INDUCTANCE AND CAPACITY 



361 



of absorption and depending only on the dielectric coefficient and 

 on the dimensions of the condenser. 



Changes of atmospheric pressure cause minute changes of 

 capacity in mica condensers which may be detected by the most 

 refined methods of measurement. The changes are subject to 

 a considerable time lag and may be of the order of magnitude, 

 1 or 2 parts in 100,000 for 1 cm. change of pressure. Usually 

 if the pressure be reduced, the condenser expands and as the 

 increase in the distance between the plates produces more effect 



0.0096mf. 



20 25 



Temperature 



A B 



FIG. 212. Characteristics of good (A) and poor (B) mica condensers. 



than their increase of size, the capacity is decreased. Firmly 

 clamped condensers are but very slightly affected. 



The characteristics of good and poor mica condensers are 

 illustrated by Fig. 212. 



Condensers of silvered mica are sometimes used, but are in- 

 ferior to those of the ordinary construction, being more unstable 

 and having a capacity dependent upon the voltage. This un- 

 stable character is probably due to deposits of silver, under flakes 

 of mica, which are imperfectly attached to the main deposit. 



