ELECTRICAL MEASUREMENTS. 



283 



A bare German silver wire WW * is stretched upon a board and 

 connected to a battery B so that an invariable current i flows 

 through it. A side circuit, containing a sensitive galvanometer 

 G and a voltaic cell of which the electromotive force, e is to be 



Fig. 209. 



measured, is connected to the wire WW by means of two slid- 

 ing contacts a and b. The sliding contact b is adjusted until 

 the galvanometer gives no deflection, then 



e = n 



(0 



where r is the resistance of the portion ab of the .German silver 

 wire. The voltaic cell e is replaced by a standard cell of which 

 the electromotive force e' is known, and the sliding contact b 

 is again adjusted until the galvanometer gives no deflection. 

 Then 



e' = r'i (ir) 



where r' is the resistance of the portion ab' of the German 

 silver wire. Dividing equation (i) by equation (ii), member by 

 member, we have 



r 



r' 



The ratio rfr' , however, is equal to the ratio of the lengths of 

 the respective portions of the wire WW, and this ratio may there- 



