328 COMPARISON OF RESISTANCES. 



A galvanometer of great resistance may directly give the resistance 

 of the battery.* The total current I is first observed, and then the 

 current / obtained by inserting between the poles, a shunt s of the 

 same order of resistance as that of the battery. We have then 



If the ratios - and - , are very small, we may write 

 8 S 



consequently 

 (14) 



Fig. 183. 



935. USE OF Two GALVANOMETERS. In the preceding methods 

 the electromotive force must be constant, which is not always the 

 case (especially with polarisable batteries) when they are traversed 

 by currents of very different strengths. 



This source of error may be eliminated by using two gal- 

 vanometers. The battery circuit contains a rheostate R (Fig. 183) 

 and a galvanometer G x ; at two points A and B of the circuit, 

 separated by a resistance r, are connected the ends of a wire 

 which contains a second galvanometer G; in this branch AGB, 

 of resistance g t the two resistances r and / are successively 

 interposed. 



By means of the rheostate the deflections of either of the two 

 galvanometers in the three experiments may be kept constant. 



* Sir W. THOMSON. Journal of the Society of Telegraph Engineers, Vol. i., 

 P- 399- 1873. 



