THE CARBON MICROPHONE 



177 



in this way a variety of contacts can be studied with the same tube. 

 A small hole in the metal cylinder permits of direct observation of 

 the contacts during measurement. Figure 14 shows how the apparatus 



Fig. 13 — Diagrammatic view of single contact device shown in Fig. 12. 



was mounted in an iron cylinder on a damped suspension to protect it 

 from acoustical and mechanical disturbance. The two microscopes 

 were used to observe the compression of the silica spring. 



We first studied the effect of voltage and temperature on contacts 

 held together with constant forces. Reversible characteristics could 

 in all cases be obtained for voltages up to 1 volt and for temperatures 

 up to about 80° C. 



Typical characteristics are shown on Fig. 15 in which the contact 

 forces were of the order of 1 dyne. On the left are plotted the re- 

 sistance-voltage characteristics and on the right the resistance- 

 temperature characteristics. All of the variables are plotted for con- 

 venience on logarithmic scales. 



