FL UCTUA TIONS IN MICROPHONES A ND OTHER RESISTA NCES 203 



the sound field produces a constant resistance modulation at the points 

 of contact between carbon granules and the granular aggregate obeys 

 ohms law over the entire voltage range. The fact that the noise 

 and the signal follow so nearly the same relationship indicates that the 

 noise also arises from resistance modulation at the points of contact 

 between carbon granules. Since a is slightly less than 2, however, the 

 noise mechanism is not entirely independent of the applied voltage. 



Noise as a function of applied voltage was also measured in single 

 contacts between carbon particles. For these observations a cantilever 

 bar device was used in which the contact can be rigidly fixed and 

 manipulated at will. This apparatus, shown in Fig. 4, consists of a 



3 



-A^ 



m\\\m 



3® 



Fig. 4 — Diagram of cantilever bar device for producing small contact displacements. 



