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The carbon microphone, also known as carbon button microphone, button microphone, or carbon transmitter, is a type of microphone, 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 toward the speaking person, acting as a diaphragm. Sound waves striking the diaphragm cause it to vibrate, exerting a varying pressure on the granules, which in turn changes the electrical resistance between the plates. Higher pressure lowers the resistance as the granules are pushed closer together. A steady direct current is passed between the plates through the granules. The varying resistance results in a modulation of the current, creating a varying electric current that reproduces the varying pressure of the sound wave. In telephony, this undulating current is directly passed through the telephone wires to the central office. In public address systems or recording devices it is amplified by an audio amplifier. 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. 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.〔Heil, B. The Microphone: A Short Illustrated History. ''QST'', 90(6), 50〕 ==History== The first microphone that enabled proper voice telephony was the (loose-contact) carbon microphone (then called transmitter). This was independently developed 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.〔(【引用サイトリンク】author=Bob Estreich )〕 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. 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.〔(Inventors Hall of Fame ), E. Berliner, filed June 1877, issued November 1891〕〔''IEEE Global History Network: Carbon Transmitter''. New Brunswick, NJ: IEEE History Center ()〕 The carbon microphone is the direct prototype of today's microphones and was critical in the development of telephony, broadcasting and the recording industries. Later, carbon granules were used between carbon buttons. Carbon microphones were widely used in telephones from 1890 until the 1980s.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Carbon microphone」の詳細全文を読む スポンサード リンク
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