The Bell System Technical Journal 



April, 1934 



The Carbon Microphone : An Account of Some Researches 

 Bearing on Its Action * 



By F. S. GOUCHER 



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. 



Introduction 



FEW electrical devices are as widely used as the "carbon micro- 

 phone" and few have given rise to as much speculation in regard 

 to their mode of action. That 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 some 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 the Franklin Institute, March 2, 1933. Published in the 

 Journal of the Franklin Institute, April, 1934. 



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