The carbon microphone, also known as carbon button microphone, button microphone, or carbon transmitter, is a transducer that converts sound to an electrical audio signal. It consists of two metal plates separated by granules of carbon. One plate is very thin and faces outward, acting as a diaphragm. When sound waves strike this plate, the pressure on the granules changes, which in turn changes the electrical resistance between the plates. Higher pressure lowers the resistance as the granules are pushed closer together. As a steady direct current is passed between the plates, the varying resistance results in a modulation of the current at the same frequency of the impinging sound waves. In telephony, this signal is directly passed through a telephone system to the central office, or it is electronically amplified in other sound systems, such as a public address system or a recording device. The frequency response of the carbon microphone, however, is limited to a narrow range, and the device produces significant electrical noise.
Before the proliferation of vacuum tube amplifiers in the 1920s, carbon microphones were the only practical means of obtaining high-level audio signals, and were widely used in telephone systems. Their low cost, inherently high output and peaked frequency response characteristic were well suited for this application. Their use in new telephone installations continued into the 1980s, long after they had been replaced by other types of microphones in other applications. In most plain old telephone service (POTS) networks, carbon-microphone based telephones can still be used without modification. Carbon microphones, usually modified telephone transmitters, were widely used in early AM radio broadcasting systems, but their limited frequency response, as well as a fairly high noise level, led to their abandonment in those applications by the late 1920s. They continued to be widely used for low-end public address, and military and amateur radio applications for some decades afterward.
The invention of the carbon transmitter, or carbon microphone, was claimed both by Thomas Alva Edison in March 1877 and separately by Emile Berliner who filed related patent applications in June 1877 and August 1879. The two sides fought a long legal battle over the patent rights. Ultimately a federal court awarded Edison full rights to the invention, stating "Edison preceded Berliner in the transmission of speech...The use of carbon in a transmitter is, beyond controversy, the invention of Edison" and the Berliner patent was ruled invalid. British courts also ruled in favor of Edison over Berliner. Having settled the Dowd suit (after Peter A. Dowd, agent of Western Union) out of court in 1881, Western Union left the telephone business, and sold Edison's patent rights and related assets to the Bell company in exchange for 20% of telephone rental receipts. Subsequently Bell telephones used the Bell receiver and the Edison transmitter. Later, carbon granules were used between carbon buttons. Carbon microphones were widely used in telephones in the United States from 1890 until the 1980s.
Use as amplifier
Carbon microphones can be used as amplifiers. This capability was used in early telephone repeaters, making long distance phone calls possible in the era before vacuum tube amplifiers. In these repeaters, a magnetic telephone receiver (an electrical-to-mechanical transducer) was mechanically coupled to a carbon microphone. Because a carbon microphone works by varying a current passed through it, instead of generating a signal voltage as with most other microphone types, this arrangement could be used to boost weak signals and send them down the line. These amplifiers were mostly abandoned with the development of vacuum tubes, which offered higher gain and better sound quality. Even after vacuum tubes were in common use, carbon amplifiers continued to be used during the 1930s in portable audio equipment such as hearing aids. The Western Electric 65A carbon amplifier was 1.2" in diameter and 0.4" high and weighed less than 1.4 ounces. Such carbon amplifiers did not require the heavy bulky batteries and power supplies used by vacuum tube amplifiers. By the 1950s, carbon amplifiers for hearing aids had been replaced by miniature vacuum tubes (only to be shortly replaced by transistors). However, carbon amplifiers are still being produced and sold.
An illustration of the amplification provided by carbon microphones was the oscillation caused by feedback, that resulted in an audible squeal from the old "candlestick telephone" if its earphone was placed near the carbon microphone.
Early radio applications
Early AM radio transmitters relied on carbon microphones for voice modulation of the radio signal. In the first long-distance audio transmissions by Reginald Fessenden in 1906, a continuous wave from an Alexanderson alternator was fed directly to the transmitting antenna through a water-cooled carbon microphone. Later systems using vacuum tube oscillators often used the output from a carbon microphone to modulate the grid bias of the oscillator or output tube to achieve modulation.
