172 BELL SYSTEM TECHNICAL JOURNAL 



Another view was that microphonic action arises from change in 

 resistivity of the solid carbon resulting from strain. This view was 

 held by Edison who doubtless believed it because of the success of 

 his microphone which was designed with the object of applying 

 pressure variation to a solid carbon block. It failed of general 



Fig. 11 — Carbon granules made from anthracite coal (X 15). 



acceptance because the effect of pressure on resistance, as shown by 

 experiment, seemed definitely to be too small. It was generally con- 

 sidered that the Edison instrument was in fact a "loose contact" 

 although Edison himself did not realize it. 



Others of the early inventors considered the contact area to be the 

 essential element — that is to say, the extent of surface or the number 

 of molecules involved in intimate contact. As Professor Sylvanus 

 Thompson expressed it in 1883, "An extremely minute motion of 

 approach or recession may suffice to alter very greatly the number of 

 molecules in contact. . . . Just as in a system of electric lamps in 

 parallel arc the resistance of the system increases when the number of 

 lamps is diminished and diminishes when the number of lamps con- 

 necting the parallel mains is increased, so it is with the molecules at 

 the two surfaces of contact." 



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