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BELL SYSTEM TECHNICAL JOURNAL 



accurac>' of 0.1 dl). Similar measurements of noise in thin metallic 

 films deposited by either the cathode sputtering or evaporation process 

 gave results in agreement with Eq. (1), the value of a lying between the 

 limits mentioned above. 



Thus it is seen that all types of granular resistance elements which 

 we have tested, namely, carbon transmitters, single carbon contacts and 

 those consisting of thin films of carbon or metal follow the same rela- 

 tionship for noise as a function of applied d.-c. voltage. 



Noise as a Function of Contact Resistance 

 The observation of contact noise as affected by contact resistance is 

 necessarily limited to loose contacts, for in fixed resistance elements 

 such as grid leaks and conducting films one has no means of inde- 

 pendently varying their resistances. One alters the resistance of a 

 single contact by the relative displacement of the two contacting 

 elements. The resistance of a multi-contact device, such as a carbon 

 microphone, may be altered either by a relative displacement of the 

 contacting particles, or by a change in the number of contacts between 

 the electrodes. The noise is affected differently by these two methods 

 of resistance change; hence one must study them separately. In this 

 section we shall be concerned only with noise as affected by resistance 

 changes due to contact displacement both in single contacts and in 

 aggregates; that due to a change in the number of contacts between the 

 electrodes will be considered in our discussion of the noise from a contact 

 assemblage. 



Figure 7 is a diagram of the input circuit used to study the relation- 



POTENTI- 

 OMETER 



STANDARD 

 SIGNAL 



TT 



^f § 



TO 



AMPLIFIER 







Fig. 7 — Diagram of the circuit used in measuring contact noise as a function of 

 resistance in granular resistance elements. 



ship between noise and resistance. By means of the potentiometer we 

 could measure both the voltage supplied to the contact circuit and that 

 across the contact or transmitter. The resistance Re is so adjusted for 

 each noise observation that one half the voltage supplied to the circuit 

 is across the contacts. The contact resistance for this condition is 

 given by R(. Also, in every case, one half the generated noise voltage 

 is impressed on the input tube of the amplifier. 



