Carbon microphone
The carbon microphone, also known as carbon button microphone, button microphone, or carbon transmitter, is a type of
Before the proliferation of vacuum tube amplifiers in the 1920s, carbon microphones were the only practical means of obtaining high-level audio signals. They were widely used in telephone systems until the 1980s, while other applications used different microphone designs much earlier. Their low cost, inherently high output and frequency response characteristic were well suited for telephony. For plain old telephone service (POTS), 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.[1]
History
The first microphone that enabled proper voice telephony was the (loose-contact) carbon microphone (then called transmitter). This was independently developed around 1878 by David Edward Hughes in England and Emile Berliner and Thomas Edison in the US. Although Edison was awarded the first patent in mid-1877, Hughes had demonstrated his working device in front of many witnesses some years earlier, and most historians credit him with its invention.[2][3][4]
Hughes' device used loosely packed carbon granules - the varying pressure exerted on the granules by the diaphragm from the acoustic waves caused the resistance of the carbon to vary proportionally, allowing a relatively accurate electrical reproduction of the sound signal. Hughes also coined the word microphone. He demonstrated his apparatus to the Royal Society by magnifying the sound of insects scratching through a sound box. Contrary to Edison, Hughes decided not to take out a patent; instead, he made his invention a gift to the world.[5]
In America, Edison and Berliner 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.[6][7]
The carbon microphone is the direct prototype of today's microphones and was critical in the development of telephony, broadcasting and the recording industries.[8] Later, carbon granules were used between carbon buttons. Carbon microphones were widely used in telephones from 1890 until the 1980s.[7]
Use as amplifier
Carbon microphones can be used as
An illustration of the amplification provided by carbon microphones was the oscillation caused by feedback, which resulted in an audible squeal from the old "candlestick telephone" if its earphone was placed near the carbon microphone.
Early radio applications
Early
Current usage
Apart from legacy telephone installations in various conditions according to areas and countries, carbon microphones may be still used today in certain niche applications although manufacturers are discontinuing distribution. For example, the Shure 104c,[12] was still in demand in the late 2010s because of its wide compatibility with existing equipment.[13]
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. The carbon microphone, by virtue of using a power supply, gives a power gain. This can readily be demonstrated by connecting a battery, microphone and earphone in series. If the microphone and earphone are brought in contact the system will oscillate. This is only possible if the power gain around the loop is greater than unity. The microphone's low-voltage performance 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 "
Carbon microphones are also widely used in safety-critical applications such as
References
- ^ Heil, B. The Microphone: A Short Illustrated History. QST, 90(6), 50
- ISBN 9780801869099.
- ^ Huurdeman, Anton (2003). The Worldwide History of Telecommunications. John Wiley & Sons.
- ^ "David Hughes". Retrieved 2013-12-30.
- ^ "A brief history of microphones" (PDF). Archived from the original (PDF) on 2013-01-24. Retrieved 2012-12-17.
- ^ Inventors Hall of Fame Archived June 10, 2006, at the Wayback Machine, E. Berliner, U.S. patent 0,463,569 filed June 1877, issued November 1891
- ^ a b IEEE Global History Network: Carbon Transmitter. New Brunswick, NJ: IEEE History Center "Carbon Transmitter - GHN". Archived from the original on 2010-03-18. Retrieved 2009-01-14.
- ^ "David Edward Hughes: Concertinist and Inventor" (PDF). Archived from the original (PDF) on 2013-12-31. Retrieved 2012-12-17.
- ^ 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 Archived 2015-09-23 at the Wayback Machine.
- 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 Archived January 15, 2014, at the Wayback Machine.
- ^ Douglas Self. "Electro-Mechanical amplifiers (Western Electric 65A carbon amplifier in 66B hearing aid)". Archived from the original on 2015-09-23. Retrieved 2010-09-14.
- ^ "GN 0686 Netcom In-Line Carbon Amplifier". Headset-Plus.com.
- ^ "Model 104C Guide". Shure Incorporated.
- ^ "527c-as-a-replacement-for-model-104c".
Bibliography
- Josephson, Matthew, Edison: A Biography, Wiley, 1992, ISBN 0-471-54806-5
External links
- 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)