The Back Electromotive Force. 147 



The lamp was made to form one side of a Wheatstone's 

 bridge, the other two sides being two resistances of 1 ohm 

 each, and a variable resistance. In one diagonal was a 

 condensor and telephones. The alternating apparatus, as 

 in the last experiment, was placed in the other diagonal. 



In this case the silence was not so good as in the former 

 experiment, but there was a decided minimum when R was 

 about "6 ohm. 



The values of the back E.M.F. from these experiments 

 with ordinary arcs agree closely with those found by Von 

 Lang and Arons for small arcs. This constancy leads to 

 the conclusion that the back E.M.F. is due to some process 

 which goes on in the arc independently of the magnitude 

 of the current which the arc is taking. Whether this 

 process is the vaporisation of carbon and whether the back 

 E.M.F. has its seat at the crater or in other parts of the arc, 

 remains yet to be determined. 



