FL UCTUA TIONS IN MICROPHONES A ND OTHER RESISTA NCES 205 



measured as a function of the voltage on the contact, the resistance 

 being held fixed. In every case the general law given by Kq. (1) was 

 found valid, a varying between the limits 1.75 and 1.95 for different 

 contacts. The results of a typical measurement on a contact having a 

 resistance of 76 ohms are shown in Fig. 5. The experimental points 

 fall on a straight line having a slope corresponding to a value of a 

 equal to 1.83. 



Figure 6 gives the results of contact noise measurements performed 

 on a commercial grid leak which was made by coating a thin layer of 



li 20 

 Q 



10 50 100 



APPLIED POTENTIAL IN VOLTS 



Fig. 6 — The mean square contact noise voltage in a 50,000-ohm carbon grid leak 

 resistor as a function of the applied voltage. 



finely divided carbon and binder on glass. The input circuit shown in 

 the insert, consists of Ri, the sample under test; Ro, a metal wire 

 resistor which produces no contact noise; and a suitable source of d.-c. 

 voltage. The resistance of both Ri and R2 was 50,000 ohms. The 

 experimental points lie on a straight line the slope of which fixes the 

 value of a at 1.90. Individual points could be reproduced within an 



