148 Dr. A. Schuster on 



Note on Mr. Frith's paper "On the True Resistance 

 and on the Back Electromotive Force of the 

 Electric Arc." By Arthur Schuster, F.R.S. 



{Received March 14-th, 1895.) 



The good agreement which has been obtained by- 

 different observers in measuring the so-called " back electro- 

 motive force " of the voltaic arc seems to shew that there 

 must be some real meaning in the result, but it would 

 be rash to take that agreement as a proof that 

 the arc is the seat of something analogous to electro- 

 lytic polarisation, as the term would seem to imply. 

 At any rate it is of importance to be clear as to what is 

 really proved and what assumed in such experiments as 

 those described in the previous paper. 



Consider part of a circuit lying between two points, 

 P and 0,and only containing metallic conductors and electro- 

 lytes or voltaic cells. The difference of potential e between 

 the points P and Q, is connected with the current strength 2, 

 the resistance r, and the sum of electromotive forces 

 included (E) by the equation 



e = E + i7- . . . . (1) 



If we vary the current and observe the change in the 

 fall of potential, a second equation is obtained which, 

 together with (1), determines both E.and r. Thus, if the 

 changes are small, we obtain from (1) 



de ,~v 



S = r • ■ • • (2) 



