first1 = Howard B.
| title = Intellectual Property Law for Engineers and Scientists
}}</ref> This is the system used in all modern radio.
With an appropriate wire [[antenna (radio)|antenna]], the Tesla coil circuit could function as such a narrow bandwidth radio transmitter.<ref name="Tilbury 2007 1" /><ref name="Pierce" /><ref name="Sprott" /><ref name="PBS" /> In his March 1893 St. Louis lecture,<ref name="Martin3" /> Tesla demonstrated a wireless system that was the first use of [[tuned circuit]]s in radio, although he used it for wireless power transmission, not radio communication.<ref name="Sterling">{{cite book
| last1 = Sterling
| first1 = Christopher H.
| title = Biographical Encyclopedia of American Radio
}}</ref><ref name="Uth" /><ref name="Aitken1">[https://books.google.com/books?id=Mez_AwAAQBAJ&pg=PA254&dq=tesla Aitken, Hugh 2014 ''Syntony and Spark: The origins of radio, p. 254-255, 259]</ref><ref name="Rockman" /><ref name="Klooster">{{cite book
}}</ref><ref name="Cheney4" /> A grounded spark-excited capacitor-tuned Tesla transformer attached to an elevated wire antenna transmitted radio waves, which were received across the room by a wire antenna attached to a receiver consisting of a second grounded resonant transformer tuned to the transmitter's frequency, which lighted a [[Geissler tube]].<ref name="Regal">{{cite book
}}</ref><ref name="Sarkar3" /><ref name="Rockman" /><ref name="Cheney4">[https://books.google.com/books?id=HIuK7iLO9zgC&pg=PA96 Cheney, Margaret (2011) ''Tesla: Man Out Of Time'', p. 96-97]</ref> This system, patented by Tesla September 2, 1897,<ref name="Patent645576" /> was the first use of the "four circuit" concept later claimed by Marconi.<ref name="FourCircuit">The "four circuit" radio system, which Marconi claimed in his 1900 patent, meant a transmitter and receiver which each contained a resonant transformer and thus were divided into primary and secondary circuits. All four circuits were tuned to the same frequency, one side by capacitors, and the other side by the capacitance of the antenna; "''the use of two high frequency circuits in the transmitter and two in the receiver, all four so adjusted to be resonant at the same frequency or multiples of it.''" {{cite web
| last =
| first =
| title = No. 369 (1943) Marconi Wireless Co. of America v. United States
| accessdate = March 14, 2017}} This was identical to the system Tesla demonstrated in 1893. The advantage of this system was that due to the resonant transformers both the receiver and transmitter had much narrower bandwidth than previous circuits.</ref><ref name="Cheney4" /><ref name="Sterling" /><ref name="Klooster" /> However, Tesla was mainly interested in wireless power and never developed a practical radio ''communication'' system.<ref name="Coe" /><ref name="Smith" /><ref name="Regal" /><ref name="Sarkar3" /> In fact, he never believed that radio waves could be used for practical communication, instead clinging to an erroneous theory that radio communication was due to currents in the Earth.<ref name="TrueWireless">{{cite journal
| accessdate = February 20, 2017}} archived on [http://www.tfcbooks.com/tesla/1919-05-00.htm tfcbooks]</ref>
Practical [[radiotelegraphy]] communication systems were developed by [[Guglielmo Marconi]] beginning in 1895. By 1897 the advantages of narrow-bandwidth (lightly damped) systems noted by Crookes were recognized, and [[resonant circuit]]s, [[capacitor]]s and [[inductor]]s, were incorporated in transmitters and receivers.<ref name="Aitken1" /> The "closed primary, open secondary" resonant transformer circuit used by Tesla proved a superior transmitter,<ref name="Klooster" /> because the loosely-coupled transformer partially isolated the oscillating primary circuit from the energy-radiating antenna circuit, reducing the damping, allowing it to produce long "ringing" waves which had a narrower bandwidth.<ref name="Fleming" /><ref name="Pierce" /><ref name="Sarkar3" /> Versions of the circuit were patented by Marconi,<ref name="Patent763772" /><ref name="Klooster" /> [[John Stone Stone]]<ref name="Patent714756">US Patent no. 714,756, John Stone Stone [https://www.google.com/patents/US714756 Method of electric signaling], filed: February 8, 1900, granted: December 2, 1902</ref> and [[Oliver Lodge]],<ref name="Patent609154">US Patent no. 609,154 Oliver Joseph Lodge, [http://www.google.com/patents/US609154 Electric Telegraphy], filed: February 1, 1898, granted: August 16, 1898</ref> and were widely used in radio for twenty years.<ref name="Uth" /><ref name="Aitken1" /><ref name="Lee" /><ref name="Sarkar3" /><ref name="Rockman" /> In 1906 [[Max Wien]] invented the quenched or "series" spark gap, which extinguished the spark after the energy had been transferred to the secondary, allowing the secondary to oscillate freely after that, reducing damping and bandwith still more.
Although their damping had been reduced as much as possible, spark transmitters still produced [[damped wave]]s which had a wide bandwidth, creating interference with other transmitters. Around 1920 they became obsolete, superseded by [[vacuum tube]] transmitters which generated [[continuous wave]]s at a single frequency, which could also be [[modulation|modulated]] to carry sound. Tesla's [[resonant transformer]] continued to be used in vacuum tube transmitters and receivers, and is a key component in radio to this day.<ref name="Belohlavek" />
During the "spark era" the radio engineering profession gave credit to Tesla,<ref name="Sarkar3" /> his circuit became known as the "Tesla coil" or "Tesla transformer".<ref name="Uth" /><ref name="Pierce" /><ref name="Mazzotto" /> However Tesla did not benefit financially, due to competing patent claims. Marconi had claimed rights to the "closed primary open secondary" transmitter circuit in his controversial 1900 "four circuit" wireless patent.<ref name="Patent763772">US Patent no. 763,772, Guglielmo Marconi, [https://www.google.com/patents/US763772 Apparatus for wireless telegraphy], filed: November 10, 1900, granted: June 28, 1904. Corresponding British patent no. 7777, Guglielmo Marconi, [http://worldwide.espacenet.com/publicationDetails/biblio?CC=GB&NR=190007777&KC=&FT=E&locale=en_EP Improvements in apparatus for wireless telegraphy], filed: April 26, 1900, granted: April 13, 1901</ref><ref name="FourCircuit" /><ref name="Klooster" /><ref name="Lee" /><ref name="Rockman" /> Tesla sued Marconi in 1915 for patent infringement, but didn't have the resources to pursue the action.<ref name="Sarkar3" /><ref name="Klooster" /><ref name="Aitken1" /><ref name="Lee" /> However in 1943, in a separate suit brought by the [[Marconi Company]] against the US government for use of its patents in WW1, the [[Supreme Court of the United States|US Supreme Court]] invalidated Marconi's 1900 patent claim to the "four circuit" concept.<ref name="Findlaw">{{cite web
| last =
| first =
| title = No. 369 (1943) Marconi Wireless Co. of America v. United States
| accessdate = March 14, 2017}}</ref><ref name="Uth" /><ref name="Lee" /><ref name="Rockman" /><ref name="Sarkar" /> The ruling cited the prior patents of Tesla, [[Oliver Lodge|Lodge]], and [[John Stone Stone|Stone]],<ref name="Sarkar3" /><ref name="Uth" /> but did not decide which of these parties had rights to the circuit.<ref name="Lee" /><ref name="Klooster" /><ref name="Rockman" /> Of course by this time the issue was moot; the patent had expired in 1915 and spark transmitters had long been obsolete.
Although there is some disagreement over the role Tesla himself played in the invention of radio,<ref name="White">{{cite web
| last = White
| first = Thomas H.
| authorlink =
| title = Nikola Tesla: The Guy Who DIDN'T "Invent Radio"
| work = United States Early Radio History
| publisher = T. H. White's personal website
| date = November 1, 2012
| url = http://earlyradiohistory.us/tesla.htm
| format =
| doi =
| accessdate = November 7, 2016}}</ref><ref name="Uth" /><ref name="Lee" /><ref name="Sarkar" /> sources agree on the importance of his circuit in early radio transmitters.<ref name="Rockman" /><ref name="Gerekos2">[http://www.teslacoildesign.com/docs/TheTeslaCoil-Gerekos.pdf Gerekos, 2012, ''The Tesla Coil'', p. 1]</ref><ref name="Sprott" /><ref name="PBS" /><ref name="Klooster" /><ref name="Sarkar3" /><ref name="Belohlavek" /> From a modern perspective, most spark transmitters could be regarded as Tesla coils.<ref name="Sprott" /><ref name="Tilbury 2007 1" />
{{breakafterimages}}
===Use in medicine===
{{multiple image
| align = center
| direction = horizontal
| header =
| image1 = Ducretet Tesla coil.jpg
| caption1 = Small Tesla coil for [[electrotherapy]], 1905. The Tesla transformer is immersed in a tank of oil for insulation to prevent arcs.
| width1 = 199
| image2 = Treatment of knee with Oudin coil.jpg
| caption2 = Effluvation treatment of knee with an [[Oudin coil]] ''(left)'', a high voltage transformer similar to a Tesla coil, 1915
| caption3 = Tesla electrotherapy coil manufactured by Adolphe Gaiffe, around 1900. The primary capacitor is in the box; the spark gap is mounted on top.
| width3 = 225
| image4 = The treatment of cancer by cytolsis. Wellcome M0015160.jpg
| caption4 = Treatment of cancer with an Oudin coil ''(left)'', 1910. The [[induction coil]] that powers the Oudin coil is behind the patient's head.
| width4 = 207
| image5 = Hercules Tesla-Thomson electrotherapy unit 1907.jpg
| caption5 = Combined Tesla / D'Arsonval / Oudin electrotherapy and x-ray outfit 1907
| width5 = 134
| footer =
}}
{{multiple image
| align = center
| direction = horizontal
| header =
| image1 = Diabetes electrotherapy with vacuum electrode 1922.jpg
| caption1 = Electrotherapy treatment of [[diabetes]] with a vacuum electrode, 1922. The series spark gap is visible mounted on the front of the machine.
| width1 = 200
| image2 = Spark diathermy machine 1921 parts labeled.jpg
| caption2 = Longwave spark diathermy machine using Tesla circuit, 1921.
| width2 = 208
| image3 = Diathermy of elbow 1945.jpg
| caption3 = Diathermy of elbow 1945
| width3 = 165
| image4 = Violet ray wand glow Hochfrequenz 309.JPG
| caption4 = Vacuum electrode "violet ray" wand in operation.
| width4 = 101
| image5 = Violet wand.JPG
| caption5 = A [[violet ray]] wand, a handheld Tesla coil sold as a quack home medical device until about 1940. Said to cure everything from [[carbuncles]] to [[lumbago]].
| width5 = 200
| footer =
}}
[[File:Tesla D'Arsonval and Oudin electrotherapy circuits.png|thumb|upright=1.8|The three circuits used in electrotherapy apparatus in the early 20th century: (1) Tesla coil, (2) D'Arsonval coil, (3) [[Oudin coil]]. In medical coils for safety ''two'' capacitors ([[Leyden jar]]s) were used, one in each branch of the primary circuit, to completely isolate the patient's body from the potentially lethal currents of the supply transformer, in case of an electrical fault.<ref name="Manders">{{cite journal
| last = Manders
| first = Horace
| title = Some phenomena of high frequency currents
| journal = Journal of Physical Therapeutics
| volume = 3
| issue = 1
| pages = 220-221
| publisher = John Bale, Sons, and Danielsson, Ltd.
Tesla had observed as early as 1891 that high frequency currents above 10 kHz did not cause the sensation of [[electric shock]], and in fact currents that would be lethal at lower frequencies could be passed through the body without apparent harm.<ref name="McGinley">McGinley, Patton H. "Tesla's contributions to electrotherapy" in {{cite book
| accessdate = June 10, 2015}} Also read at the 8th annual meeting of The American Electro-Therapeutic Association, Buffalo, New York, Sept. 13-15, 1898</ref><ref name="Rhees">{{cite journal
| last1 = Rhees
| first1 = David J.
| title = Electricity - "The greatest of all doctors": An introduction to "High Frequency Oscillators for Electro-therapeutic and Other Purposes"
| journal = Proceedings of the IEEE
| volume = 87
| issue = 7
| pages = 1277-1281
| publisher = Inst. of Electrical and Electronic Engineers
| accessdate = September 20, 2015}}</ref> He experimented on himself, and found daily applications of high voltage relieved [[depression]] after his lab burned down in 1895.<ref name="Carlson8" >[https://books.google.com/books?id=5I5c9j8BEn4C&pg=PA217 Carlson 2013 ''Tesla: Inventor of the Electrical Age, p. 217]</ref> He was one of the first to observe the heating effect of high frequency currents on the body, the basis of [[diathermy]].<ref name="Kovács">{{cite book
| last1 = Kovács
| first1 = Richard
| title = Electrotherapy and Light Therapy, 5th Ed.
}}</ref><ref name="Cheney2">[https://books.google.com/books?id=HIuK7iLO9zgC&pg=PA103 Cheney (2011) ''Tesla:Man Out of Time'', p. 103]</ref> During his highly-publicized early 1890s demonstrations he passed hundreds of thousands of volts through his body.<ref name="Goldman" /><ref name="Carlson2" /> With characteristic hyperbole he called electricity "the greatest of all doctors"<ref name="Carlson8" /> and suggested burying wires under classrooms so its stimulating effect would improve performance of "dull" schoolchildren.<ref name="Cheney2" /><ref name="Gilliams">{{cite journal
| last1 = Gilliams
| first1 = E. Leslie
| title = Tesla's Plan Of Electrically Treating School Children
| journal = Popular Electricity
| volume =
| issue =
| pages = 813-814
| publisher = The Popular Electricity Publishing Co.
| accessdate = April 30, 2016}}</ref> Tesla wrote a pioneering paper in 1898 on the medical uses of high frequency currents<ref name="Tesla1898" /><ref name="Saberton">he also wrote a second earlier medical paper: Tesla, N. "High frequency currents for medical purposes" in ''Electrical Engineer'', 1891, cited in Saberton, Claude (1920) ''[https://books.google.com/books?id=swI1AQAAMAAJ&pg=PA131&dq=tesla Diathermy in Medical and Surgical Practice]'', published by Paul B. Hoeber, New York, p. 131</ref><ref name="Rhees" /> but did no further work on the subject.
