(Also known as a Carbon Button Microphone). An obsolete form of microphone in which carbon granules are contained between two metal contact plates, one of which acts as the diaphragm and moves in response to sound waves. The microphone has to be biased with a DC voltage which causes a current to pass from one metal contact plate, through the carbon granules, to the other metal contact plate. The varying pressure exerted on the carbon granules by the moving diaphgram causes a varying resistance and thus a varying current which is analogous to the sound waves. Carbon Button Microphones were used in the very early days of sound recording and broadcasting, as well as in domestic telephones up until the 1980s when electret capsules became more commonplace.