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BELL SYSTEM TECHNICAL JOURNAL 



the actual microphone and by coating these with a conducting layer 

 of graphite and lacquer we are able to make them behave electrically 

 as well as elastically in accordance with our simple theory. When 

 placed in the model the aggregate is compressed cyclically by means 

 of the piston which acts as a diaphragm, producing a change of re- 

 sistance in the current path around the insulating barrier. The 

 curves shown in Figs. 27 and 28 show typical resistance-force cycles, 

 obtained with the model and the actual instrument under conditions 

 wherein the reactive forces are mainly elastic. The similarity of 

 these characteristics is striking. The existence of the loops indicates 

 that the reactive forces are not entirely elastic and that the behavior 

 is modified by friction, as in the case of single contacts. 



f) 95 



2 



Z 90 



70 



0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 



FORCE IN GRAMS 



Fig. 28 — Resistance-force cycle obtained with a standard transmitter. 



In conclusion it seems fair to say that our experiments on "loose 

 contacts" under conditions which are equivalent to those under which 

 they operate in actual microphones have given a satisfactory picture 

 of the essential nature of such contacts, and their mode of operation 

 when strained, both from the elastic and the electrical point of view. 

 The electrical current is carried through regions in intimate contact 

 and changes in resistance under strain are due both to a variation in 

 the number of microscopic hills which form the carbon surface and to 

 area changes at the junctions of these hills arising from their elastic 

 deformation in accordance with the well known laws of elasticity. 