A few other researchers were also experimentally applying high frequency currents to the body at this time.<ref name="Morton">{{cite journal
| last1 = Morton
| first1 = W. J.
| title = A brief glance at electricity in medicine
| journal = Transactions of the American Inst. of Electrical Engineers
}}</ref> [[Elihu Thomson]], the co-inventor of the Tesla coil, was one, so in medicine the Tesla coil became known as the "Tesla-Thomson apparatus".<ref name="Strong" /> In France, from 1889 physician and pioneering biophysicist [[Jacques Arsene d'Arsonval|Jacques d'Arsonval]] had been documenting the physiological effects of high frequency current on the body, and had made the same discoveries as Tesla.<ref name="D'Arsonval1893">{{cite journal
| last1 = D'Arsonval
| first1 = A.
| title = Physiological action of currents of great frequency
| journal = Modern Medicine and Bacteriological World
| accessdate = November 22, 2015}}, translated by J. H. Kellogg</ref><ref name="Kovács" /><ref name="Ho" /> During his 1892 European trip Tesla met with D'Arsonval and was flattered to find they were using similar circuits. D'Arsonval's spark-excited resonant circuits ''(above)'' did not produce as high voltage as the Tesla transformer.<ref name="Strong" /> In 1893 French physician [[Paul Marie Oudin]] added a "resonator" coil to the D'Arsonval circuit to create the high voltage [[Oudin coil]],<ref name="Ho" /><ref name="JMartin">{{cite book
| last1 = Martin
| first1 = James M.
| title = Practical electro-therapeutics and X-ray therapy
}}, p.189 fig. 98</ref> a circuit very similar to the Tesla coil, which was widely used for treating patients in Europe.<ref name="Strong" />
During this period, people were fascinated by the new technology of electricity, and many believed it had miraculous curative or "vitalizing" powers.<ref name="Peña">{{cite book
| last1 = De la Peña
| first1 = Carolyn Thomas
| title = The Body Electric: How Strange Machines Built the Modern American
}}</ref> [[Medical ethics]] were also looser, and doctors could experiment on their patients. By the turn of the century, application of high voltage, "high frequency" currents to the body had become part of a [[Victorian era]] medical field, part legitimate experimental medicine and part [[quack medicine]],<ref name="Rhees" /> called ''[[electrotherapy]]''.<ref name="Peña" /><ref name="Martin">{{cite book
| last1 = Martin
| first1 = James M.
| title = Practical electro-therapeutics and X-ray therapy
| accessdate = September 5, 2015}}</ref> Manufacturers produced medical apparatus to generate "Tesla currents", "D'Arsonval currents", and "Oudin currents" for physicians. In electrotherapy, a pointed electrode attached to the high voltage terminal of the coil was held near the patient, and the luminous [[brush discharge]]s from it (called "''effluves''") were applied to parts of the body to treat a wide variety of medical conditions. In order to apply the electrode directly to the skin, or tissues inside the mouth, anus or vagina, a "vacuum electrode" was used, consisting of a metal electrode sealed inside a partially evacuated glass tube, which produced a dramatic violet glow. The glass wall of the tube and the skin surface formed a [[capacitor]] which limited the current to the patient, preventing discomfort. These "[[violet ray]]" wands were later sold as a quack home medical device.<ref name="Behary">{{cite web
| accessdate = October 13, 2015}}</ref><ref>The small high voltage coils in these home violet ray wands resembled [[induction coil]]s more than Tesla coils; they had iron core transformers and mechanical interrupters and produced lower voltages than Tesla coils</ref>
The popularity of electrotherapy peaked after [[World War 1]],<ref name="Kovács" /><ref name="Peña" /> but by the 1920s authorities began to crack down on fraudulent medical treatments, and electrotherapy largely became obsolete. A part of the field that survived was ''[[diathermy]]'', the application of high frequency current to heat body tissue, pioneered by German physician Karl Nagelschmidt in 1907 using Tesla coils.<ref name="Kovács" /><ref name="Ho" /> By 1930 "long wave" (0.5~2 MHz) Tesla coil diathermy machines were being replaced by "short wave" (10~100 MHz) vacuum tube diathermy machines,<ref name="Kovács" /><ref name="Ho" /> but Tesla coils continued to be used in both diathermy<ref name="Kovács" /> and quack medical devices like [[violet ray]]<ref name="Behary" /> until World War 2.
During the 1920s and 30s all unipolar (single terminal) high voltage medical coils came to be called Oudin coils, so today's unipolar Tesla coils are sometimes referred to as "Oudin coils".<ref name="Behary2007">{{cite mailing list
| caption2 = "Electrice" lighting a candle with brush discharge from her fingers.<ref name="Electrice" /> The current came from the electric chair she is touching, which is connected to the Tesla coil in the background.
| width2 = 269
| image3 = Preacher shooting sparks from fingers 1938.jpg
| image4 = Transtrom's Tesla coil stunts - brush discharge from hand.jpg
| caption4 = Demonstrating 10 inch (25 cm) brush discharge from hand, 1913<ref name="Transtrom" />
| width4 = 120
| image5 = Transtrom's Tesla coil stunts - lighting bulb in mouth.jpg
| caption5 = RF current from Tesla coil lights the bulb's filament as it passes through the wire to charge and discharge the performer's body, which acts as a capacitor plate.<ref name="Transtrom" />
| width5 = 228
| footer = Turn-of-the-century sideshow performers did stunts with Tesla coils that would be considered extremely dangerous today. '''DON'T TRY THESE'''
}}
The Tesla coil's spectacular displays of sparks, and the fact that its currents could pass through the human body without causing [[electric shock]], led to its use in the entertainment business.
In the early 20th century it appeared in [[traveling carnival]]s, [[freak show]]s and circus and carnival [[sideshow]]s, which often had an act in which a performer would pass high voltages through his body<ref name="Goldman">{{cite book
| last1 = Goldman
| first1 = Harry
| title = Kenneth Strickfaden, Dr. Frankenstein's Electrician
}}</ref> Performers such as "Dr. Resisto", "The Human Dynamo", "Electrice", "The Great Volta", and "Madamoiselle Electra" would have their body connected to the high voltage terminal of a hidden Tesla coil, causing sparks to shoot from their fingertips and other parts of their body, and [[Geissler tube]]s to light up when held in their hand or even brought near them.<ref name="Transtrom">Many of these stunts are demonstrated and explained in {{cite book
| last1 = Transtrom
| first1 = Henry L.
| title = Electricity at high pressures and frequencies
}}. Bradbury has said that this was based on a real performer, Mr. Electrico, part of a seedy traveling carnival, whom he met as a boy in 1932 in Waukegan, Illinois. [http://www.raybradbury.com/inhiswords02.html Bradbury, Ray (December 2001) ''In his words'' blog, Ray Bradbury personal website] and [http://www.theparisreview.org/interviews/6012/the-art-of-fiction-no-203-ray-bradbury Weller, Sam (Spring 2010) "Ray Bradbury interview, The Art of Fiction No. 203", ''The Paris Review'', No. 192, published by Antonio Weiss, New York].</ref> They could also light candles or cigarettes with their fingers.<ref name="Electrice" /> Although they didn't usually cause electric shocks, RF arc discharges from the bare skin could cause painful burns; to prevent them performers sometimes wore metal thimbles on their fingertips<ref name="Electrice" /> ''(Rev. Moon, center image above, is using them)''. These acts were extremely dangerous and could kill the performer if the Tesla coil was misadjusted.<ref name="Gangi" /> In [[carny]] lingo this was called an "electric chair act" because it often included a spark-laced "[[electrocution]]" of the performer in an [[electric chair]],<ref name="Gangi" /><ref name="Nickell" /> exploiting public fascination with this exotic new method of [[capital punishment]], which had become the United States' dominant method of execution around 1900. Today entertainers still perform high voltage acts with Tesla coils,<ref name="Curioddities">Danielle Stamp AKA 'Miss Electra' {{cite book
| title = Ripley's Believe It Or Not! Curioddities
| accessdate = October 21, 2015}}</ref> but modern [[bioelectromagnetics]] has brought a new awareness of the hazards of Tesla coil currents, and allowing them to pass through the body is today considered extremely dangerous.
Tesla coils were also used as dramatic [[Theatrical property|props]] in early mystery and science fiction [[motion picture]]s, starting in the [[silent era]].<ref name="Goldman" /> The crackling, writhing sparks emanating from the electrode of a giant Tesla coil became Hollywood's iconic symbol of the "[[mad scientist]]'s" lab, recognized throughout the world.<ref name="Skal">{{cite book
| last1 = Skal
| first1 = David J.
| title = Screams of Reason: Mad Science and Modern Culture
}}</ref> This was probably because the eccentric [[Nikola Tesla]] himself, with his famous high voltage demonstrations and his mysterious Colorado Springs laboratory, was one of the main prototypes from which the "mad scientist" [[stock character]] originated.<ref name="Skal" /><ref name="VanRiper">{{cite book |last=Van Riper |first=A. Bowdoin |url=https://books.google.com/books?id=ABtJPIcVtBoC |title=A Biographical Encyclopedia of Scientists and Inventors in American Film and TV since 1930 |year=2011 |publisher=Scarecrow Press |pages=150 |isbn=978-0-8108-8128-0 }}</ref> Some early films in which Tesla coils appeared were ''Wolves of Kultur'' (1918), ''The Power God'' (1926), ''[[Metropolis (1927 film)|Metropolis]]'' (1927), ''[[Frankenstein (1931 film)|Frankenstein]]'' (1931) and its many sequels such as ''[[Son of Frankenstein]]'' (1939), ''[[The Mask of Fu Manchu]]'' (1932), ''[[Chandu the Magician (film)|Chandu the Magician]]'' (1932), ''[[The Lost City (1935 serial)|The Lost City]]'' (1935), and ''[[The Clutching Hand]]'' (1936)<ref name="Tibbetts">William Luddington, "Mr. Electricity: The Multi-Volted Career of Kenneth Strickfaden" in {{cite book
}}</ref><ref name="Goldman" /> and many later films and television shows. By the 1980s, effects like high voltage sparks were being added to movies by [[computer generated imagery|CGI]] as [[visual effects]] in [[post-production]], eliminating the need for dangerous high voltage Tesla coils on sets.
The Tesla coils for many of these movies were constructed by [[Kenneth Strickfaden]] (1896-1984) who, beginning with his spectacular effects in the 1931 ''Frankenstein'', became Hollywood's preeminent electrical [[special effects]] expert.<ref name="Goldman" /><ref name="Hanson">{{cite journal
| accessdate = October 1, 2015}}</ref> His large "Meg Senior" Tesla coil seen in many of these movies consisted of a 6 foot 1000 turn conical secondary and a 10 turn primary, connected to a capacitor through a rotary spark gap, powered by a 20 kV transformer.<ref name="Hanson" /> It could produce 6 foot sparks. Some of his last gigs were the reassembly of the original 1931 ''Frankenstein'' high voltage apparatus for the Mel Brooks satire ''[[Young Frankenstein]]'' (1974), and construction of a million volt Tesla coil which produced 12 foot sparks for a 1976 stage show by the rock band ''[[Kiss (band)|Kiss]]''.<ref name="Tibbetts" />
{{breakafterimages}}
=== Use in education ===
{{multiple image
| align = right
| direction = horizontal
| header =
| image1 = Tesla coil at Griffith Observatory LA.jpg
| caption1 = Million volt [[Griffith Park Observatory]] coil, Los Angeles. Over 100 years old, it is one of the oldest working Tesla coils.
| width1 = 167
| image2 = High frequency inductance experiment 1906.jpg
| caption2 = Demonstration of inductance with a Tesla coil, 1906.<ref name="Collins">{{cite journal
| accessdate = December 15, 2016}}</ref> RF current will not pass through the heavy copper wire because of the bend, and passes through the lamp instead.
| width2 = 140
}}
[[File:Small Tesla coil kit 1918.jpg|thumb|Small educational Tesla coil kit, 1918]]
Ever since Tesla's 1890s lectures, Tesla coils have been used as attractions in educational exhibits and [[science fair]]s. They have become a way to counter the stereotype that science is boring.<ref name="Goldman2" /> In the early 20th century, experts like [[Henry Transtrom]] and [[Earle Ovington]] gave high voltage demonstrations at "electric fairs".<ref name="Transtrom" /> High school classes built Tesla coils.
From 1933 into the 1980s, between movie jobs Hollywood special effects expert [[Kenneth Strickfaden|Ken Strickfaden]] would take his high voltage apparatus on the road in an exhibition called "Science on Parade" and later "The Kenstric Space Age Science Show" to high schools, colleges, World Fairs and expositions.<ref name="Goldman2">[https://books.google.com/books?id=_xkZIjk3lXAC&pg=PA66 Goldman (2005) ''Kenneth Strickfaden, Dr. Frankenstein's Electrician'', p. 62-68]</ref> These spectacular shows, which reached 48 states, had a seminal influence on the birth of the modern "coiling" movement.<ref name="Tibbetts" /> A number of present-day Tesla hobbyists such as William Wysock say they were inspired to build Tesla coils by seeing Strickfaden's show.<ref name="Goldman2" />
One of the oldest and best-known coils still in operation is the "GPO-1" at [[Griffith Park Observatory]] in Los Angeles. It was originally one of a pair of coils built in 1910 by [[Earle L. Ovington]], a friend of Tesla and manufacturer of high voltage electrotherapy apparatus.<ref name="Gurstelle">{{cite book
| last1 = Gurstelle
| first1 = William
| title = Adventures from the Technology Underground
| accessdate = November 14, 2015}}</ref><ref name="Goldman"/> For a number of years Ovington displayed them at the December electrical trade show at [[Madison Square Garden]] in New York City, using them for demostrations of high voltage science, which Tesla himself sometimes attended.<ref name="Griffith" /> Called the ''Million Volt Oscillator'', the twin coils were installed on the balcony at the show. Every hour the lights were dimmed and the public was treated to a display of 10 foot arcs. Ovington gave the coils to his friend Dr. [[Frederick Finch Strong]], a leading figure in the alternative health field of [[electrotherapy]]. In 1937 Strong donated the coils to the Griffith Observatory. The museum didn't have room to display both, but one coil was restored by Kenneth Strickfaden and has been in daily operation ever since.<ref name="Goldman"/> It consists of a 48 in. (1.2 m) high conical secondary coil topped by a 12 in. (30 cm) diameter copper ball electrode, with a 9-turn spiral primary of 2 in. copper strip, a glass plate capacitor (replacing the original Leyden jars), and rotary spark gap.<ref name="Gurstelle" /> Its output has been estimated at 1.3 million volts.<ref name="Griffith" />
{{breakafterimages}}
===Later uses===
[[File:Tuve & Breit Tesla coil particle accelerator 1928.jpg|thumb|Breit and Tuve's 5 MV Tesla coil used as particle accelerator, 1928]]
In addition to its use in [[spark-gap transmitter|spark-gap radio transmitters]] and [[electrotherapy]] described above, the Tesla coil circuit was also used in the early 20th century in [[x-ray machine]]s, [[ozone generator]]s for water purification, and [[induction heating]] equipment. However in the 1920s [[vacuum tube]] [[electronic oscillator|oscillators]] replaced it in all these applications.<ref name="Tilbury 2007 1" /> The [[triode]] vacuum tube was a much better radio frequency current generator than the noisy, hot, [[ozone]]-producing spark, and could produce [[continuous wave]]s. After this, industrial use of the Tesla coil was mainly limited to a few specialized applications which were suited to its unique characteristics, such as high voltage insulation testing.
