The carbon microphone, also known as a carbon button microphone (or sometimes just a button microphone) or a carbon transmitter, is a sound-to-electrical signal transducer consisting of two metal plates separated by granules of carbon.

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One plate faces outward and acts 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.) 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.

Before the proliferation of vacuum tube amplifiers in the 1920s, carbon microphones were the only practical means of obtaining 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for this application, and their use in new telephone installations continued up to the 1980s, long after they had been replaced by other types of microphones in other applications. Carbon microphones were widely used in early AM radio broadcasting systems (usually modified telephone microphones), but their limited frequency response, as well as a fairly high noise level, led to their abandonment for that use by the late 1920s. They continued to be widely used for low-end public address, and military and amateur radio applications for some decades afterwards.

The invention of the carbon microphone (then called a “transmitter”) was claimed both by Thomas Alva Edison in March 1878 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 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. 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.

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 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.

One 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.