﻿738 Mr. G. Breifc on the Effective 



Again, if the periphery is grounded and the centre is not, 



di 

 formula (20) applies, and in that formula V =L — , as is 



seen from (10) by setting Y = when .r = a. This gives 



c o = ^ m 



for the capacity with periphery grounded. 



Finally, if the coil is insulated and the condenser is 

 unshielded, as much current enters the coil as leaves it ; so 

 that (20) and (21) must give the same value for C . 



Multiplying (20) by 2 and adding to (21), it is found 

 that 



Co = ~ . ■ (24) 



if the coil is ungrounded. 



It is worth mentioning that if C be eliminated from (20) 



y 2 



and (21), it is found that — j~- = o, which, in virtue of the 



T di o 



identity '" 



1 Y i (ji)dp = 0, 



10 



shows in a different way that the coil is insulated. 



Expressing the results in micromicrofarads, the capacity 



is 



when grounded at centre 0'567 Ka /Hfxf, 



when grounded at periphery . . . 0*330 Ka yuy-tf, 



when insulated 0*252 Ka fju/jui . 



Now, according to the results of a previous calculation *, 

 the effective capacity of a pancake coil of small depth when 

 insulated is 0*437 Ka. 



Thus, so far as the effective capacity is concerned, there 

 is an advantage in using pancake coils of large depth as 

 compared to pancake coils of small depth. 



Experimental Verification. 



The formulas (22), (23) have been verified experimentally 

 on a coil which is shown in fig. 3. This coil is not circular 



* See G. Breit, I. c. 



.1 



