Determination of Coefficients of Mutual Induction, 413 



where log Gr has the values : — 



No. of coils. logG. 



2 _-24797 



18 1-29373 



We can now proceed to the problem. 



Two coils, P, S, are placed side by side, and a current passed 

 through P ; then, on making the contact, a ballistic galvano- 

 meter in the circuit of S is deflected. The earth induction- 

 coil forms part of the circuit of S. 



So far we have the equations 



C = GHtan0, (1) 



q=¥» ^ 



with reading = a minutes of arc. 



If we now give the earth induction-coil a half turn, we have 



_2NAH 

 ^o— — ^ — , (o, 



with reading = /3 minutes of arc. 



Whence Q_ MGHang 



wnence ^ - 2NA 



a 

 by the principle of the ballistic galvanometer. And 



-iv=, * 



The 2 disappears from the numerator if the observation is 

 made by reversal. 



In order that H may be the same for the tangent-galvano- 

 meter and the earth induction-coil, it is necessary to remove 

 the tangent-galvanometer and put the earth induction-coil 

 in its place before making the observation 0. 



There are two classes of cases which cannot be treated by 

 this most simple form of the method : — 



(a) Where the quantity of electricity to be dealt with is too 

 large for the galvanometer, even with the coil of highest 

 resistance. And 



(b) where the resistance of one of the coils to be determined 

 is so great that no earth induction-coil could throw a sufficient 

 quantity of electricity through it. 



In both these cases we have to introduce a large resistance 



