The Back Electromotive Force. 141 



One diagonal contained a Galvanometer G, the fixed 

 coil of an Electro-dynamometer D, and a variable re- 

 sistance T. 



The other diagonal contained the secondary coil of an 

 induction apparatus I. The swinging coil of the dynamo- 

 meter was joined in series to the secondary of another 

 induction coil J, the primary of which was in circuit with a 

 Dubois apparatus and an interrupter. 



With the arc in circuit, the resistances were arranged so 

 that the deflection of the dynamometer remained constant 

 when the induction apparatus was set to work. The carbons 

 were then short circuited, and the equivalent resistances 

 inserted at S. 



Arons gives two measurements. 



At 3*4 amperes the back E.M.F. was 40*6 volts and the 

 resistance 2 - i ohms. 



At 4 - i amperes the back E.M.F. was 39*6 volts and the 

 resistance v6 ohms. 



Although the results of these experiments seem to show 

 decisively that in the arc we have to deal with a back 

 electromotive force, one often finds doubts expressed in 

 the technical press as to whether this back E.M.F. really 

 exists. In order to throw additional light on the subject the 

 following experiments were commenced on arcs corres- 

 ponding more nearly to the arcs in common use than those 

 experimented on by Von Lang and Arons. 



A number of methods were tried in order to measure 

 the real resistance of the arc. The first was a modification 

 of Mance's method, by placing the arc in one side of 

 a Wheatstone's bridge, a galvanometer in one diagonal 

 and a key in the other, and adjusting the resistances 

 in the sides till closing the key had no effect on the 

 galvanometer. This, however, had to be abandoned on 

 account of the alteration in current through the arc on 

 closing the key. 



