The Back Electromotive Force. 143 



through the galvanometer which prevented the balancing 

 of the resistances being carried out. This led to the use of 

 an alternating E.M.F. instead of the battery, and a pair of 

 telephones instead of the galvanometer in the balancing 

 circuit. It was soon found, however, that to render the 

 telephones serviceable a condenser had to be placed in 

 series with them, thus preventing any continuous current 

 passing through the telephone circuit when A and B differed 

 in potential. 



The use of alternating current involved the balancing 

 of self induction as well as of resistance to obtain silence 

 in the telephones. This was effected by making the circuit 

 as symmetrical as possible, by balancing the working 

 dynamo in one side of the bridge by an idle armature in 

 the other, and by the use of an adjustable self induction 

 formed by inserting an iron cylinder more or less into a 

 solenoid. 



Fig. 4 represents, diagrammatically, the apparatus with 

 these alterations introduced. 



On the right is the working continuous current circuit, 

 consisting of a shunt wound dynamo D,, in series with 26 

 accumulators E, an arc lamp X, an ammeter C, and a 

 resistance R. 



The Voltameter V can be connected through the key K 

 either to the terminals of the lamps or to the battery. 



On the left is the testing circuit, made to resemble the 

 other as much as possible. It consists of a stationary 

 dynamo D y/ exactly similar to D /5 a resistance R //5 equal to 

 R y , a variable self induction L, and an adjustable resistance 



These two circuits are in the arms of a Wheatstone's 

 bridge, the other two arms bring the two halves of a 

 stretched platinoid wire PQ, P being kept equal to Q. 



Across one diagonal of this bridge is a condenser M 

 in series with a pair of telephones T. The other diagonal 