In 1926, pioneering accelerator physicists [[Merle Tuve]] and [[Gregory Breit]] built a 5 million volt Tesla coil as a linear [[particle accelerator]].<ref name="Breit">{{cite journal
| last1 = Breit
| first1 = G. M.
| last2 = Tuve
| first2 = M. A.
| last3 = Dahl
| first3 = O.
| title = A laboratory method of producing high potentials
}}</ref> The bipolar coil consisted of a pyrex tube a meter long wound with 8000 turns of fine wire, with round corona caps on each end, and a 5 turn spiral primary coil surrounding it at the center. It was operated in a tank of insulating oil pressurized to 500 psi which allowed it to reach a potential of 5.2 megavolts. Although it was used for a short period in 1929-30 it was not a success because the particles' acceleration had to be completed within the brief period of a half cycle of the RF voltage.
In 1970 Robert K. Golka built a replica of Tesla's huge Colorado Springs magnifying transmitter in a shed at [[Wendover Air Force Base]], Utah, using data he found in Tesla's lab notes archived at the [[Nikola Tesla Museum]] in [[Beograd]], Serbia.<ref name="Shunamen" /><ref name="Golka">{{cite journal
| last1 = Golka
| first1 = Robert K.
| title = Project Tesla - In Search of an Answer to Our Energy Needs
| accessdate = September 4, 2015}}</ref><ref name="Reed" /> This was one of the first experiments with the magnifier circuit since Tesla's time. The coil generated 12 million volts. Golka used it to try to duplicate Tesla's reported synthesis of [[ball lightning]].
This article may be too long to read and navigate comfortably. Consider splitting content into sub-articles, condensing it, or adding subheadings. Please discuss this issue on the article's talk page.(October 2016)
Homemade Tesla coil in operation, showing brush discharges from the toroid. The high electric field causes the air around the high voltage terminal to ionize and conduct electricity, allowing electricity to leak into the air in colorful corona discharges, brush discharges and streamer arcs. Tesla coils are used for entertainment at science museums and public events, and for special effects in movies and television.
Tesla coils can produce output voltages from 50 kilovolts to several million volts for large coils.[15][17][19] The alternating current output is in the low radio frequency range, usually between 50 kHz and 1 MHz.[17][19] Although some oscillator-driven coils generate a continuous alternating current, most Tesla coils have a pulsed output;[15] the high voltage consists of a rapid string of pulses of radio frequency alternating current.
The common spark-excited Tesla coil circuit, shown below, consists of these components:[16][20]
A high voltage supply transformer(T), to step the AC mains voltage up to a high enough voltage to jump the spark gap. Typical voltages are between 5 and 30 kilovolts (kV).[20]
A spark gap(SG) that acts as a switch in the primary circuit
The Tesla coil (L1, L2), an air-core double-tuned resonant transformer, which generates the high output voltage.
Optionally, a capacitive electrode (top load) (E) in the form of a smooth metal sphere or torus attached to the secondary terminal of the coil. Its large surface area suppresses premature corona discharge and streamer arcs, increasing the Q factor and output voltage.
Resonant transformer
Many resonances are observed on the secondary coil[21]
Unipolar Tesla coil circuit. C2 is not an actual capacitor but represents the capacitance of the secondary windings L2, plus the capacitance to ground of the toroid electrode E.
Properly descriptions of the secondary coil stray capacitance
Tesla coil is also called resonant transformer, oscillation transformer or radio-frequency (RF) transformer. The transformer of Tesla coil has lower coupling coefficient between coils than an ordinary transformer used in AC power circuits.[22][23][24]
Because the coupling coefficient is low, the resonant transformer seems to have low energy transfer efficiency, but the actual energy transfer efficiency is surprisingly good. This is because it is designed to temporarily store electrical energy in the tank circuit on the secondary coil. The tank circuit functions as an LC circuit (resonant circuit, tuned circuit) that stores electric energy by the short-circuit inductance of the secondary coil and the stray capacitance generated between the secondary coil windings. The primary coil(L1) consisting of a relatively few turns of heavy copper wire or tubing, is connected to a capacitor(C1) through the spark gap(SG).[15][16] The secondary coil(L2) consists of many turns (hundreds to thousands) of fine wire on a hollow cylindrical form inside the primary. The secondary is not connected to an actual capacitor, but it also functions as an LC circuit, the short-circuit inductance of (L2) resonates with stray capacitance (C2), the sum of the stray parasitic capacitance between the windings of the coil, and the capacitance of the toroidal metal electrode attached to the high voltage terminal. When energy tuned to the serial resonant frequency 1' of the secondary coil is given from the spark gap, the secondary coil efficiently absorbs the energy and stores it in the tank circuit. In this case, when the resonance circuit on the primary side is tuned so as to have the same frequency as the resonance frequency of the secondary coil, much energy is transfer to the secondary side. Generally, in the spark gap type, since the frequency of the driving energy can not be concentrated to frequency 1' , many standing waves frequency 2', 3 ', 4'... Are generated on the secondary coil.
The peculiar design of the coil is dictated by the need to achieve low resistive energy losses (high Q factor) at high frequencies,[17] which results in the largest secondary voltages:
Ordinary power transformers have an iron core to increase the magnetic coupling between the coils. However at high frequencies an iron core causes energy losses due to eddy currents and hysteresis, so it is not used in the Tesla coil.[24]
Ordinary transformers are designed to be "tightly coupled". Due to the iron core and close proximity of the windings, they have a high mutual inductance(M), the coupling coefficient is close to unity 0.95 - 1.0, which means almost all the magnetic field of the primary winding passes through the secondary.[22][24] The Tesla transformer in contrast is "loosely coupled",[15][24] the primary winding is larger in diameter and spaced apart from the secondary,[16] so the mutual inductance is lower and the coupling coefficient is only 0.05 to 0.2. Generally, this means that only 5% to 20% of the magnetic field of each coil passes through the other when the secondary coil is open circuited.[15][20] However, in the Tesla coil, the secondary coil resonates at the serial resonant frequency 1'. In this state, the current phases of the primary coil and the secondary coil current are in phase. Then the magnetic phases of each coils are also synchronized and the mutual flux increases. As a result, the primary coil and the secondary coil are strongly coupled. In this manner, energy exchange between the primary coil and the secondary coil is performed, which allows the oscillating energy to stay in the secondary circuit longer before it returns to the primary and begins dissipating in the spark.
Each winding is also limited to a single layer of wire, which reduces proximity effect losses. The primary carries very high currents. Since high frequency current mostly flows on the surface of conductors due to skin effect, it is often made of copper tubing or strip with a large surface area to reduce resistance, and its turns are spaced apart, which reduces proximity effect losses and arcing between turns.[25][26]
Unipolar coil design widely used in modern coils. The primary is the flat red spiral winding at bottom, the secondary is the vertical cylindrical coil wound with fine red wire. The high voltage terminal is the aluminum torus at the top of the secondary coil.
Bipolar coil, used in the early 20th century. There are two high voltage output terminals, each connected to one end of the secondary, with a spark gap between them. The primary is 12 turns of heavy wire, which is located at the midpoint of the secondary to discourage arcs between the coils.
The output circuit can have two forms:
Unipolar - One end of the secondary winding is connected to a single high voltage terminal, the other end is grounded. This type is used in modern coils designed for entertainment. The primary winding is located near the bottom, low potential end of the secondary, to minimize arcs between the windings. Since the ground (Earth) serves as the return path for the high voltage, streamer arcs from the terminal tend to jump to any nearby grounded object.
Bipolar - Neither end of the secondary winding is grounded, and both are brought out to high voltage terminals. The primary winding is located at the center of the secondary coil, equidistant between the two high potential terminals, to discourage arcing.
Operation cycle
The circuit operates in a rapid, repeating cycle in which the supply transformer (T) charges the primary capacitor (C1) up, which then discharges in a spark through the spark gap, creating a brief pulse of oscillating current in the primary circuit which excites a high oscillating voltage across the secondary:[18][20][24][27]
Current from the supply transformer (T) charges the capacitor (C1) to a high voltage.
When the voltage across the capacitor reaches the breakdown voltage of the spark gap (SG) a spark starts, reducing the spark gap resistance to a very low value. This completes the primary circuit and current from the capacitor flows through the primary coil (L1). The current flows rapidly back and forth between the plates of the capacitor through the coil, generating radio frequency oscillating current in the primary circuit at the circuit's resonant frequency.
The oscillating magnetic field of the primary winding induces an oscillating current in the secondary winding (L2), by Faraday's law of induction. Over a number of cycles, the energy in the primary circuit is transferred to the secondary. The total energy in the tuned circuits is limited to the energy originally stored in the capacitor C1, so as the oscillating voltage in the secondary increases in amplitude ("ring up") the oscillations in the primary decrease to zero ("ring down"). Although the ends of the secondary coil are open, it also acts as a tuned circuit due to the capacitance (C2), the sum of the parasitic capacitance between the turns of the coil plus the capacitance of the toroid electrode E. Current flows rapidly back and forth through the secondary coil between its ends. Because of the small capacitance, the oscillating voltage across the secondary coil which appears on the output terminal is much larger than the primary voltage.
The secondary current creates a magnetic field that induces voltage back in the primary coil, and over a number of additional cycles the energy is transferred back to the primary. This process repeats, the energy shifting rapidly back and forth between the primary and secondary tuned circuits. The oscillating currents in the primary and secondary gradually die out ("ring down") due to energy dissipated as heat in the spark gap and resistance of the coil.
When the current through the spark gap is no longer sufficient to keep the air in the gap ionized, the spark stops ("quenches"), terminating the current in the primary circuit. The oscillating current in the secondary may continue for some time.
The current from the supply transformer begins charging the capacitor C1 again and the cycle repeats.
This entire cycle takes place very rapidly, the oscillations dying out in a time of the order of a millisecond. Each spark across the spark gap produces a pulse of damped sinusoidal high voltage at the output terminal of the coil. Each pulse dies out before the next spark occurs, so the coil generates a string of damped waves, not a continuous sinusoidal voltage.[18] The high voltage from the supply transformer that charges the capacitor is a 50 or 60 Hz sine wave. Depending on how the spark gap is set, usually one or two sparks occur at the peak of each half-cycle of the mains current, so there are more than a hundred sparks per second. Thus the spark at the spark gap appears continuous, as do the high voltage streamers from the top of the coil.
The supply transformer (T) secondary winding is connected across the primary tuned circuit. It might seem that the transformer would be a leakage path for the RF current, damping the oscillations. However its large inductance gives it a very high impedance at the resonant frequency, so it acts as an open circuit to the oscillating current. If the supply transformer has inadequate leakage inductance, radio frequency chokes are placed in its secondary leads to block the RF current.
Oscillation frequency
To produce the largest output voltage, the primary and secondary tuned circuits are adjusted to resonance with each other. The resonant frequencies of the primary and secondary circuits, and , are determined by the inductance and capacitance in each circuit[17][18][22][28]
Generally the secondary is not adjustable, so the primary circuit is tuned, usually by a moveable tap on the primary coil L1, until it resonates at the same frequency as the secondary
Thus the condition for resonance between primary and secondary is
The resonant frequency of Tesla coils is in the low radio frequency (RF) range, usually between 50 kHz and 1 MHz. However, because of the impulsive nature of the spark they produce broadband radio noise, and without shielding can be a significant source of RFI, interfering with nearby radio and television reception.
Output voltage
Large coil producing 3.5 meter (10 foot) streamer arcs, indicating a potential of millions of volts.
In a resonant transformer the high voltage is produced by resonance; the output voltage is not proportional to the turns ratio, as in an ordinary transformer.[24][29] It can be calculated approximately from conservation of energy. At the beginning of the cycle, when the spark starts, all of the energy in the primary circuit is stored in the primary capacitor . If is the voltage at which the spark gap breaks down, which is usually close to the peak output voltage of the supply transformer T, this energy is
During the "ring up" this energy is transferred to the secondary circuit. Although some is lost as heat in the spark and other resistances, in modern coils, over 85% of the energy ends up in the secondary.[18] At the peak () of the secondary sinusoidal voltage waveform, all the energy in the secondary is stored in the capacitance between the ends of the secondary coil
Assuming no energy losses, . Substituting into this equation and simplifying, the peak secondary voltage is[17][18][24]
The second formula above is derived from the first using .[24] Since the capacitance of the secondary coil is very small compared to the primary capacitor, the primary voltage is stepped up to a high value.[18]
It might seem that the output voltage could be increased indefinitely by reducing and . However, as the output voltage increases, it reaches the point where the air next to the high voltage terminal ionizes and corona discharges, brush discharges and streamer arcs break out from the secondary coil. This happens when the electric field strength exceeds the dielectric strength of the air, about 30 kV per centimeter, and occurs first at sharp points and edges on the high voltage terminal. The resulting energy loss damps the oscillation, so the above lossless model is no longer accurate, and the voltage does not reach the theoretical maximum above.[18][24][25]
The top load or "toroid" electrode
Solid state DRSSTC Tesla coil with pointed wire attached to toroid to produce brush discharge
Most unipolar Tesla coil designs have a spherical or toroidal shaped metal electrode on the high voltage terminal. Although the "toroid" increases the secondary capacitance, which tends to reduce the peak voltage, its main effect is that its large diameter curved surface reduces the potential gradient (electric field) at the high voltage terminal, increasing the voltage threshold at which corona and streamer arcs form. Suppressing premature air breakdown and energy loss allows the voltage to build to higher values on the peaks of the waveform, creating longer, more spectacular streamers.[24]
If the top electrode is large and smooth enough, the electric field at its surface may never get high enough even at the peak voltage to cause air breakdown, and air discharges will not occur. Some entertainment coils have a sharp "spark point" projecting from the torus to start discharges.
Types
The term "Tesla coil" is applied to a number of high voltage resonant transformer circuits.
Tesla coil circuits can be classified by the type of "excitation" they use, what type of circuit is used to apply current to the primary winding of the resonant transformer:[30][31]
Spark-excited or Spark Gap Tesla Coil (SGTC) - This type uses a spark gap to switch pulses of current through the primary, exciting oscillation in the transformer. This pulsed (disruptive) drive creates a pulsed high voltage output. Spark gaps have disadvantages due to the high primary currents they must handle. They produce a very loud noise while operating, noxious ozone gas, and high temperatures which often require a cooling system. The energy dissipated in the spark also reduces the Q factor and the output voltage.
Static spark gap - This is the most common type, which was described in detail in the previous section. It is used in most entertainment coils. An AC voltage from a high voltage supply transformer charges a capacitor, which discharges through the spark gap. The spark rate is not adjustable but is determined by the line frequency. Multiple sparks may occur on each half-cycle, so the pulses of output voltage may not be equally-spaced.
Static triggered spark gap - Commercial and industrial circuits often apply a DC voltage from a power supply to charge the capacitor, and use high voltage pulses generated by an oscillator applied to a triggering electrode to trigger the spark.[15] This allows control of the spark rate and exciting voltage. Commercial spark gaps are often enclosed in an insulating gas atmosphere such as sulfur hexafluoride, reducing the length and thus the energy loss in the spark.
