The Back Electromotive Force. 145 



arrangement gives any desired alternating E.M.F. in the 

 testing circuit. In practice an E.M.F. of about 5 volts 

 was found to give the best results. 



The experiment is conducted thus : The right hand 

 continuous current circuit is run with the switch S open, 

 till the potential at the terminals of the arc exactly equals 

 the potential of the cells, while the lamp is at the same time 

 taking the required current as measured by the ammeter C. 

 This is effected by altering the speed of the engine driving 

 D y in the first place, and finally by feeding the lamp, which 

 is done by hand. 



When this adjustment is made, the points A and B are 

 at the same potential and the switch S is closed. R //; and 

 L are then adjusted till there is silence in the telephones T, 

 which are protected from the effects of any slight lack of 

 adjustment of the continuous current circuit by the con- 

 densor M. 



In practice complete silence is never obtained in the 

 telephones, it is replaced by a minimum which is accom- 

 panied by a change of note in the humming produced by 

 the alternator. 



That the position of this minimum is identical with the 

 position for complete silence, was proved by replacing the 

 arc by its apparent resistance, when, by a proper adjustment 

 of L, an absolute silence was obtained. L was then altered 

 till a minimum could be found ; this minimum, however, 

 gave the same value for R ; „, as in the former case. 



This balance having been obtained, all the resistances 

 being known, the resistance of the battery being found by 

 its drop of potential when giving current, the real resist- 

 ance of the arc is readily found. 



A number of experiments were made under increasingly 

 favourable conditions, all of which showed that the true 

 resistance lay somewhere about '6 ohms. As the circuit 

 was made more and more symmetrical, the self inductions 



