Fleming valve

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The first prototype Fleming valves, built October 1904.
Early commercial Fleming valves used in radio receivers, 1919
Fleming valve schematic from US Patent 803,684.

The Fleming valve, also called the Fleming oscillation valve, was a thermionic valve (or vacuum tube) diode invented in 1904 by John Ambrose Fleming as a detector (rectifier) for use in radio receivers. It was the first type of thermionic diode, which was later widely used as a rectifier in the power supplies of consumer electronic equipment such as radios and televisions until the semiconductor diode replaced it in the 1960s. It was also the first vacuum tube, the forerunner of amplifying vacuum tubes like the triode which dominated electronics for 50 years. The IEEE has described it as "one of the most important developments in the history of electronics",[1] and it is on the List of IEEE Milestones for electrical engineering.

How it works[edit]

The valve consists of an evacuated, sealed glass bulb containing two electrodes: a cathode in the form of a heated filament, a loop of carbon or fine tungsten wire similar to that used in light bulbs of the time; and an anode (called the plate), a metal plate. In early versions this was a flat plate next to the filament, but in later versions it was a cylinder of sheet metal surrounding the filament. In some versions, a grounded copper screen surrounded the bulb, to shield against external electric fields from nearby static charges.

In operation, a separate current through the filament heats it until it glows. The hot filament emits electrons into the space of the tube, in a process called thermionic emission. The alternating (AC) voltage to be rectified is applied between the filament and the plate. When the plate has a positive voltage with respect to the filament, the electrons are attracted to it, creating a current of electrons through the tube from filament to plate. However, when the plate has a negative voltage, the electrons are not attracted to it, so there is no current through the tube; unlike the filament, the plate does not produce electrons. So current can pass through the tube only in one direction; it rectifies an AC voltage to a pulsing DC current The load is connected in series with the tube and AC source.

This simple picture was complicated to some extent by the presence of residual air in the tube, because the vacuum pumps of the time were not able to create as high a vacuum as exists in modern vacuum tubes. At high voltages, the tube could become unstable and oscillate, but this occurred at voltages far above those normally used.

History[edit]

The Fleming valve was the first practical application of thermionic emission, discovered in 1873 by Frederick Guthrie. As a result of his work on the incandescent light bulb, Thomas Edison made his own discovery of the phenomenon in 1880, which led to it being called the Edison effect. Edison was granted a patent for this device as part of an electrical indicator in 1884, but did not hit upon any practical use. Professor Fleming of University College London consulted for the Edison Electric Light Company from 1881-1891, and subsequently for the Marconi Wireless Telegraph Company.

In 1901 Fleming designed a transmitter for Guglielmo Marconi to attempt transmission of radio waves across the Atlantic from Poldhu, England, to Nova Scotia, Canada. The distance between the two points was about 3,500 kilometres (2,200 mi). Although the contact, reported November 12, 1901, was widely heralded as a great scientific advance at the time, there is also some skepticism about the claim, because the received signal, the three dots of the Morse code letter "S", was so weak the primitive receiver had difficulty distinguishing it from atmospheric radio noise caused by static discharges, leading later critics to suggest it may have been random noise. Regardless, it was clear that reliable transatlantic communication with the existing transmitter required more sensitive receiving apparatus.

Thermionic diode valves derived from the Fleming valve, from the 1930s (left) to the 1970s (right)

The receiver for the transatlantic demonstration employed a coherer, which had poor sensitivity and degraded the tuning of the receiver. This led Fleming to look for a detector which was more sensitive and reliable while at the same time being better suited for use with tuned circuits.[2][3] In 1904 Fleming tried an Edison effect bulb for this purpose, and found that it worked well to rectify high frequency oscillations and thus allow detection of the rectified signals by a galvanometer. On November 16, 1904, he applied for a US patent for what he termed an oscillation valve. This patent was subsequently issued as number 803,684 and found immediate utility in the detection of messages sent by Morse code.

Oscillation valves[edit]

The Fleming valve proved to be the start of a technological revolution. After reading Fleming's 1905 paper on his oscillation valve, American engineer Lee DeForest in 1906 created a three-element vacuum tube, the Audion, by adding a wire grid between cathode and anode. It was the first electronic amplifying device, allowing the creation of amplifiers and continuous wave oscillators. De Forest quickly refined his device into the triode, which became the basis of long-distance telephone and radio communications, radars, and early digital computers for 50 years, until the advent of the transistor in the 1960s. Fleming sued De Forest for infringing his valve patents, resulting in decades of expensive and disruptive litigation, which were not settled until 1943 when the United States Supreme Court ruled Fleming's patent invalid.[4]

Power applications[edit]

Later, when vacuum tube equipment began to be powered from wall power by transformers instead of batteries, the Fleming valve was developed into a rectifier to produce the DC plate (anode) voltage required by other vacuum tubes. Around 1914 Irving Langmuir at General Electric developed a high voltage version called the Kenotron which was used to power x-ray tubes. As a rectifier, the tube was used for high voltage applications but its high internal resistance made it inefficient in low voltage, high current applications. Up until vacuum tube equipment was replaced by transistors in the 1970s, radios and televisions usually had one or more diode tubes.

References and notes[edit]

Citations[edit]

  1. ^ "Milestones:Fleming Valve, 1904". IEEE Global History Network. IEEE. Retrieved 29 July 2011. 
  2. ^ Radio Communications: A Brief Synopsis
  3. ^ John Ambrose Fleming (1849-1945) By W A Atherton, Published in Wireless World August 1990
  4. ^ The Supreme Court invalidated the patent because of an improper disclaimer and later maintained the technology in the patent was known art when filed. For more see, Misreading the Supreme Court: A Puzzling Chapter in the History of Radio. Mercurians.org.

Patents[edit]

Issued
  • U.S. Patent 803,684 - Instrument for converting alternating electric currents into continuous currents (Fleming valve patent)
Cited by

External links[edit]