Dielectric Constant and Electrical Conductivity of Mica . 113 



capacity of the system. The multiple key D, when in the 

 position shown, connects the small leyden-jar E and the 

 mica test-condenser F to the high pressure source. As 

 already mentioned, E was subsequently replaced by an air- 

 condenser. Any leakage through the mica is measured by 



Fig. 1. 



the galvanometer Gr, provided that the potential difference 

 across the mica is constant. When D is in the position 

 shown, the galvanometers H and I are short-circuited. On 

 releasing a trigger the key discharges E and F throngh H 

 and I respectively, the short-circuit connexions having first 

 been broken. J and K are two condensers, each being about 

 5 microfarads, L and M are two self-inductances. This 

 arrangement prevents large potential differences from oc- 

 curring between the terminals of the galvanometers during 

 discharge, while not affecting the quantities passing through 

 them. A similar arrangement protects the galvanometer Gr 

 in the event of the mica in F being pierced. N is an earthed 

 guard-ring which prevents any surface leak over the mica 

 from reaching G. The point is connected to earth, P 

 and Q are two high resistances to prevent surges, as 

 explained later. 



The first step in the construction of the test condenser was 

 to obtain a uniform sheet of mica about 6 or 7 cm. square 

 and from 0*01 to 0*02 mm. thick. The sheets, obtained by 

 careful splitting, were examined under a microscope between 



Phil. Mag. S. 6. Vol. 32. No. 187. July 1910. I 



i 



