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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 outward, acting 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. As a steady direct current is passed between the plates, the varying resistance results in a modulation of the current at the same frequency of the impinging sound waves. In telephony, this signal is directly passed through a telephone system to the central office, or it is electronically amplified in other sound systems, such as a public address system or a recording device. The frequency response of the carbon microphone, however, is limited to a narrow range, and the device produces significant electrical noise. Wikipedia