Rotary spark gap - These use a spark gap consisting of electrodes around the periphery of a wheel rotated by a motor, which create sparks when they pass by a stationary electrode. Tesla used this type on his big coils, and they are used today on large entertainment coils. The rapid separation speed of the electrodes quenches the spark quickly, allowing "first notch" quenching, making possible higher voltages. The wheel is usually driven by a synchronous motor, so the sparks are synchronized with the AC line frequency, the spark occurring at the same point on the AC waveform on each cycle, so the primary pulses are repeatable.
Switched or Solid State Tesla Coil (SSTC) - These use power semiconductor devices, usually thyristors or transistors such as MOSFETs or IGBTs,[15] to switch pulses of current from a DC power supply through the primary winding. They provide pulsed (disruptive) excitation without the disadvantages of a spark gap: the loud noise and high temperatures. They allow fine control of the voltage, pulse rate and exciting waveform. This type is used in most commercial, industrial, and research applications[15] as well as higher quality entertainment coils.
A simplest single resonant solid state Tesla coil circuit in which the ground end of the secondary supplies the feedback current phase to the transistor oscillatorSingle resonant solid state Tesla coil (SRSSTC) - In this circuit the primary does not have a capacitor and so is not a tuned circuit; only the secondary is. The pulses of current to the primary from the switching transistors excite resonance in the secondary tuned circuit. Single tuned SSTCs are simpler, but don't have as high a Q and cannot produce as high voltage from a given input power as the DRSSTC.
Dual Resonant Solid State Tesla Coil (DRSSTC) - A double tuned Tesla transformer driven by solid state switching supply. This functions similarly to the double tuned spark excited circuits.
Singing Tesla coil or musical Tesla coil - This is a Tesla coil which can be played like a musical instrument, with its high voltage discharges reproducing simple musical tones. The drive current pulses applied to the primary are modulated at an audio rate by a solid state "interrupter" circuit, causing the arc discharge from the high voltage terminal to emit sounds. Only tones and simple chords have been produced so far; the coil cannot function as a loudspeaker, reproducing complex music or voice sounds. The sound output is controlled by a keyboard or MIDI file applied to the circuit through a MIDI interface. Two modulation techniques have been used: AM (amplitude modulation of the exciting voltage) and PFM (pulse-frequency modulation). These are mainly built as novelties for entertainment.
Continuous wave - In these the transformer is driven by a feedback oscillator, which applies a sinusoidal current to the transformer. The primary tuned circuit serves as the tank circuit of the oscillator, and the circuit resembles a radio transmitter. Unlike the previous circuits which generate a pulsed output, they generate a continuous sine wave output. Power vacuum tubes are often used as active devices instead of transistors because they are more robust and tolerant of overloads. In general, continuous excitation produces lower output voltages from a given input power than pulsed excitation.
Tesla circuits can also be classified by how many coils (inductors) they contain:[32][33]
Two coil or double-resonant circuits - Virtually all present Tesla coils use the two coil resonant transformer, consisting of a primary winding to which current pulses are applied, and a secondary winding that produces the high voltage, invented by Tesla in 1891. The term "Tesla coil" normally refers to these circuits.
Three coil, triple-resonant, or magnifier circuits - These are circuits with three coils, based on Tesla's "magnifying transmitter" circuit which he began experimenting with sometime before 1898 and installed in his Colorado Springs lab 1899-1900, and patented in 1902.[34][35][36] They consist of a two coil air-core step-up transformer similar to the Tesla transformer, with the secondary connected to a third coil not magnetically coupled to the others, called the "extra" or "resonator" coil, which is series-fed and resonates with its own capacitance. The presence of three energy-storing tank circuits gives this circuit more complicated resonant behavior. It is the subject of research, but has been used in few practical applications.
History
Henry Rowland's 1889 spark-excited resonant transformer,[37] a predecessor to the Tesla coil.[38]
Steps in Tesla's development of the Tesla transformer around 1891.[39] (1) Closed-core transformers used at low frequencies, (2-7) rearranging windings for lower losses, (8) removed iron core, (9) partial core, (10-11) final conical Tesla transformer, (12-13) Tesla coil circuits
Nikola Tesla patented the Tesla coil circuit April 25, 1891.[40][41] and first publicly demonstrated it May 20, 1891 in his lecture "Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination" before the American Institute of Electrical Engineers at Columbia College, New York.[42][43][44] Although Tesla patented many similar circuits during this period, this was the first that contained all the elements of the Tesla coil: high voltage primary transformer, capacitor, spark gap, and air core "oscillation transformer".
Invention
First drawing of Tesla coil circuit from Tesla's April 25, 1891 patent.[41]
Drawing of Tesla coil circuit from Tesla's May 20, 1891 lecture at Columbia College, New York.[42]
Elihu Thomson's Tesla coil, published February 1892, identical to Tesla's except for a compressed air spark blowout (J).[45]
Tesla's background was in the new field of alternating current power systems, so he understood transformers and resonance.[47][44] In 1888 he decided that high frequencies were the most promising field for research, and set up a laboratory at 33 South Fifth Avenue, New York, where he researched them, initially repeating Hertz's experiments.
Tesla first developed alternators as sources of high frequency current, but by 1890 found they were limited to frequencies of about 20 kHz.[44] In search of higher frequencies he turned to spark-excited resonant circuits.[47] Tesla's innovation was in applying resonance to transformers.[49]Transformers functioned differently at high frequencies than at the low frequencies used in power systems; the iron core in the Ruhmkorff coil transformers he had been using caused energy losses due to eddy currents and hysteresis.[47] Tesla[39][49][44] and Elihu Thomson[38][50][51] independently developed a new type of transformer without an iron core, the "oscillation transformer".
This occurred in 1891 while Tesla was developing a "wireless" lighting system, with a gas discharge light bulb that would glow in an electrostatic field from a high voltage, high frequency power source.[47][44] Tesla powered his Ruhmkorff coil with his high frequency alternator but found that the high frequency current overheated the iron core and melted the insulation between the primary and secondary windings. To fix this problem Tesla changed the design so that there was an air gap instead of insulating material between the windings, and made the iron core adjustable so it could be moved in or out of the coil.[52] Tesla also found he needed to put the capacitor normally used in the Ruhmkorff circuit between his alternator and the coil's primary winding to avoid burning out the coil. By adjusting the coil and capacitor Tesla found he could take advantage of the resonance set up between the two to achieve even higher frequencies.[53] He found that the highest voltages were generated when the primary winding in the "closed" tuned circuit with the capacitor and spark gap, was in resonance with the "open" secondary winding.[49][44]
One of Tesla's early coils at his New York lab in 1892, with a conical secondary.
Prototype "magnifying transmitter"[54][55] in Tesla's New York lab around 1898 producing 2.5 million volts. The round "spiderweb" secondary coil is visible in background
Compact coil designed by Tesla for use as an ozone generator for water treatment[56]
Tesla was not the first to invent this circuit.[57][51]Henry Rowland built a spark-excited resonant transformer circuit (above) in 1889[38] and Elihu Thomson had experimented with similar circuits in 1890, including one which could produce 64 inch (1.6 m) sparks,[45][58][59][37] and other sources confirm Tesla was not the first.[50][60][51] However he was the first to see practical applications for it and patent it. Tesla did not perform detailed mathematical analyses of the circuit, relying instead on trial and error and his intuitive understanding of resonance.[44] He even realized that the secondary coil functioned as a quarter-wave resonator; he specified the length of the wire in the secondary coil must be a quarter wavelength at the resonant frequency.[61][44] The first mathematical analyses of the circuit were done by Anton Oberbeck (1895)[62][51] and Paul Drude (1904).[63][40]
Tesla's demonstrations
Tesla demonstrating wireless lighting at his 1891 lecture at Columbia College.[64].[65] The two metal sheets are connected to a Tesla coil oscillator, which applies a high radio frequency oscillating voltage. The oscillating electric field between the sheets ionizes the low pressure gas in the two long Geissler tubes he is holding, causing them to glow by fluorescence, similar to neon lights, without wires.
A charismatic showman and self-promoter, in 1891-1893 Tesla used the Tesla coil in dramatic public lectures demonstrating the new science of high voltage, high frequency electricity.[64] The radio frequency AC electric currents produced by a Tesla coil did not behave like the DC or low frequency AC current scientists of the time were familiar with. In lectures at Columbia College May 20, 1891,[42] scientific societies in Britain and France during a 1892 European speaking tour,[66] the Franklin Institute, Philadelphia in February 1893, and the National Electric Light Association, St. Louis in March 1893,[67] he impressed audiences with spectacular brush discharges and streamers, heated iron by induction heating, showed RF current could pass through insulators and be conducted by a single wire without a return path, and powered light bulbs and motors without wires.[64] He demonstrated that high frequency currents often did not cause the sensation of electric shock, applying hundreds of thousands of volts to his own body,[68][64] causing his body to light up with a glowing corona discharge in the darkened room. These lectures brought the Tesla "oscillator" to the attention of the scientific community and made Tesla internationally famous.[69][48]
Light bulb (bottom) powered wirelessly by "receiver" coil tuned to resonance with the huge "magnifying transmitter" coil at Tesla's Colorado Springs lab, 1899.[70]
Tesla's proposed wireless power system, from his 1897 patent.[71] The transmitter (left) consists of a Tesla coil (A,C) driving an elevated capacitive terminal (B) suspended by a balloon (D). The receiver (right) is a similar terminal and resonant transformer.
Tesla employed the Tesla coil in his efforts to achieve wireless power transmission,[72] his lifelong dream. In the period 1891 to 1900 he used it to perform some of the first experiments in wireless power,[73][74][75] transmitting radio frequency power across short distances by inductive coupling between coils of wire.[74][75][76] In his early 1890s demonstrations such as those before the American Institute of Electrical Engineers[76] and at the 1893 Columbian Exposition in Chicago he lit light bulbs from across a room.[75] He found he could increase the distance by using a receiving LC circuit tuned to resonance with the Tesla coil's LC circuit,[49] transferring energy by resonant inductive coupling.[75] At his Colorado Springs laboratory during 1899-1900, by using voltages of the order of 10 million volts generated by his enormous magnifying transmitter coil (described below), he was able to light three incandescent lamps at a distance of about 100 feet (30 m).[70][6][77] Today the resonant inductive coupling discovered by Tesla is a familiar concept in electronics, widely used in IF transformers and short range wireless power transmission systems[75][78] such as cellphone charging pads.
It is now understood that inductive and capacitive coupling are "near-field" effects,[75] so they cannot be used for long-distance transmission.[70][79][80][81] However, Tesla was obsessed with developing a long range wireless power transmission system which could transmit power from power plants directly into homes and factories without wires, described in a visionary June 1900 article in Century Magazine; "The Problem of Increasing Human Energy".[82] He claimed to be able to transmit power on a worldwide scale, using a method that involved conduction through the Earth and atmosphere.[71][83][84][72][85] Tesla believed that the entire Earth could act as an electrical resonator, and that by driving current pulses into the Earth at its resonant frequency from a grounded Tesla coil with an elevated capacitance, the potential of the Earth could be made to oscillate, creating global standing waves, and this alternating current could be received with a capacitive antenna tuned to resonance with it at any point on Earth.[86][87][88][83] Another of his ideas was that transmitting and receiving terminals could be suspended in the air by balloons at 30,000 feet (9,100 m) altitude, where the air pressure is lower.[87][54][71][72] At this altitude, he thought, a layer of electrically conductive rarefied air would allow electricity to be sent at high voltages (hundreds of millions of volts) over long distances. Tesla envisioned building a global network of wireless power stations, which he called his "World Wireless System", which would transmit both information and electric power to everyone on Earth.[89]
Magnifying transmitter
Circuit of magnifying transmitter at Tesla's Colorado Springs laboratory.[20][35]C2 represents the parasitic capacitance between the windings of coil L3.
Tesla's wireless research required increasingly high voltages, and he had reached the limit of the voltages he could generate within the space of his New York lab. Between 1899-1900 he built a laboratory in Colorado Springs and performed experiments on wireless transmission there.[35] The Colorado Springs laboratory had one of the largest Tesla coils ever built, which Tesla called a "magnifying transmitter" as it was intended to transmit power to a distant receiver.[90] With an input power of 300 kilowatts it could produce potentials of the order of 10 million volts,[35][86] at frequencies of 50-150 kHz, creating huge "lightning bolts" reportedly up to 135 feet long.[17] During experiments, it caused an overload which set fire to the alternator of the Colorado Springs power company, destroying it, and Tesla had to rebuild the alternator.[17]
In the magnifying transmitter, Tesla used a modified design (see circuit) which he had been experimenting with since before 1898 and patented in 1902,[34][29] different from his previous double-tuned circuits. In addition to the primary (L1) and secondary (L2) coils, it had a third coil (L3) which he called the "extra" coil, not magnetically coupled to the others, attached to the top terminal of the secondary.[35] When driven by the secondary it produced high voltage by resonance, being adjusted to resonate with its own parasitic capacitance(C2)[35] The use of a series-fed resonator coil to generate high voltages was independently discovered by Paul Marie Oudin in 1893 and employed in his Oudin coil.[91]
The Colorado Springs apparatus consisted of a 51 foot (15.5 m) diameter Tesla transformer composed of a secondary winding (L2) of 50 turns of heavy wire wound on an 8 foot (2.4 m) high circular "fence" around the periphery of the lab, and a single-turn primary (L1) either mounted on the fence or buried in the ground under it.[92][93] The primary was connected to a bank of oil capacitors(C1) to make a tuned circuit, with a rotary spark gap (SG), powered by 20 to 40 kilovolts from a powerful utility step-up transformer (T). The top of the secondary was connected to the 100-turn 8 ft (2.4 m) diameter "extra" or "resonator" coil (L3) in the center of the room. It's high voltage end was connected to a telescoping 143 foot (43.6 m) "antenna" rod with a 30 inch (1 m) metal ball on top which could project through the roof of the lab.
Famous image of magnifying transmitter in operation with Tesla sitting next to it. This is a "trick" photo, a double exposure; Tesla was not in the room when the coil was operating.
Coil in operation, at -12 million volts. The 10 ft diameter "extra" coil is shown. The 51 ft. diameter secondary coil is visible dimly in background, and in the previous photo.
Discharge of same coil with a metal sphere capacitive terminal.
Primary circuit, showing oil capacitor bank (boxes, foreground), 40 kV supply transformer and rotary spark gap (rear), and part of secondary winding (wall, left)
The huge "magnifying transmitter" coil at Tesla's Colorado Springs laboratory, 1899-1900, photos by photographer Dickenson Alley December 1899. The long arcs shown above were not a feature of the normal operation of the transmitter because they wasted energy; for these photos Tesla forced the machine to produce arcs by switching the power rapidly on and off.
Wardenclyffe Tower wireless station, one of the largest Tesla coils ever built, intended as a prototype radiotelegraphy and wireless power transmitter, built by Tesla at Shoreham, NY, 1901-1902. It was never completed.
