390 COMPARISON OF RESISTANCES. 



condition cannot generally be realised, and without extinguishing 

 the sound in the telephone, we obtain thus only a minimum of 

 sound. 



Instead of altering the relative position of the principal coils, 

 which would give rise to too sudden changes, Mr. Hughes found 

 it more convenient to have recourse to an auxiliary apparatus 

 which he calls a Sonometer. 



This apparatus consists of two identical coils H and H', fixed 

 at the ends of a horizontal scale, between which a coil K may 

 move parallel to itself; the former are inserted in the inducing 

 circuit, and the movable coil in the induced circuit. The actions 

 of H and H' on K being of contrary signs, the resultant is null 

 when the coil K is exactly in the middle of the scale ; the 

 induction increases on one system or the other according as the 

 coil is displaced to the right or the left. That position of the 

 coil, which in each case produces silence, furnishes an arbitrary 

 measure of the effect produced by the conductor. 



Another mode of measurement consists in merely modifying 

 the balance by introducing a conductor between the coils A' and a'. 

 Mr. Hughes uses a zinc strip cut in the shape of a very acute 

 wedge which he slides along; this is graduated, so that it is easy 

 to determine the thickness of the plate which produces silence 

 and balances the effect produced by the conductor C. 



We may also place below the system A'a a small copper disc, 

 or a ring of the same metal, movable about a horizontal axis. 

 The action is null when the plane of the disc is perpendicular to 

 that of the coils, and is a maximum for the parallel position. 



It can be understood that this arrangement is better than the 

 sonometer for compensating the action of a metal plate as it gives 

 rise to a phenomenon of the same kind. If the letters with an 

 accent denote quantities relative to the new conductor C', the 

 equations (44) should be replaced by the following: 



_ di dy , dy' 



l dt +m ^- m Tt 



(46) ^y ' 



A S +M 



.d' d\ 



