DIFFERENTIAL GALVANOMETER. 465 



of the discharge introduces a source of error which it is difficult to 

 eliminate. We know further (884) that the use of a shunt may 

 give rise to serious errors. 



1053. This method gives a means of determining a capacity C 

 in absolute units in the electromagnetic system. For if the de- 

 flection 8 of the galvanometer for the current produced by the 

 electromotive force V in a resistance R be directly or indirectly 

 produced, we shall have 



,.4,. 



and therefore 



The capacity is thus determined by the resistance R and the 

 time of oscillation T of the needle. 



This is the method employed by the late Fleeming Jenkin,* for 

 instance, in the name of the British Association, to determine the 

 absolute value of a condenser which was to serve as standard, the 

 capacity being near 10 microfarads (io~ 14 C.G.S. units). 



The experiment presents great difficulties with condensers having 

 a solid dielectric. Unless we make the oscillations of the needle 

 extremely slow, which is inconvenient in practice, and which reduces 

 the swings, we can never be sure that the duration of the discharge 

 is only a very small fraction of the time of oscillation (889). More- 

 over, in consequence of the absorption of electricity by the dielectric, 

 the capacity appears as a function of the time of charge and of dis- 

 charge. 



Thus in Jenkin's experiments with a Thomson's astatic galvano- 

 meter (849), in which the moment of inertia of the movable 

 system had been so increased as to raise the time of oscillation to 

 about 20 seconds, the charge was obtained by a battery of 20 

 DanielPs elements. The condenser was discharged after being 

 charged a minute. According as the discharge contact was 17, 

 3-4, or 5 seconds, the deflection was 156, 161, 164, or 166 divisions, 

 this latter being the same as for a permanent contact. Here again 

 it is necessary to specify the duration of the discharge. 



1054. DIFFERENTIAL GALVANOMETER. When two capacities 

 are equal, and after having charged them by the same battery 

 they are discharged through a differential galvanometer, the needle 



* JENKIN. British Association Report, Dundee, 1867. Reprint, p. 146. 

 VOL. II. HH 



