RESISTANCE OF A GALVANOMETER. 



357 



and, for r = oo , 



from this we get 



N a + b ' 



being the error of adjustment in the branch b\ 

 Supposing b = b' and a=a = ml>, we get 



E 



The denominator is a minimum for m z = , and the 



b 



difference is greater for a given relative error as to b', the feebler 

 is the resistance R of the battery. 



Fig. 1 88. 



962. RESISTANCE OF A BATTERY. MANCE'S METHOD. Let us 

 suppose the resistance to be measured is at the same time the 

 seat of an electromotive force (in the case of a voltaic couple, for 

 instance), we may again* place this resistance in a lateral branch b 

 (Fig. 1 88), the galvanometer on the diagonal R in the place of 

 the battery, and finally substitute a break K for the galvanometer 

 in the bridge r. 



The resistance b' is adjusted so that the deflection is constant, 

 when the key is opened or closed; the relations ab = ab' is then 

 satisfied. For it is only in this case that, the two branches r and 



* MANCE. Proceedings of the Roy. Soc., Vol. xix., p. 248. 1871. 



