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The carbon microphone is also known as a carbon button microphone (or sometimes just a button microphone) or a carbon transmitter. It consists of two metal plates separated by granules of carbon. When sound waves strike one of the plates, the pressure on the granules changes, and in turn the electrical resistance between the plates changes. A direct current is passed from one plate to the other, and the changing resistance results in a changing current, which can be passed through a telephone system, or used in other ways in electronics systems to change the sound into an electrical signal.
Carbon microphones have the advantages of low cost, high output level, and low impedance. However, they suffer from very low quality of sound reproduction and limited frequency response, as well as a high noise (hiss) level, so they were abandoned for radio broadcasting after the 1920s, and were not used for public address and amateur radio after the 1930s.Heil, B. The Microphone: A Short Ilustrated History. QST, 90(6), 50
History
The invention of the carbon microphone was claimed both by Thomas Alva Edison in March 1878IEEE Virtual Museum: Carbon Transmitter. New Brunswick, NJ: IEEE History Center and separately by Emile Berliner who filed related patent applications in June 1877 and August 1879.[http://www.invent.org/Hall_Of_Fame/13.html Inventors Hall of Fame, E. Berliner, filed June 1877, issued November 1891 The two inventors fought a long legal battle over the patent rights. Ultimately a federal court awarded Edison full rights to the invention of the carbon microphone, saying "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. Edison's patent rights were sold to the Bell company, and subsequently Bell telephones used the Bell receiver and the Edison transmitter. (Josephson, pp. 147-151). Later, carbon granules were used in place of carbon buttons. Carbon microphones were widely used in telephones in the United States from 1890 until the 1980s.
Carbon microphones used as amplifiers
One of the surprising attributes of carbon microphones is that they can actually be used as amplifiers. This capability was used in early telephone repeaters, making long distance phone calls possible in the era before vacuum tubes. 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, not by generating a signal voltage, this arrangement could be used to boost weak signals and send them down the line. These amplifiers were abandoned with the development of vacuum tubes, which offered a more efficient method of amplification and better sound quality.
References
See also
External links
- A. Edison, Improvement in Speaking Telephones (compressed lamp black button insulated from diaphram), filed March 1878, issued April, 1878
- E. Berliner, Improvement in Electrical Contact Telephones (carbon diaphram with carbon contact pin), filed August 1879, issued December 16, 1879
Kulkornsmikrofon | Micrófono de carbón | Micrófono de carbón
"Carbon microphone"