MEASUREMENT OF A CAPACITY. 563 



added to the resistance of the wire g, we have, if G and G' are the 

 constants of the two coils, 



G 



consequently 



E RG'j+- 



a = = 

 e x G s 



In order to determine the ratio , the same current is passed 



G 



through the two coils, with a suitable shunt on that with the fine 

 wire, so as to bring the needle to zero. 



The coefficient of self-induction M is determined by elliptic 

 functions. For two circles of radius a and a', whose currents are 

 in opposite directions, we have, as a function of quantities which 

 have been defined above (763), 



aa 



and the calculation for the two coils may be made by the method 

 of Lord Rayleigh (765). 



1134. MEASUREMENT OF A CAPACITY. The measurement of a 

 capacity may be determined directly in electrostatic units. The 

 simplest and most certain method is that of two parallel plates, 

 one of which is surrounded by a guard-ring, in order to avoid the 

 influence of the edges. 



In order to determine the same capacity in electromagnetic units, 

 the method will be employed which we have pointed out in 1053 

 and the following. The measurement reduces itself to that of a 

 resistance and of a time. With a single discharge, let T be the 

 time of oscillation of the needle of a galvanometer, a the swing 

 produced by the discharge of a condenser, the armatures of which 



002 



