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XIY. JSfote on a Method of Determining Coefficients of Mutual 

 Induction. By Gr. Caeey Foster, F.R.S.* 



THE determination in absolute measure of the coefficient 

 of mutual induction of two electric circuits by the ordi- 

 nary method founded on the throw of a ballistic galvanometer 

 is a somewhat complicated matter, necessarily occupying a 

 good deal of time. But the process may be greatly simplified 

 if we have available a condenser of which the capacity is 

 accurately known. For instance, if P and s are two coils 

 whose coefficient of mutual induction is required, let them 

 first be joined up, as indicated in fig. 1 — P in a primary 



Fig. 1. 



circuit containing a battery, B, and make-and-break key, K ; 

 and s in a secondary circuit of total resistance r including a 

 ballistic galvanometer G. Then, on closing or opening the 

 key K, a momentary deflection of the galvanometer will 

 occur in consequence of its being traversed by a quantity 

 of electricity Q given by 



Q = M? 



r 



where M is the coefficient of mutual induction between P and 

 s, and 7 the strength of the primary current. Next, leaving 

 the primary circuit unaltered, let connections be made as in- 

 dicated by fig. 2, where c is a condenser of known capacity 



Fur. 2. 



B-= 



C, and A and D are two points in the primary circuit sepa- 

 rated by a resistance R. Then, on making or breaking contact 



* Communicated by the Physical Society : read November 27, 1886. 

 Phil, Mag. S. 5. Vol. 23. No. 141. Feb. 1887. K 