Circuit reportedly used in Wardenclyffe plant
In 1901, convinced his wireless theories were correct, Tesla with financing from banker J. P. Morgan began construction of a high-voltage wireless station, now called the Wardenclyffe Tower, at Shoreham, New York.[83][94] Although it was built as a transatlantic radiotelegraphy station, Tesla also intended it to transmit electric power without wires as a prototype transmitter for his proposed "World Wireless System".[90][89] Essentially an enormous Tesla coil, it consisted of a powerhouse with a 400 horsepower generator and a 187 foot (57 m) tower topped by a 68 foot (21 m) diameter metal dome capacitive electrode.[90][95] Underneath the surface Tesla installed an elaborate ground system that he said was needed to "grip the earth" to create the oscillating earth currents which he believed would transmit the power.
By 1904 his investors had pulled out[89] and the facility was never completed; it was torn down in 1916.[84][90] Although Tesla seems to have believed his wireless power ideas were proven,[96] he had a history of making claims that he had not confirmed by experiment,[97][98] and there seems to be no evidence that he ever transmitted significant power beyond the short-range demonstrations mentioned above.[70][99][74][96][47][100][101][102][103] The few reports of long-distance power transmission by Tesla are not from reliable sources. For example, a widely repeated myth is that in 1899 he wirelessly lit 200 light bulbs at a distance of 26 miles (42 km).[99][96] There is no independent confirmation of this supposed demonstration;[99][96] Tesla did not mention it,[96] and it does not appear in his laboratory notes.[86][104] It originated in 1944 from Tesla's first biographer, John J. O'Neill,[6] who said he pieced it together from "fragmentary material... in a number of publications".[105]
In the 100 years since, others such as Robert Golka[92][106][107] have built equipment similar to Tesla's, but long distance power transmission has not been demonstrated,[108][75][6][96] and the scientific consensus is his World Wireless system would not have worked.[14][73][74][84][96][100][109][103] Contemporary scientists point out that while Tesla's coils (with appropriate antennas) can function as radio transmitters, transmitting energy in the form of radio waves, the frequency he used, around 150 kHz, is far too low for practical long range power transmission.[74][96][101] At these wavelengths the radio waves spread out in all directions and cannot be focused on a distant receiver.[73][74][96][100][109] Tesla's world power transmission scheme remains today what it was in Tesla's time: a bold, fascinating dream.[84][100]
Powerful spark-gap transmitter, showing series spark gaps (horizontal cylindrical objects), Leyden jar capacitors (vertical cylinders, rear), and resonant transformer (top)
Spark transmitter circuit from Marconi's 1900 patent.[110] It's similarity to a Tesla coil can be seen; the only difference is the addition of a variable inductor (g) to tune the antenna (f) to resonance.[111][69]
"[The Tesla coil] was invented not for wireless but for making vacuum lamps glow without external electrodes, and it later played a principal part in other hands in the operation of big spark stations." --William H. Eccles, 1933[112]
One of the largest applications of the Tesla coil circuit was in early radio transmitters called spark gap transmitters. The first radio wave generators, invented by Heinrich Hertz in 1887, were spark gaps connected directly to antennas, powered by induction coils.[113][114][48] Because they lacked a resonant circuit, these transmitters produced highly damped radio waves. As a result their transmissions occupied an extremely wide bandwidth of frequencies. When multiple transmitters were operating in the same area their frequencies overlapped and they interfered with one another, causing garbled reception. There was no way for a receiver to select one signal over another.[114][113]
In 1892 William Crookes, a friend of Tesla, had given a lecture[115] on the uses of radio waves in which he suggested using resonance to reduce the bandwidth in transmitters and receivers. By using resonant circuits, different transmitters could be "tuned" to transmit on different frequencies. With narrower bandwidth, separate transmitter frequencies would no longer overlap, so a receiver could receive a particular transmission by "tuning" its resonant circuit to the same frequency as the transmitter.[113][48][111] This is the system used in all modern radio.
With an appropriate wire antenna, the Tesla coil circuit could function as such a narrow bandwidth radio transmitter.[10][50][17][1] In his March 1893 St. Louis lecture,[67] Tesla demonstrated a wireless system that was the first use of tuned circuits in radio, although he used it for wireless power transmission, not radio communication.[69][48][116][111][117][118] A grounded spark-excited capacitor-tuned Tesla transformer attached to an elevated wire antenna transmitted radio waves, which were received across the room by a wire antenna attached to a receiver consisting of a second grounded resonant transformer tuned to the transmitter's frequency, which lighted a Geissler tube.[119][113][111][118] This system, patented by Tesla September 2, 1897,[71] was the first use of the "four circuit" concept later claimed by Marconi.[120][118][69][117] However, Tesla was mainly interested in wireless power and never developed a practical radio communication system.[96][8][119][113] In fact, he never believed that radio waves could be used for practical communication, instead clinging to an erroneous theory that radio communication was due to currents in the Earth.[121]
Practical radiotelegraphy communication systems were developed by Guglielmo Marconi beginning in 1895. By 1897 the advantages of narrow-bandwidth (lightly damped) systems noted by Crookes were recognized, and resonant circuits, capacitors and inductors, were incorporated in transmitters and receivers.[116] The "closed primary, open secondary" resonant transformer circuit used by Tesla proved a superior transmitter,[117] because the loosely-coupled transformer partially isolated the oscillating primary circuit from the energy-radiating antenna circuit, reducing the damping, allowing it to produce long "ringing" waves which had a narrower bandwidth.[51][50][113] Versions of the circuit were patented by Marconi,[110][117]John Stone Stone[122] and Oliver Lodge,[123] and were widely used in radio for twenty years.[48][116][72][113][111] In 1906 Max Wien invented the quenched or "series" spark gap, which extinguished the spark after the energy had been transferred to the secondary, allowing the secondary to oscillate freely after that, reducing damping and bandwith still more.
Although their damping had been reduced as much as possible, spark transmitters still produced damped waves which had a wide bandwidth, creating interference with other transmitters. Around 1920 they became obsolete, superseded by vacuum tube transmitters which generated continuous waves at a single frequency, which could also be modulated to carry sound. Tesla's resonant transformer continued to be used in vacuum tube transmitters and receivers, and is a key component in radio to this day.[14]
During the "spark era" the radio engineering profession gave credit to Tesla,[113] his circuit became known as the "Tesla coil" or "Tesla transformer".[48][50][12] However Tesla did not benefit financially, due to competing patent claims. Marconi had claimed rights to the "closed primary open secondary" transmitter circuit in his controversial 1900 "four circuit" wireless patent.[110][120][117][72][111] Tesla sued Marconi in 1915 for patent infringement, but didn't have the resources to pursue the action.[113][117][116][72] However in 1943, in a separate suit brought by the Marconi Company against the US government for use of its patents in WW1, the US Supreme Court invalidated Marconi's 1900 patent claim to the "four circuit" concept.[124][48][72][111][13] The ruling cited the prior patents of Tesla, Lodge, and Stone,[113][48] but did not decide which of these parties had rights to the circuit.[72][117][111] Of course by this time the issue was moot; the patent had expired in 1915 and spark transmitters had long been obsolete.
Although there is some disagreement over the role Tesla himself played in the invention of radio,[125][48][72][13] sources agree on the importance of his circuit in early radio transmitters.[111][27][17][1][117][113][14] From a modern perspective, most spark transmitters could be regarded as Tesla coils.[17][10]
Use in medicine
Small Tesla coil for electrotherapy, 1905. The Tesla transformer is immersed in a tank of oil for insulation to prevent arcs.
Effluvation treatment of knee with an Oudin coil(left), a high voltage transformer similar to a Tesla coil, 1915
Tesla electrotherapy coil manufactured by Adolphe Gaiffe, around 1900. The primary capacitor is in the box; the spark gap is mounted on top.
Treatment of cancer with an Oudin coil (left), 1910. The induction coil that powers the Oudin coil is behind the patient's head.
Combined Tesla / D'Arsonval / Oudin electrotherapy and x-ray outfit 1907
Electrotherapy treatment of diabetes with a vacuum electrode, 1922. The series spark gap is visible mounted on the front of the machine.
Longwave spark diathermy machine using Tesla circuit, 1921.
Diathermy of elbow 1945
Vacuum electrode "violet ray" wand in operation.
A violet ray wand, a handheld Tesla coil sold as a quack home medical device until about 1940. Said to cure everything from carbuncles to lumbago.
The three circuits used in electrotherapy apparatus in the early 20th century: (1) Tesla coil, (2) D'Arsonval coil, (3) Oudin coil. In medical coils for safety two capacitors (Leyden jars) were used, one in each branch of the primary circuit, to completely isolate the patient's body from the potentially lethal currents of the supply transformer, in case of an electrical fault.[126]
Tesla had observed as early as 1891 that high frequency currents above 10 kHz did not cause the sensation of electric shock, and in fact currents that would be lethal at lower frequencies could be passed through the body without apparent harm.[127][128][129] He experimented on himself, and found daily applications of high voltage relieved depression after his lab burned down in 1895.[130] He was one of the first to observe the heating effect of high frequency currents on the body, the basis of diathermy.[131][132] During his highly-publicized early 1890s demonstrations he passed hundreds of thousands of volts through his body.[68][64] With characteristic hyperbole he called electricity "the greatest of all doctors"[130] and suggested burying wires under classrooms so its stimulating effect would improve performance of "dull" schoolchildren.[132][133] Tesla wrote a pioneering paper in 1898 on the medical uses of high frequency currents[128][134][129] but did no further work on the subject.
A few other researchers were also experimentally applying high frequency currents to the body at this time.[135][136][137][38][138]Elihu Thomson, the co-inventor of the Tesla coil, was one, so in medicine the Tesla coil became known as the "Tesla-Thomson apparatus".[38] In France, from 1889 physician and pioneering biophysicist Jacques d'Arsonval had been documenting the physiological effects of high frequency current on the body, and had made the same discoveries as Tesla.[139][131][138] During his 1892 European trip Tesla met with D'Arsonval and was flattered to find they were using similar circuits. D'Arsonval's spark-excited resonant circuits (above) did not produce as high voltage as the Tesla transformer.[38] In 1893 French physician Paul Marie Oudin added a "resonator" coil to the D'Arsonval circuit to create the high voltage Oudin coil,[138][140] a circuit very similar to the Tesla coil, which was widely used for treating patients in Europe.[38]
During this period, people were fascinated by the new technology of electricity, and many believed it had miraculous curative or "vitalizing" powers.[141]Medical ethics were also looser, and doctors could experiment on their patients. By the turn of the century, application of high voltage, "high frequency" currents to the body had become part of a Victorian era medical field, part legitimate experimental medicine and part quack medicine,[129] called electrotherapy.[141][91][135] Manufacturers produced medical apparatus to generate "Tesla currents", "D'Arsonval currents", and "Oudin currents" for physicians. In electrotherapy, a pointed electrode attached to the high voltage terminal of the coil was held near the patient, and the luminous brush discharges from it (called "effluves") were applied to parts of the body to treat a wide variety of medical conditions. In order to apply the electrode directly to the skin, or tissues inside the mouth, anus or vagina, a "vacuum electrode" was used, consisting of a metal electrode sealed inside a partially evacuated glass tube, which produced a dramatic violet glow. The glass wall of the tube and the skin surface formed a capacitor which limited the current to the patient, preventing discomfort. These "violet ray" wands were later sold as a quack home medical device.[142][143]
The popularity of electrotherapy peaked after World War 1,[131][141] but by the 1920s authorities began to crack down on fraudulent medical treatments, and electrotherapy largely became obsolete. A part of the field that survived was diathermy, the application of high frequency current to heat body tissue, pioneered by German physician Karl Nagelschmidt in 1907 using Tesla coils.[131][138] By 1930 "long wave" (0.5~2 MHz) Tesla coil diathermy machines were being replaced by "short wave" (10~100 MHz) vacuum tube diathermy machines,[131][138] but Tesla coils continued to be used in both diathermy[131] and quack medical devices like violet ray[142] until World War 2.
During the 1920s and 30s all unipolar (single terminal) high voltage medical coils came to be called Oudin coils, so today's unipolar Tesla coils are sometimes referred to as "Oudin coils".[144]
Use in show business
"Electrice" sideshow performer being "electrocuted" 1914[145]
"Electrice" lighting a candle with brush discharge from her fingers.[145] The current came from the electric chair she is touching, which is connected to the Tesla coil in the background.
Evangelist Irwin Moon shooting "lightning bolts" from fingers, 1938.
Demonstrating 10 inch (25 cm) brush discharge from hand, 1913[146]
RF current from Tesla coil lights the bulb's filament as it passes through the wire to charge and discharge the performer's body, which acts as a capacitor plate.[146]
Turn-of-the-century sideshow performers did stunts with Tesla coils that would be considered extremely dangerous today. DON'T TRY THESE
The Tesla coil's spectacular displays of sparks, and the fact that its currents could pass through the human body without causing electric shock, led to its use in the entertainment business.
In the early 20th century it appeared in traveling carnivals, freak shows and circus and carnival sideshows, which often had an act in which a performer would pass high voltages through his body[68][147][145][148][149] Performers such as "Dr. Resisto", "The Human Dynamo", "Electrice", "The Great Volta", and "Madamoiselle Electra" would have their body connected to the high voltage terminal of a hidden Tesla coil, causing sparks to shoot from their fingertips and other parts of their body, and Geissler tubes to light up when held in their hand or even brought near them.[146][150] They could also light candles or cigarettes with their fingers.[145] Although they didn't usually cause electric shocks, RF arc discharges from the bare skin could cause painful burns; to prevent them performers sometimes wore metal thimbles on their fingertips[145](Rev. Moon, center image above, is using them). These acts were extremely dangerous and could kill the performer if the Tesla coil was misadjusted.[148] In carny lingo this was called an "electric chair act" because it often included a spark-laced "electrocution" of the performer in an electric chair,[148][149] exploiting public fascination with this exotic new method of capital punishment, which had become the United States' dominant method of execution around 1900. Today entertainers still perform high voltage acts with Tesla coils,[151][152] but modern bioelectromagnetics has brought a new awareness of the hazards of Tesla coil currents, and allowing them to pass through the body is today considered extremely dangerous.
Tesla coils were also used as dramatic props in early mystery and science fiction motion pictures, starting in the silent era.[68] The crackling, writhing sparks emanating from the electrode of a giant Tesla coil became Hollywood's iconic symbol of the "mad scientist's" lab, recognized throughout the world.[153] This was probably because the eccentric Nikola Tesla himself, with his famous high voltage demonstrations and his mysterious Colorado Springs laboratory, was one of the main prototypes from which the "mad scientist" stock character originated.[153][154] Some early films in which Tesla coils appeared were Wolves of Kultur (1918), The Power God (1926), Metropolis (1927), Frankenstein (1931) and its many sequels such as Son of Frankenstein (1939), The Mask of Fu Manchu (1932), Chandu the Magician (1932), The Lost City (1935), and The Clutching Hand (1936)[155][68] and many later films and television shows. By the 1980s, effects like high voltage sparks were being added to movies by CGI as visual effects in post-production, eliminating the need for dangerous high voltage Tesla coils on sets.
