388 COMPARISON OF RESISTANCES. 



of resistances and of the coefficients of mutual induction, whatever 

 moreover be the coefficients of self-induction. 



The galvanometer cannot be used if the inducing current, in- 

 stead of the sudden changes in direction given by the opening or 

 closing of the circuit, undergoes variations which succeed rapidly 

 in contrary directions, like those produced by a vibrating break, 

 or a microphone introduced into the circuit, or an electrometer 

 with alternating currents. In this case recourse must be had to 

 an instrument such as the telephone, which is sensitive merely to 

 instantaneous variations of the current. A differential telephone, 

 for instance, would be silent if the currents induced at each instant 



are equal, which implies that the coefficients of the two systems are 

 equal (505). 



It is, moreover, immaterial whether the differential telephone is 

 worked by currents induced in the coils a and #', or by two 

 branches of the inducing circuit which respectively traverse the 

 two coils A and A'.* 



987. It is on this principle of the equilibrium of two circuits 

 that is based the arrangement employed by Mr. Hughes, f and 

 known as the induction balance, for comparing the electrical pro- 

 perties, and in particular the resistance of bodies which are met 

 with in any given form. 



* CHRYSTAL. Phil. Trans. Roy. Soc. Edin., Vol. xxix., p. 609. 1880. 

 t HUGHES. Phil. Mag. [5], Vol. n., p. 50. 1879. 



