Electrical Conductivity of Mica in Intense Fields. 115 



with a micrometer-gauge and found to be 0*74 mm. So, the 

 thickness of this sheet being known to be O0617 mm., we 

 can find the thickness of any other sheet by comparison, 

 since for a given part of the spectrum, b\ varies inversely 

 as t. The thickness of any sheet measured by this method 

 always agreed with that obtained with the spherometer 

 within the limits of error of the latter. The results were 

 .apparently reliable to within about 0*0001 mm. 



The conducting sheets of the condenser were tinfoil disks 

 about 1*15 cm. in diameter cut out with a sharp cork- borer. 

 They were stuck on the opposite faces of the mica with 

 seccotine, being pressed into close contact by means of a 

 roller, care being taken to bring them exactly opposite to 

 each other. When the seccotine was perfectly dry, the free 

 surface of the mica was well washed under a tap to remove 

 all traces of seccotine, and finally rinsed in distilled water 

 aud dried by gentle heat. The mica sheet was then stuck 

 on the iron guard-ring with seccotine, as shown in fig. 2. 



'* 



Fig. 2. 



The insulated wire shown pressing lightly against the lower 

 disk was eventually connected to the galvanometer Gr, while 

 the small wire tripod standing on the upper disk was con- 

 nected to the key D (fig. 1). In the earlier experiments the 

 •surfaces of the mica surrounding the disks were exposed to 

 the air. In this case the capacity of the condenser was 

 found to increase with pressure for voltages greater than 

 1000. This increase apparently depended on the hygro- 

 metric state of the air, as it was considerably reduced, 

 though not entirely removed, by placing the condenser in a 



12 