The Tesla coils for many of these movies were constructed by Kenneth Strickfaden (1896-1984) who, beginning with his spectacular effects in the 1931 Frankenstein, became Hollywood's preeminent electrical special effects expert.[68][156] His large "Meg Senior" Tesla coil seen in many of these movies consisted of a 6 foot 1000 turn conical secondary and a 10 turn primary, connected to a capacitor through a rotary spark gap, powered by a 20 kV transformer.[156] It could produce 6 foot sparks. Some of his last gigs were the reassembly of the original 1931 Frankenstein high voltage apparatus for the Mel Brooks satire Young Frankenstein (1974), and construction of a million volt Tesla coil which produced 12 foot sparks for a 1976 stage show by the rock band Kiss.[155]
Use in education
Million volt Griffith Park Observatory coil, Los Angeles. Over 100 years old, it is one of the oldest working Tesla coils.
Demonstration of inductance with a Tesla coil, 1906.[157] RF current will not pass through the heavy copper wire because of the bend, and passes through the lamp instead.
Small educational Tesla coil kit, 1918
Ever since Tesla's 1890s lectures, Tesla coils have been used as attractions in educational exhibits and science fairs. They have become a way to counter the stereotype that science is boring.[158] In the early 20th century, experts like Henry Transtrom and Earle Ovington gave high voltage demonstrations at "electric fairs".[146] High school classes built Tesla coils.
From 1933 into the 1980s, between movie jobs Hollywood special effects expert Ken Strickfaden would take his high voltage apparatus on the road in an exhibition called "Science on Parade" and later "The Kenstric Space Age Science Show" to high schools, colleges, World Fairs and expositions.[158] These spectacular shows, which reached 48 states, had a seminal influence on the birth of the modern "coiling" movement.[155] A number of present-day Tesla hobbyists such as William Wysock say they were inspired to build Tesla coils by seeing Strickfaden's show.[158]
One of the oldest and best-known coils still in operation is the "GPO-1" at Griffith Park Observatory in Los Angeles. It was originally one of a pair of coils built in 1910 by Earle L. Ovington, a friend of Tesla and manufacturer of high voltage electrotherapy apparatus.[159][160][68] For a number of years Ovington displayed them at the December electrical trade show at Madison Square Garden in New York City, using them for demostrations of high voltage science, which Tesla himself sometimes attended.[160] Called the Million Volt Oscillator, the twin coils were installed on the balcony at the show. Every hour the lights were dimmed and the public was treated to a display of 10 foot arcs. Ovington gave the coils to his friend Dr. Frederick Finch Strong, a leading figure in the alternative health field of electrotherapy. In 1937 Strong donated the coils to the Griffith Observatory. The museum didn't have room to display both, but one coil was restored by Kenneth Strickfaden and has been in daily operation ever since.[68] It consists of a 48 in. (1.2 m) high conical secondary coil topped by a 12 in. (30 cm) diameter copper ball electrode, with a 9-turn spiral primary of 2 in. copper strip, a glass plate capacitor (replacing the original Leyden jars), and rotary spark gap.[159] Its output has been estimated at 1.3 million volts.[160]
Later uses
Breit and Tuve's 5 MV Tesla coil used as particle accelerator, 1928
In addition to its use in spark-gap radio transmitters and electrotherapy described above, the Tesla coil circuit was also used in the early 20th century in x-ray machines, ozone generators for water purification, and induction heating equipment. However in the 1920s vacuum tubeoscillators replaced it in all these applications.[10] The triode vacuum tube was a much better radio frequency current generator than the noisy, hot, ozone-producing spark, and could produce continuous waves. After this, industrial use of the Tesla coil was mainly limited to a few specialized applications which were suited to its unique characteristics, such as high voltage insulation testing.
In 1926, pioneering accelerator physicists Merle Tuve and Gregory Breit built a 5 million volt Tesla coil as a linear particle accelerator.[161][162][163] The bipolar coil consisted of a pyrex tube a meter long wound with 8000 turns of fine wire, with round corona caps on each end, and a 5 turn spiral primary coil surrounding it at the center. It was operated in a tank of insulating oil pressurized to 500 psi which allowed it to reach a potential of 5.2 megavolts. Although it was used for a short period in 1929-30 it was not a success because the particles' acceleration had to be completed within the brief period of a half cycle of the RF voltage.
In 1970 Robert K. Golka built a replica of Tesla's huge Colorado Springs magnifying transmitter in a shed at Wendover Air Force Base, Utah, using data he found in Tesla's lab notes archived at the Nikola Tesla Museum in Beograd, Serbia.[92][106][107][36] This was one of the first experiments with the magnifier circuit since Tesla's time. The coil generated 12 million volts. Golka used it to try to duplicate Tesla's reported synthesis of ball lightning.
Electric discharge showing the lightning-like plasma filaments from a 'Tesla coil'Tesla coil (discharge).Tesla coil in terrarium (I)
Modern high-voltage enthusiasts usually build Tesla coils similar to some of Tesla's "later" 2-coil air-core designs. These typically consist of a primary tank circuit, a series LC (inductance-capacitance) circuit composed of a high-voltage capacitor, spark gap and primary coil, and the secondary LC circuit, a series-resonant circuit consisting of the secondary coil plus a terminal capacitance or "top load". In Tesla's more advanced (magnifier) design, a third coil is added. The secondary LC circuit is composed of a tightly coupled air-core transformer secondary coil driving the bottom of a separate third coil helical resonator. Modern 2-coil systems use a single secondary coil. The top of the secondary is then connected to a topload terminal, which forms one 'plate' of a capacitor, the other 'plate' being the earth (or "ground"). The primary LC circuit is tuned so that it resonates at the same frequency as the secondary LC circuit. The primary and secondary coils are magnetically coupled, creating a dual-tuned resonant air-core transformer. Earlier oil-insulated Tesla coils needed large and long insulators at their high-voltage terminals to prevent discharge in air. Later Tesla coils spread their electric fields over larger distances to prevent high electrical stresses in the first place, thereby allowing operation in free air. Most modern Tesla coils also use toroid-shaped output terminals. These are often fabricated from spun metal or flexible aluminum ducting. The toroidal shape helps to control the high electrical field near the top of the secondary by directing sparks outward and away from the primary and secondary windings.
A more complex version of a Tesla coil, termed a "magnifier" by Tesla, uses a more tightly coupled air-core resonance "driver" transformer (or "master oscillator") and a smaller, remotely located output coil (called the "extra coil" or simply the resonator) that has a large number of turns on a relatively small coil form. The bottom of the driver's secondary winding is connected to ground. The opposite end is connected to the bottom of the extra coil through an insulated conductor that is sometimes called the transmission line. Since the transmission line operates at relatively high RF voltages, it is typically made of 1" diameter metal tubing to reduce corona losses. Since the third coil is located some distance away from the driver, it is not magnetically coupled to it. RF energy is instead directly coupled from the output of the driver into the bottom of the third coil, causing it to "ring up" to very high voltages. The combination of the two-coil driver and third coil resonator adds another degree of freedom to the system, making tuning considerably more complex than that of a 2-coil system. The transient response for multiple resonance networks (of which the Tesla magnifier is a sub-set) has only recently been solved.[164] It is now known that a variety of useful tuning "modes" are available, and in most operating modes the extra coil will ring at a different frequency than the master oscillator.[165]
Modern transistor or vacuum tube Tesla coils do not use a primary spark gap. Instead, the transistor(s) or vacuum tube(s) provide the switching or amplifying function necessary to generate RF power for the primary circuit. Solid-state Tesla coils use the lowest primary operating voltage, typically between 155 and 800 volts, and drive the primary winding using either a single, half-bridge, or full-bridge arrangement of bipolar transistors, MOSFETs or IGBTs to switch the primary current. Vacuum tube coils typically operate with plate voltages between 1500 and 6000 volts, while most spark gap coils operate with primary voltages of 6,000 to 25,000 volts. The primary winding of a traditional transistor Tesla coil is wound around only the bottom portion of the secondary coil. This configuration illustrates operation of the secondary as a pumped resonator. The primary 'induces' alternating voltage into the bottom-most portion of the secondary, providing regular 'pushes' (similar to providing properly timed pushes to a playground swing). Additional energy is transferred from the primary to the secondary inductance and top-load capacitance during each "push", and secondary output voltage builds (called 'ring-up'). An electronic feedback circuit is usually used to adaptively synchronize the primary oscillator to the growing resonance in the secondary, and this is the only tuning consideration beyond the initial choice of a reasonable top-load.
Demonstration of the Nevada Lightning Laboratory 1:12 scale prototype twin Tesla Coil at Maker Faire 2008
In a dual resonant solid-state Tesla coil (DRSSTC), the electronic switching of the solid-state Tesla coil is combined with the resonant primary circuit of a spark-gap Tesla coil. The resonant primary circuit is formed by connecting a capacitor in series with the primary winding of the coil, so that the combination forms a series tank circuit with a resonant frequency near that of the secondary circuit. Because of the additional resonant circuit, one manual and one adaptive tuning adjustment are necessary. Also, an interrupter is usually used to reduce the duty cycle of the switching bridge, to improve peak power capabilities; similarly, IGBTs are more popular in this application than bipolar transistors or MOSFETs, due to their superior power handling characteristics. A current-limiting circuit is usually used to limit maximum primary tank current (which must be switched by the IGBT's) to a safe level. Performance of a DRSSTC can be comparable to a medium-power spark-gap Tesla coil, and efficiency (as measured by spark length versus input power) can be significantly greater than a spark-gap Tesla coil operating at the same input power.
A large Tesla coil of more modern design often operates at very high peak power levels, up to many megawatts (millions of watts[166]). It is therefore adjusted and operated carefully, not only for efficiency and economy, but also for safety. If, due to improper tuning, the maximum voltage point occurs below the terminal, along the secondary coil, a discharge (spark) may break out and damage or destroy the coil wire, supports, or nearby objects.
Typical circuit configuration Here, the spark gap shorts the high frequency across the first transformer that is supplied by alternating current. An inductance, not shown, protects the transformer. This design is favoured when a relatively fragile neon sign transformer is used.
Alternative circuit configuration With the capacitor in parallel to the first transformer and the spark gap in series to the Tesla-coil primary, the AC supply transformer must be capable of withstanding high voltages at high frequencies.
Tesla experimented with these, and many other, circuit configurations (see right). The Tesla coil primary winding, spark gap and tank capacitor are connected in series. In each circuit, the AC supply transformer charges the tank capacitor until its voltage is sufficient to break down the spark gap. The gap suddenly fires, allowing the charged tank capacitor to discharge into the primary winding. Once the gap fires, the electrical behavior of either circuit is identical. Experiments have shown that neither circuit offers any marked performance advantage over the other.
However, in the typical circuit, the spark gap's short circuiting action prevents high-frequency oscillations from 'backing up' into the supply transformer. In the alternate circuit, high amplitude high frequency oscillations that appear across the capacitor also are applied to the supply transformer's winding. This can induce corona discharges between turns that weaken and eventually destroy the transformer's insulation. Experienced Tesla coil builders almost exclusively use the top circuit, often augmenting it with low pass filters (resistor and capacitor (RC) networks) between the supply transformer and spark gap to help protect the supply transformer. This is especially important when using transformers with fragile high-voltage windings, such as neon sign transformers (NSTs). Regardless of which configuration is used, the HV transformer must be of a type that self-limits its secondary current by means of internal leakage inductance. A normal (low leakage inductance) high-voltage transformer must use an external limiter (sometimes called a ballast) to limit current. NSTs are designed to have high leakage inductance to limit their short circuit current to a safe level.
Tuning precautions
The primary coil's resonant frequency is tuned to that of the secondary, by using low-power oscillations, then increasing the power (and retuning if necessary) until the system operates properly at maximum power. While tuning, a small projection (called a "breakout bump") is often added to the top terminal in order to stimulate corona and spark discharges (sometimes called streamers) into the surrounding air. Tuning can then be adjusted so as to achieve the longest streamers at a given power level, corresponding to a frequency match between the primary and secondary coil. Capacitive 'loading' by the streamers tends to lower the resonant frequency of a Tesla coil operating under full power. A toroidal topload is often preferred to other shapes, such as a sphere. A toroid with a major diameter that is much larger than the secondary diameter provides improved shaping of the electrical field at the topload. This provides better protection of the secondary winding (from damaging streamer strikes) than a sphere of similar diameter. And, a toroid permits fairly independent control of topload capacitance versus spark breakout voltage. A toroid's capacitance is mainly a function of its major diameter, while the spark breakout voltage is mainly a function of its minor diameter.
Air discharges
A small, later-type Tesla coil in operation: The output is giving 43-cm sparks. The diameter of the secondary is 8 cm. The power source is a 10 000 V,60 Hzcurrent-limited supply.
While generating discharges, electrical energy from the secondary and toroid is transferred to the surrounding air as electrical charge, heat, light, and sound. The process is similar to charging or discharging a capacitor, except that a Tesla coil uses AC instead of DC. The current that arises from shifting charges within a capacitor is called a displacement current. Tesla coil discharges are formed as a result of displacement currents as pulses of electrical charge are rapidly transferred between the high-voltage toroid and nearby regions within the air (called space charge regions). Although the space charge regions around the toroid are invisible, they play a profound role in the appearance and location of Tesla coil discharges.
When the spark gap fires, the charged capacitor discharges into the primary winding, causing the primary circuit to oscillate. The oscillating primary current creates an oscillating magnetic field that couples to the secondary winding, transferring energy into the secondary side of the transformer and causing it to oscillate with the toroid capacitance to ground. Energy transfer occurs over a number of cycles, until most of the energy that was originally in the primary side is transferred to the secondary side. The greater the magnetic coupling between windings, the shorter the time required to complete the energy transfer. As energy builds within the oscillating secondary circuit, the amplitude of the toroid's RF voltage rapidly increases, and the air surrounding the toroid begins to undergo dielectric breakdown, forming a corona discharge.
As the secondary coil's energy (and output voltage) continue to increase, larger pulses of displacement current further ionize and heat the air at the point of initial breakdown. This forms a very electrically conductive "root" of hotter plasma, called a leader, that projects outward from the toroid. The plasma within the leader is considerably hotter than a corona discharge, and is considerably more conductive. In fact, its properties are similar to an electric arc. The leader tapers and branches into thousands of thinner, cooler, hair-like discharges (called streamers). The streamers look like a bluish 'haze' at the ends of the more luminous leaders. The streamers transfer charge between the leaders and toroid to nearby space charge regions. The displacement currents from countless streamers all feed into the leader, helping to keep it hot and electrically conductive.