Apart from legacy telephone installations in Third World countries, carbon microphones are still used today in certain niche applications in the developed world. An example is the Shure 104c, which is still in demand because of its wide compatibility with existing equipment.
The principal advantage of carbon microphones over other microphone designs is that they can produce high-level audio signals from very low DC voltages, without needing any form of additional amplification or batteries. This is particularly useful in remote locations served by very long telephone lines, where the electrical resistance of the wires can lead to severe DC voltage drop. Most all-electronic telephones need at least three volts DC to work, and so will often become useless in such situations, whereas carbon transmitter telephones will continue to work down to a fraction of a volt. Even where they do work, electronic telephones also suffer from the so-called "cliff effect", whereby they abruptly stop working when the line voltage falls below the critical level. In particular, this means that one telephone on a "party line" may tend to "hog" all the line current, cutting the others off. With carbon microphones, all receivers on the same line will still operate, albeit with reduced output).
Carbon microphones are also widely used in safety-critical applications such as mining and chemical manufacturing, where higher line voltages cannot be used, due to the risk of sparking and consequent explosions . Carbon-based telephone systems are also resistant to damage from high-voltage transients, such as those produced by lightning strikes, and electromagnetic pulses of the type generated by nuclear explosions, and so are still maintained as backup communication systems in critical military installations.
- Heil, B. The Microphone: A Short Illustrated History. QST, 90(6), 50
- IEEE Global History Network: Carbon Transmitter. New Brunswick, NJ: IEEE History Center 
- Inventors Hall of Fame, E. Berliner, U.S. Patent 0,463,569 filed June 1877, issued November 1891
- Josephson, pp. 147-151
- Examples of amplifiers that were based on carbon microphones:
- By 1904 Herbert E. Shreeve had developed a carbon amplifier for relaying telephone signals over long distances. Also, the (U.S.) Western Electric company used a carbon amplifier in its No. 66B hearing aid. See: Electro-mechanical amplifiers.
- A mechanical telephone relay, which was invented by Herbert E. Shreeve, an engineer with the Western Electric Co., was patented in 1905. See: Herbert E. Shreeve, "Telephone-current reinforcer or relay," U.S. patent no. 791,655 (filed: July 8, 1904 ; issued: June 6, 1905). See also: Herbert E. Shreeve, "Telephone-current reinforcer or relay," U.S. patent no. 791,656 (filed: Feb. 28, 1904 ; issued: June 6, 1905).
- The Acousticon hearing aid employed a carbon amplifier. See: The Hearing Aid Museum.
- Other early hearing aids that were based on carbon amplifiers are mentioned here: Washington University School of Medicine: Timeline of hearing devices and early deaf education
- In 1924, the Siemens Corp. of Germany patented a carbon-amplifier hearing aid. See: Siemens Hearing Instruments: 1924 carbon amplifier.
- Douglas Self. "Electro-Mechanical amplifiers (Western Electric 65A carbon amplifier in 66B hearing aid)".
- "GN 0686 Netcom In-Line Carbon Amplifier". Headset-Plus.com.
- "Model 104C Guide" (PDF). Shure Incorporated.
- Josephson, Matthew, Edison: A Biography, Wiley, 1992, ISBN 0-471-54806-5
|Wikimedia Commons has media related to Carbon microphones.|
- Edison's invention of the carbon (graphite) microphone by Frank Dyer
- T A Edison, U.S. Patent 0,474,230 Speaking Telegraph (graphite microphone), filed April 1877, issued May 1892
- T A Edison, U.S. Patent 0,203,016 Improvement in Speaking Telephones (compressed lamp black button insulated from diaphragm), filed March 1878, issued April, 1878
- T A Edison, U.S. Patent 0,222,390 Carbon Telephone (carbon granules microphone), filed Nov 1878, issued Dec 1879
- E. Berliner, U.S. Patent 0,222,652 Improvement in Electrical Contact Telephones (carbon diaphragm with carbon contact pin), filed August 1879, issued December 16, 1879
- A C White, U.S. Patent 0,485,311 Telephone (solid back carbon microphone), filed March 24, 1892, issued November 1, 1892 (Bell engineer)