41 8 MEASUREMENT OF ELECTROMOTIVE FORCES. 



the condition when the values of the total resistance of the circuit 

 ABC are successively R and R', we have 



e x x' x - x 



E R + / R' + / R'-R' 



Instead of comparing the electromotive force e with that which 

 gives the principal current, and which, for that very reason, is badly 

 defined, it is better to work by substitution, and to place successively 

 in the circuit AGC the two electromotive forces e and e' to be com- 

 pared.* As the two experiments give the ratios and , the ratio - 



hi h, e 



is at once obtained. 



In the second arrangement, for instance, if the value of R is 

 constant, and x and x' are the values of the resistance AC, which 



Fig. 205. 



correspond to equilibrium for the two currents e and e\ we have 

 simply 



e x 



If the wire is homogeneous, the electromotive forces are propor- 

 tional to the corresponding distances from the point of contact to the 

 point A. The rheostate R serves to regulate the resistance so that 

 the distances x and x' t which satisfy the preceding equation, are 

 comprised within the length of the wire AB. 



The galvanometer may clearly be replaced by an electrometer, 

 and particularly by Lippmann's capillary electrometer. 



1013. The two experiments may be made simultaneously; the 

 two couples e and *', connected at the point A by one pole, are 



* PELLATT. Ann. de Chim. et Phys. [5], Vol. xxiv., p. 5, 1881. Journal 

 de Phys., Vol. IX., p. 145. 



