464 MEASUREMENT OF CAPACITY. DIELECTRICS. 



electrometer. This residue does not, however, occur in the pre- 

 sent case ; for the discharge observed corresponds to the same 

 fall of potential from the primitive value V to that which can no 

 longer produce contact with the gold leaf. 



As the experiment has a certain duration, the total charge is 

 thus measured. If we wish to know the instantaneous discharge, 

 it will be sufficient} after having determined the total charge, to 

 electrify the condenser afresh to the same potential, then to dis- 

 charge it for a very short time, and finally to determine the residual 

 discharge. The difference of the two values thus obtained repre- 

 sents the instantaneous discharge. 



In place of the total charges, we may solely determine the rate 

 of leakage, or the lengths of time / which correspond to the same 

 flow V -V of potential. For if R is the resistance of the con- 

 necting wire, and C the capacity in question, we have (985) 



the time t is thus proportional to the capacity. This method only 

 requires a delicate electroscope, without any attention being needed 

 as to its graduation. 



1052. If the discharge of a condenser is determined by a bal- 

 listic galvanometer (883), the swing a of the needle, corrected for 

 damping and graduation, gives the equation 



0) 



As the potential is the same in two successive experiments, the 

 ratio of the capacities is equal to the ratio of the swings. 



In order to compare very unequal capacities, which would give 

 very different swings, it would be useful to take electromotive forces 

 V and V, which are in a known relation for instance, with the 

 numbers n and ' of couples of the same kind. We shall then 

 have 



Ca V'n'a 



The experiment is very exact when the capacities are very near 

 and of the same kind. For very different capacities, the duration 



