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

A great variety of speculations in regard to the physics of microphonic action has arisen because of the complexity of behavior when current passes through a so-called “loose contact” which forms the essential element in a carbon microphone. Technical difficulties arising from the minuteness of the contact forces and movements between contacts when in a sensitive microphonic state have retarded the establishment of a quantitative theory. Recent studies of carbon contacts have led to a satisfactory picture of the nature of such contacts and their mode of operation when strained, both from the elastic and the electrical point of view. The surfaces of the carbon particles are microscopically rough and when two such surfaces are brought together under the action of compressional forces, both the number of hills in intimate contact and the contact area between hills vary through deformations which are primarily elastic. Changes in electrical resistance under strain are consistent with the assumption that current passes through the regions in intimate contact.