The Carbon Microphone: An Account of Some Researches Bearing on Its Action

EW electrical devices are as widely used as the "carbon microphone" and few have given rise to as much speculation in regard to their mode of action. T h a t the problem has proved elusive is shown by the fact that in Bell Telephone Laboratories it has been regarded as perennial. However, recent researches have thrown a considerable amount of light upon it and it therefore seems fitting to bring before you this evening a brief survey of the subject and an account of sbme of the latest experimental work. The widespread use of the "carbon microphone"--it is employed almost exclusively throughout the world in commercial telephone service--is due primarily to its unique property of being its own amplifier. In converting acoustical into electrical waves, it magnifies the energy about one thousandfold. Other microphones, such as the condenser or electromagnetic type, are unable to do this and so require separate amplifiers when used in practice. For this reason, it seems unlikely that the carbon microphone will be supplanted in the near future for at least the great bulk of telephone work. * Presented before t h e Franklin Institute, March 2, 1933. Journal of the Franklin Institute, April, 1934. 163 Published in the