Carbon Microphone

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. 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 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for this application, and their use in new telephone installations continued up to the 1980s, long after they had been replaced by other types of microphones in other applications. (In most Western copper-wire telephone networks, old fashioned carbon-microphone based telephones can still be used without modification.) 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 afterward.

Read more about Carbon MicrophoneHistory, Carbon Microphones Used As Amplifiers, Current Usage

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Invention Of The Telephone - Variable Resistance Transmitters - Carbon Microphone - Thomas Edison
... Alva Edison took the next step in improving the telephone with his invention in 1878 of the carbon grain "transmitter" (microphone) that provided a ... Edison discovered that carbon grains, squeezed between two metal plates, had a variable electrical resistance that was related to the pressure ... The carbon microphone was further improved by Emile Berliner, Francis Blake, David E ...
Carbon Microphone - Current Usage
... telephone installations in Third World countries, carbon microphones are still used today in certain niche applications in the developed world ... The principal advantage carbon microphones have over other microphone types is that they can produce high-level audio signals from very low DC ... need at least 3 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 ...
Timeline Of The Telephone - 1876 To 1878
... October 1876 Thomas Edison tests his first carbon microphone ... in transmitting over wires many articulated sentences" using carbon granules as a pressure sensitive variable resistance under the pressure of a diaphragm (Josephson. 4 March 1877 Emile Berliner invents a microphone based on "loose contact" between two metal electrodes, an improvement on the Reis' Telephone, and in April 1877 files a caveat of an ...
History Of The Telephone - Early Commercial Instruments
... Most, however, used Edison/Berliner carbon transmitters, which were much louder than the other kinds, even though they required induction coils, actually acting as impedance ... In 1877 and 1878, Edison invented and developed the carbon microphone used in all telephones along with the Bell receiver until the 1980s ... a federal court ruled in 1892 that Edison and not Emile Berliner was the inventor of the carbon microphone ...