The primary break rate of sparking Tesla coils is slow compared to the resonant frequency of the resonator-topload assembly. When the switch closes, energy is transferred from the primary LC circuit to the resonator where the voltage rings up over a short period of time up culminating in the electrical discharge. In a spark gap Tesla coil, the primary-to-secondary energy transfer process happens repetitively at typical pulsing rates of 50–500 times per second, depending on the frequency of the input line voltage. At these rates, previously-formed leader channels do not get a chance to fully cool down between pulses. So, on successive pulses, newer discharges can build upon the hot pathways left by their predecessors. This causes incremental growth of the leader from one pulse to the next, lengthening the entire discharge on each successive pulse. Repetitive pulsing causes the discharges to grow until the average energy available from the Tesla coil during each pulse balances the average energy being lost in the discharges (mostly as heat). At this point, dynamic equilibrium is reached, and the discharges have reached their maximum length for the Tesla coil's output power level. The unique combination of a rising high-voltage radio frequency envelope and repetitive pulsing seem to be ideally suited to creating long, branching discharges that are considerably longer than would be otherwise expected by output voltage considerations alone. High-voltage, low-energy discharges create filamentary multibranched discharges which are purplish-blue in colour. High-voltage, high-energy discharges create thicker discharges with fewer branches, are pale and luminous, almost white, and are much longer than low-energy discharges, because of increased ionisation. A strong smell of ozone and nitrogen oxides will occur in the area. The important factors for maximum discharge length appear to be voltage, energy, and still air of low to moderate humidity. There are comparatively few scientific studies about the initiation and growth of pulsed lower-frequency RF discharges, so some aspects of Tesla coil air discharges are not as well understood when compared to DC, power-frequency AC, HV impulse, and lightning discharges.
Applications
Tesla coil circuits were used commercially in sparkgap radio transmitters for wireless telegraphy until the 1920s,[1][10][11] and in electrotherapy and pseudomedical devices such as violet ray. Today, although small Tesla coils are used as leak detectors in scientific high vacuum systems[9] and igniters in arc welders,[167] their main use is entertainment and educational displays, Tesla coils are built by many high-voltage enthusiasts, research institutions, science museums, and independent experimenters. Although electronic circuit controllers have been developed, Tesla's original spark gap design is less expensive and has proven extremely reliable.
Entertainment
Tesla coils are very popular devices among certain electrical engineers and electronics enthusiasts. Builders of Tesla coils as a hobby are called "coilers". A very large Tesla coil, designed and built by Syd Klinge, is shown every year at the Coachella Valley Music and Arts Festival, in Coachella, Indio, California, USA. People attend "coiling" conventions where they display their home-made Tesla coils and other electrical devices of interest. Austin Richards, a physicist in California, created a metal Faraday Suit in 1997 that protects him from Tesla Coil discharges. In 1998, he named the character in the suit Doctor MegaVolt and has performed all over the world and at Burning Man 9 different years.
Low-power Tesla coils are also sometimes used as a high-voltage source for Kirlian photography.[168]
Tesla coils can also be used to generate sounds, including music, by modulating the system's effective "break rate" (i.e., the rate and duration of high power RF bursts) via MIDI data and a control unit. The actual MIDI data is interpreted by a microcontroller which converts the MIDI data into a PWM output which can be sent to the Tesla coil via a fiber optic interface.[169][170] The YouTube video Super Mario Brothers theme in stereo and harmony on two coils shows a performance on matching solid state coils operating at 41 kHz. The coils were built and operated by designer hobbyists Jeff Larson and Steve Ward. The device has been named the Zeusaphone, after Zeus, Greek god of lightning, and as a play on words referencing the Sousaphone. The idea of playing music on the singing Tesla coils flies around the world and a few followers[171] continue the work of initiators. An extensive outdoor musical concert has demonstrated using Tesla coils during the Engineering Open House (EOH) at the University of Illinois at Urbana-Champaign. The Icelandic artist Björk used a Tesla coil in her song "Thunderbolt" as the main instrument in the song. The musical group ArcAttack uses modulated Tesla coils and a man in a chain-link suit to play music.
The world's largest currently existing two-coil Tesla coil is a 130,000-watt unit, part of a 38-foot-tall (12 m) sculpture titled Electrum owned by Alan Gibbs and currently resides in a private sculpture park at Kakanui Point near Auckland, New Zealand.[172] The most powerful conical Tesla coil (1.5 million volts) was installed in 2002 at the Mid-America Science Museum in Hot Springs, Arkansas.[173] This is a replica of the Griffith Observatory conical coil installed in 1936.
Vacuum system leak detectors
Scientists working with high vacuum systems test for the presence of tiny pin holes in the apparatus (especially a newly blown piece of glassware) using high-voltage discharges produced by a small handheld Tesla coil. When the system is evacuated the high voltage electrode of the coil is played over the outside of the apparatus. The discharge travels through any pin hole immediately below it, producing a corona discharge inside the evacuated space which illuminates the hole, indicating points that need to be annealed or reblown before they can be used in an experiment.
Wireless power
Tesla demonstrating wireless power transfer during an 1891 lecture at Columbia College. The two metal sheets are connected to a Tesla coil oscillator, which applies high-voltage radio frequency alternating current. An oscillating electric field between the sheets ionizes the low-pressure gas in the two long Geissler tubes in his hands, causing them to glow.An incandescent light bulb is powered at a distance of 1,938 feet (591 m) using a Tesla coil "receiver" tuned to resonance with the Colorado Springs magnifying transmitter.[174][175]
In the period from 1891 to 1904 Tesla used his resonant transformer circuit to perform experiments in wireless power transfer.[73][74][75] With this basic resonance transformer design concept he was able to transmit electrical energy over short distances without interconnecting wires by resonant magneto-inductive coupling.[74][75][176][76][177] The transformer's primary LC circuit acted as a transmitter. The transformer's secondary LC circuit was tuned to the primary LC circuit's resonant frequency and acted as a receiver. He found the Tesla coil transformer itself could be configured as a wireless transmitter and used to transfer energy by resonant electro-inductive coupling. While demonstrating this technology during lectures before the American Institute of Electrical Engineers in 1891, the Institution of Electrical Engineers in 1892, and at the 1893 Columbian Exposition in Chicago he was able to wirelessly power lamps from across the stage and out into the room.[178][179][180] At his Colorado Springs laboratory (1899-1900), he assembled an improved version of his resonance transformer called a Tesla coil magnifying transmitter, capable of producing voltages on the order of 10 megavolts. In one demonstration, using just the oscillator's primary LC circuit energized to only one-twentieth of the oscillator's full capacity, he was able to light three incandescent lamps by resonant magneto-inductive coupling at a distance of about one hundred feet (30 m).[181][82] The resonant inductive coupling technique pioneered by Tesla has recently become a central concept in modern wireless power development and is being widely used in short- and mid-range wireless transmission systems[75] like cellphone charging pads. The resonant magnetic induction and resonant capacitive induction coupling techniques make use of "near-field" effects,[75] meaning the energy transferred decreases with the sixth power of the distance between transmitter and receiver,[75][79][80][81] so they are not suitable for the long range transmission of electrical energy.
Tesla's top priority at Colorado Springs was the development of a wireless system that could transmit electrical energy over great distances, as proposed in his 1900 Century magazine article.[182][183][84][85] He claimed that it is possible to transmit energy on a worldwide scale, applying a method that involves electrical conduction through the earth and the periodic alteration of Earth's electrostatic charge.[83][184][185] In 1900 he received the patents SYSTEM OF TRANSMISSION OF ELECTRICAL ENERGY and APPARATUS FOR TRANSMISSION OF ELECTRICAL ENERGY.[186][187] They describe two hypothetical wireless stations, one consisting of a large Tesla coil magnifying transmitter and the other a similar Tesla coil receiver, each with an elevated air terminal electrode suspended by balloon at an altitude of 30,000 feet (9,100 m), where the atmospheric pressure is lower. Tesla believed atmospheric ionization would allow energy to be transmitted at high voltages (millions of volts) over long distances by electrical conduction. Another claim was that such a high elevation of the air terminal electrodes is not absolutely necessary.[188] By 1901 Tesla had come to believe the entire planet could be made to act as a giant electrical resonator and that by driving sufficiently powerful current pulses into Earth at a harmonic of its fundamental resonant frequency using a grounded Tesla coil transmitter working against a relatively short elevated capacitive air terminal electrode, its natural electrostatic potential could be made to oscillate, and this alternating current could be received at any location with a Tesla coil receiver and similar capacitive air terminal electrode arrangement tuned to resonance with the transmitter.[87][189] Tesla demonstrated the coupling of a Tesla coil transmitter and a Tesla coil receiver in an experiment at his Experimental Station in Colorado Springs, transmitting enough electrical energy to power a 10-watt incandescent lamp at a distance of 1,938 feet (591 m) from the station's magnifying transmitter.[174][175]
Tesla claimed his ideas were proven but there is little direct corroboration that he ever transferred power beyond the above-mentioned demonstrations.[74][100][49][97][96][101][190][191][102] His belief that long range wireless energy transmission is possible by means of the magnifying transmitter was based upon empiricalscientific evidence gathered at Colorado Springs.[174][175][192] While he claimed to have created terrestrial stationary waves, he was never able to demonstrate a fully developed prototypeWorld Wireless system based upon the earth resonance principle.[47] The only known report of the long range transmission and reception of electrical energy by Tesla himself is a statement made to attorney Drury W. Cooper that in 1899 he collected quantitative transmission-reception data at a distance of about 10 miles (16 km).[193] There are four other reports of Tesla having achieved long range wireless power transfer. The first is the wireless operation of lamps and electric motors at a distance of 15 miles (24 km).[194] The second is the wireless transfer of "power enough to light a lamp at 30 km" (19 miles).[195] The third is the wireless transfer of sufficient power to light an incandescent lamp of approximately 10 W at a distance of 1,938 feet (591 m) from the magnifying transmitter's ground plate to the point of reception.[174][175] The fourth is a statement that was made by Tesla biographer John J. O'Neill, based upon "fragmentary material published in a number of publications," that Tesla lit 200 incandescent lamps at a distance of 26 miles (42 km).[96][191][196][197] Tesla does not mention this demonstration in his writings; it does not appear in his Colorado laboratory notes,[174][175] There is no independent confirmation of it having taken place[96][198] but recent mathematical analysis suggest it could have been possible using the 1899 Colorado Springs magnifying transmitter.[199]
Student conducting Tesla coil streamers through his body, 1909
The 'skin effect'
The dangers of contact with high-frequency electric current are sometimes perceived as being less than at lower frequencies, because the subject usually does not feel pain or a 'shock'. This is often erroneously attributed to skin effect, a phenomenon that tends to inhibit alternating current from flowing inside conducting media. It was thought that in the body, Tesla currents travelled close to the skin surface, making them safer than lower-frequency electric currents.
Although skin effect limits Tesla currents to the outer fraction of an inch in metal conductors, the 'skin depth' of human flesh is deeper than that of a metallic conductor due to higher resistivity and lower permittivity. Calculations of skin depth of body tissues at the frequency of Tesla coils show that it can be greater than the thickness of the body.[200][201][202] Thus there seems to be nothing to prevent high-frequency Tesla currents from passing through deeper portions of a subject's body, such as vital organs and blood vessels, which may be better conducting. The reason for the lack of pain is that a human being's nervous system does not sense the flow of potentially dangerous electric currents above 15–20 kHz; essentially, for nerves to be activated, a significant number of ions must cross their membranes before the current (and hence voltage) reverses. Since the body no longer provides a warning 'shock', novices may touch the output streamers of small Tesla coils without feeling painful shocks. However, anecdotal evidence among Tesla coil experimenters indicates temporary tissue damage may still occur and be observed as muscle pain, joint pain, or tingling for hours or even days afterwards. This is believed to be caused by the damaging effects of internal current flow, and is especially common with continuous wave, solid state or vacuum tube Tesla coils operating at relatively low frequencies (tens to hundreds of kHz). It is possible to generate very high frequency currents (tens to hundreds of MHz) that do have a smaller penetration depth in flesh. These are often used for medical and therapeutic purposes such as electrocauterization and diathermy. The designs of early diathermy machines were based on Tesla coils or Oudin coils.
Large Tesla coils and magnifiers can deliver dangerous levels of high-frequency current, and they can also develop significantly higher voltages (often 250,000–500,000 volts, or more). Because of the higher voltages, large systems can deliver higher energy, potentially lethal, repetitive high-voltage capacitor discharges from their top terminals. Doubling the output voltage quadruples the electrostatic energy stored in a given top terminal capacitance. Professionals usually use other means of protection such as a Faraday cage or a metallic mail suit to prevent dangerous currents from entering their bodies.
The most serious dangers associated with Tesla coil operation are associated with the primary circuit. It is capable of delivering a sufficient current at a significant voltage to stop the heart of a careless experimenter. Because these components are not the source of the trademark visual or auditory coil effects, they may easily be overlooked as the chief source of hazard. Should a high-frequency arc strike the exposed primary coil while, at the same time, another arc has also been allowed to strike to a person, the ionized gas of the two arcs forms a circuit that may conduct lethal, low-frequency current from the primary into the person.
Further, great care must be taken when working on the primary section of a coil even when it has been disconnected from its power source for some time. The tank capacitors can remain charged for days with enough energy to deliver a fatal shock. Proper designs always include 'bleeder resistors' to bleed off stored charge from the capacitors. In addition, a safety shorting operation is performed on each capacitor before any internal work is performed.[203]
^"Tesla coil". Museum of Electricity and Magnetism, Center for Learning. National High Magnetic Field Laboratory website, Florida State Univ. 2011. Retrieved September 12, 2013.
^"Instruction and Application Manual"(PDF). Model 10-206 Tesla Coil. Science First, Serrata, Pty. educational equipment website. 2006. p. 2. Retrieved September 12, 2013.
^ abcd"Unfortunately, the common misunderstanding by most people today is that the Tesla coil is merely a device that produces a spectacular exhibit of sparks which tittilates audiences. Nevertheless, its circuitry is fundamental to all radio transmission" Belohlavek, Peter; Wagner, John W (2008). Innovation: The Lessons of Nikola Tesla. Blue Eagle Group. p. 110. ISBN9876510096. Cite error: The named reference "Belohlavek" was defined multiple times with different content (see the help page).
^ abcDenicolai, Marco (May 30, 2001). "Tesla Transformer for Experimentation and Research"(PDF). Thesis for Licentiate Degree. Electrical and Communications Engineering Dept., Helsinki Univ. of Technology, Helsinki, Finland: 2–6. Retrieved July 26, 2015. {{cite journal}}: Cite journal requires |journal= (help)
^ abcGerekos, Christopher (2012). "The Tesla Coil"(PDF). Thesis. Physics Dept., Université Libre de Bruxelles, Brussels, Belgium: 20–22. Archived from the original(PDF) on October 1, 2015. Retrieved July 27, 2015. {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |deadurl= ignored (|url-status= suggested) (help), reprinted on The Zeus Tesla Coil, HazardousPhysics.com
^The lecture is reprinted in Tesla, Nikola (2007). The Nikola Tesla Treasury. Wilder Publications. pp. 68–107. ISBN1934451894. The Tesla coil illustration is shown p. 103, fig. 32
^ abThomson, Elihu (February 20, 1892). "Induction by high potential discharges". Electrical World. 19 (8). New York: W. J. Johnson Co.: 116–117. Retrieved November 21, 2015.
