SPECIFIC INDUCTIVE CAPACITY. 475 



sent into a ballistic galvanometer, will give 

 w = CRI = --<* 



G 7T a 



Suppose that the circuit of the primary current contains a coil, 

 in front of which is another coil forming part of a closed circuit, 

 the resistance of which is r, and the coefficient of mutual induction 

 of the two .coils M. When the principal current is made or 

 broken, the induced discharge is 



, MI 



r J 



and if we measure it by the swing #', which it produces in the same 

 galvanometer, we have also 



MI_HT , 



~7~~G ^ a ' 



It follows that 



CRr_a r- M - 



The two systems of discharge may still be transformed into con- 

 tinuous currents.* That of the condenser being repeated n times 

 per second, and the induced discharge n' times per second, if * and i' 

 are the mean currents in the same galvanometer, 



/ M n' i 



-, or C = --. 



With a suitable mechanical arrangement, which establishes suit- 

 able connections, the two kinds of discharges may be sent alternately 

 in the galvanometer, and the resistances, or the values of M and C, 

 regulated so that the needle is at zero; we have n = ri, /=*', and 

 therefore 



1064. SPECIFIC INDUCTIVE CAPACITY. By comparison with 

 solid or liquid dielectrics, the specific inductive capacity of air may 



* RO'ITI. Atti del . /. Veneto [6], Vol. n. 1884. 