^ abcde"Tesla is entitled to either distinct priority or independent discovery of" three concepts in wireless theory: "(1) the idea of inductive coupling between the driving and the working circuits (2) the importance of tuning both circuits, i.e. the idea of an 'oscillation transformer' (3) the idea of a capacitance loaded open secondary circuit" Wheeler, L. P. (August 1943). "Tesla's contribution to high frequency". Electrical Engineering. 62 (8). IEEE: 355–357. doi:10.1109/EE.1943.6435874. ISSN0095-9197. Cite error: The named reference "Wheeler" was defined multiple times with different content (see the help page).
^Tesla stated in Nikola Tesla My Inventions - Ch. 5: The Magnifying Transmitter, Electrical Experimenter, Vol. 7, No. 2, June 1919, p. 112, that this picture showed a prototype of his magnifying transmitter, a smaller version of the apparatus installed in his Colorado Springs lab.
^Tesla, Nikola (July 1919). "Electrical Oscillators"(PDF). Electrical Experimenter. 7 (3). New York: Experimenter Publishing Co.: 228–229, 259–260. Retrieved August 20, 2015.
^"Transformer". Encyclopaedia Britannica, 10th Ed. Vol. 33. The Encyclopaedia Britannica Co. 1903. p. 426. Retrieved May 1, 2015.
^Thomson, Elihu (April, 1893). "High Frequency Electric Induction". Technology Quarterly and Proceedings of Society of Arts. 6 (1). Boston: Massachussets Inst. of Technology: 50–59. Retrieved November 22, 2015. {{cite journal}}: Check date values in: |date= (help)
^Thomson, Elihu (July 23, 1906). "Letter to Frederick Finch Strong". The Electrotherapy Museum website. Jeff Behary, Bellingham, Washington, USA. Reproduced by permission of The American Philosophical Society. Retrieved August 20, 2015. In this letter Thomson lists papers he published in technical journals which support his claim to priority in inventing the "Tesla coil" resonant transformer circuit
^Fessenden, Reginald A. (August 1908). "Wireless Telephony". Telephony. 16 (2). Chicago, Illinois: The Telephony Publishing Co.: 75. Retrieved May 2, 2015.
^"The length of the...coil in each transformer should be approximately one quarter of the wave length of the electric disturbance in the circuit, this estimate being based on the velocity of propagation of the disturbaiice through the coil itself..." US Patent No. 645576, Nikola Tesla, System of transmission of electrical energy, filed September 2, 1897; granted March 20, 1900
^ abcdThe longest wireless power transmission for which there is credible evidence is probably Tesla's picture of a receiving coil with a 10 watt light bulb lit by power transmitted from his 300,000 watt magnifying transmitter, which he claimed was at a distance of 1938 feet (591 m). Tesla, Nikola; Marincic, Aleksandar; Popovic, Vojin; Ciric, Milan (2008). From Colorado Springs to Long Island : research notes : Colorado Springs 1899-1900, New York 1900-1901. Belgrade: Nikola Tesla Museum. p. 169. ISBN9788681243442. This represents a throughput efficiency of only 0.0033%, which would seem to be adequately explained by conventional power transfer by radio waves.
^ abcdefghijklLee, C.K.; Zhong, W.X.; Hui, S.Y.R. (September 5, 2012). Recent Progress in Mid-Range Wireless Power Transfer(PDF). The 4th Annual IEEE Energy Conversion Congress and Exposition (ECCE 2012). Raleigh, North Carolina: Inst. of Electrical and Electronic Engineers. pp. 3819–3821. Retrieved November 4, 2014. Cite error: The named reference "LeeZhongHui" was defined multiple times with different content (see the help page).
^Tesla was notoriously secretive about the distance he could transmit power. One of his few disclosures of details was in the caption of fig. 7 of his noted magazine article: The Problem of Increasing Human Energy, Century magazine, June 1900. The caption reads: "EXPERIMENT TO ILLUSTRATE AN INDUCTIVE EFFECT OF AN ELECTRICAL OSCILLATOR OF GREAT POWER - The photograph shows three ordinary incandescent lamps lighted to full candle-power by currents induced in a local loop consisting of a single wire forming a square of fifty feet each side, which includes the lamps, and which is at a distance of one hundred feet from the primary circuit energized by the oscillator. The loop likewise includes an electrical condenser, and is exactly attuned to the vibrations of the oscillator, which is worked at less than five percent of its total capacity."
^ abTesla, Nikola (June 1900). "The Problem of Increasing Human Energy". Century Magazine. New York: The Century Co. Retrieved November 20, 2014. Cite error: The named reference "Tesla1900" was defined multiple times with different content (see the help page).
^ abcdeBroad, William J. (May 4, 2009). "A Battle to Preserve a Visionary's Bold Failure". New York Times. New York: The New York Times Co. pp. D1. Retrieved November 19, 2014. Cite error: The named reference "Broad" was defined multiple times with different content (see the help page).
^Tesla, Nikola (March 8, 1907). "Tuned Lightning". English Mechanic and World of Science. Retrieved October 18, 2015., reprinted in Tesla, Nikola (2012). The Nikola Tesla Treasury. Start Publications LLC. p. 526. ISBN1627932569.
^ abHawkins, Lawrence A. (February 1903). "Nikola Tesla: His Work and Unfulfilled Promises". The Electrical Age. 30 (2): 107–108. Retrieved November 4, 2014. Cite error: The named reference "Hawkins" was defined multiple times with different content (see the help page).
^ abcBrown, William C. (1984). "The history of power transmission by radio waves". MTT-Trans. on Microwave Theory and Technique. 32 (9). Inst. of Electrical and Electronic Engineers: 1230–1234. Retrieved November 20, 2014. Cite error: The named reference "Brown" was defined multiple times with different content (see the help page).
^ ab"Life and Legacy: Colorado Springs". Tesla: Master of Lightning - companion site for 2000 PBS television documentary. PBS.org, Public Broadcasting Service website. 2000. Retrieved November 19, 2014. Cite error: The named reference "ColoradoSpringsPBS" was defined multiple times with different content (see the help page).
^ ab"Dennis Papadopoulos interview". Tesla: Master of Lightning - companion site for 2000 PBS television documentary. PBS.org, Public Broadcasting Service website. 2000. Retrieved November 19, 2014.
^Crookes, William (February 1, 1892). "Some Possibilities of Electricity". The Fortnightly Review. 51. London: Chapman and Hall: 174–176. Retrieved August 19, 2015.
^ abThe "four circuit" radio system, which Marconi claimed in his 1900 patent, meant a transmitter and receiver which each contained a resonant transformer and thus were divided into primary and secondary circuits. All four circuits were tuned to the same frequency, one side by capacitors, and the other side by the capacitance of the antenna; "the use of two high frequency circuits in the transmitter and two in the receiver, all four so adjusted to be resonant at the same frequency or multiples of it." "No. 369 (1943) Marconi Wireless Co. of America v. United States". United States Supreme Court decision. Findlaw.com website. June 21, 1943. Retrieved March 14, 2017. This was identical to the system Tesla demonstrated in 1893. The advantage of this system was that due to the resonant transformers both the receiver and transmitter had much narrower bandwidth than previous circuits.
^Tesla, Nikola (May 1919). "The True Wireless"(PDF). Electrical Experimenter. 7 (1). New York: Experimenter Publishing Co.: 28–30, 61. Retrieved February 20, 2017. archived on tfcbooks
^US Patent no. 714,756, John Stone Stone Method of electric signaling, filed: February 8, 1900, granted: December 2, 1902
^US Patent no. 609,154 Oliver Joseph Lodge, Electric Telegraphy, filed: February 1, 1898, granted: August 16, 1898
^Manders, Horace (August 1, 1902). "Some phenomena of high frequency currents". Journal of Physical Therapeutics. 3 (1). London: John Bale, Sons, and Danielsson, Ltd.: 220–221. Retrieved December 2, 2014.
^McGinley, Patton H. "Tesla's contributions to electrotherapy" in Childress, David Hatcher, Ed. (2000). The Tesla Papers. Adventures Unlimited Press. pp. 162–167. ISBN0932813860.{{cite book}}: CS1 maint: multiple names: authors list (link)
^ abTesla, Nikola (November 17, 1898). "High frequency oscillators for electro-therapeutic and other purposes". The Electrical Engineer. 26 (550): 477–481. Retrieved June 10, 2015. Also read at the 8th annual meeting of The American Electro-Therapeutic Association, Buffalo, New York, Sept. 13-15, 1898
^he also wrote a second earlier medical paper: Tesla, N. "High frequency currents for medical purposes" in Electrical Engineer, 1891, cited in Saberton, Claude (1920) Diathermy in Medical and Surgical Practice, published by Paul B. Hoeber, New York, p. 131
^ abMorton, W. J. (January 17, 1893). "A brief glance at electricity in medicine". Transactions of the American Inst. of Electrical Engineers. New York: AIEE: 576–578. Retrieved September 21, 2015. Cite error: The named reference "Morton" was defined multiple times with different content (see the help page).
^Batten, George B. (October 15, 1926). "President's Address"(PDF). Proc. of the Royal Society of Medicine - Electro-therapeutics section. 20 (1). London: 33–34. Retrieved September 22, 2015.
^D'Arsonval, A. (August 1893). "Physiological action of currents of great frequency". Modern Medicine and Bacteriological World. 2 (8). Modern Medicine Publishing Co.: 200–203. Retrieved November 22, 2015., translated by J. H. Kellogg
^ abBehary, Jeff (1997). "Violet Ray Misconceptions". The Electrotherapy Museum. Jeff Behary's website. Retrieved October 13, 2015.
^The small high voltage coils in these home violet ray wands resembled induction coils more than Tesla coils; they had iron core transformers and mechanical interrupters and produced lower voltages than Tesla coils
^Behary, Jeff (Sun, 1 July 2007 06:56:03 -0600 (MDT)). "RE: Oudin coil". Tesla Coil Mailing List (Mailing list). Retrieved 16 November 2015. {{cite mailing list}}: Check date values in: |date= (help); Unknown parameter |mailinglist= ignored (|mailing-list= suggested) (help)
^"Madamoiselle Electra" (October 1911). "How I Give the Public Electric Thrills". Popular Electricity. 4 (6). Chicago: Popular Electricity Publishing Co.: 507–510. Retrieved September 25, 2015.
^ abcWilliam Luddington, "Mr. Electricity: The Multi-Volted Career of Kenneth Strickfaden" in Tibbetts, John C.; Welsh, James M., Ed. (2010). American Classic Screen Profiles. Scarecrow Press. pp. 202–208. ISBN0810876779.{{cite book}}: CS1 maint: multiple names: authors list (link)
^ abHanson, Eugene M. (September 1949). "High-Voltage Magic". Popular Mechanics. 92 (3). Chicago, USA: The Popular Mechanics Co.: 140–142. Retrieved October 1, 2015.
^Collins, Archie Frederick (January 27, 1906). "High-Potential Discharges". Scientific American. 94 (4). New York: Munn and Co.: 92. Retrieved December 15, 2016.
Starting from two facts that the earth is a conductor insulated in space, and that a body cannot be charged without causing an equivalent displacement of electricity in the earth, I undertook to construct a machine suited for creating as large a displacement as possible of the earth's electricity.
^The Feynman Lectures on Physics, R.P. Feynman, R.B. Leighton, M. Sands, Addison-Wesley Publishing Co., 1964, Vol. 2, chapter 9.
At that time I was absolutely sure that I could put up a commercial plant, if I could do nothing else but what I had done in my laboratory on Houston Street; but I had already calculated and found that I did not need great heights to apply this method. My patent says that I break down the atmosphere "at or near" the terminal. If my conducting atmosphere is 2 or 3 miles above the plant, I consider this very near the terminal as compared to the distance of my receiving terminal, which may be across the Pacific. That is simply an expression. I saw that I would be able to transmit power provided I could construct a certain apparatus -- and I have, as I will show you later. I have constructed and patented a form of apparatus which, with a moderate elevation of a few hundred feet, can break the air stratum down.
^Eccles, W. H. (1943). "Dr. Nikola Tesla". Nature. 13 (II). London: 189. (Reprinted in W. H. Eccles (1961). Tribute to Nikola Tesla. Beograd: Nikola Tesla Museum.)
^Corum, Kenneth L.; Corum, James F. (June 1, 2016). "Bell Labs and the Radio Surface Wave Propagation Experiment". In Valone, Thomas (ed.). Nikola Tesla's Electricity Unplugged: Wireless Transmission of Power as the Master of Lightning Intended. Adventures Unlimited Press. ISBN978-1939149572.
^History of Wireless By Tapan K. Sarkar, et al. ISBN 0-471-78301-3
^A Multifrequency electro-magnetic field generator that is capable of generating electro-magnetic radial fields, horizontal fields and spiral flux fields that are projected at a distance from the device and collected at the far end of the device by an antenna.
Nicholson, Paul, "Tesla Secondary Simulation Project" (Current state of the art in rigorously describing Tesla coil secondary behavior through theoretical analysis, simulation and testing of results in practice)
"The Development of High Frequency Currents for Practical Application"., The Electrical World, Vol 32, No. 8.
"Boundless Space: A Bus Bar". The Electrical World, Vol 32, No. 19.
Other publications
A. L. Cullen, J. Dobson, "The Corona Breakdown of Aerials in Air at Low Pressures". Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, Vol. 271, No. 1347 (February 12, 1963), pp. 551–564
Bieniosek, F. M., "Triple Resonance Pulse Transformer Circuit". Review of Scientific Instruments, 61 (6).
Corum, J. F., and K. L. Corum, "RF Coils, Helical Resonators and Voltage Magnification by Coherent Spatial Modes". IEEE, 2001.
de Queiroz, Antonio Carlos M., "Synthesis of Multiple Resonance Networks". Universidade Federal do Rio de Janeiro, Brazil. EE/COPE.
Haller, George Francis, and Elmer Tiling Cunningham, "The Tesla high frequency coil, its construction and uses". New York, D. Van Nostrand company, 1910.
Hartley, R. V. L., "Oscillations with Non-linear Reactances". Bell System Technical Journal, Alcatel-Lucent, 1936 (3), 424-440.
Norrie, H. S., "Induction Coils: How to make, use, and repair them". Norman H. Schneider, 1907, New York. 4th edition.
Reed, J. L., "Greater voltage gain for Tesla transformer accelerators", Review of Scientific Instruments, 59, p. 2300, (1988).
Reed, J. L., "Tesla transformer damping", Review of Scientific Instruments, 83, 076101-1 (2012).
Reed, J. L., "Conduction-coupled Tesla transformer," Review of Scientific Instruments, 86,035113 (2015)