Akt. III. — On a Modification of Mance's Method of 

 Measuring the Resistance of a Battery. 



By F. J. Pirani, M.A. 



[Read 19th May, 1881.] 



Mance's method of measuring the resistance of a battery is 

 based on the following proposition : — 



Let A, B, C, D be four points connected by six conductors, 

 as in the figure, a battery being in he branch AB. 



Then, if the resistance of AB is to that of BC as the 

 resistance of AD is to that of DC, the current in AC will be 

 independent of the resistance of BD ; and, vice versa, if the 

 current in AC is independent of the resistance of BD, the 

 above relation exists between the resistances of AB, BC, 

 AD, DC. 



If, then, we adjust the resistances of BC, AD, and DC 

 until the current in AC is unaffected by varying the resist- 

 ance of BD, and if we know the resistance of BC and the 

 ratio of the resistance of AD to that of DC, the resistance 

 of AB is found by a simple calculation. 



In the method as originally proposed, a galvanometer is 

 placed in AC, and a key in BD ; and the resistances of BC, 

 AD, and DC are adjusted until no alteration in the deflection 

 of the galvanometer is produced by making and breaking 

 contact by means of the key. 



In this application of the principle the greatest sensitive- 

 ness is obtained when the resistance of AD is equal to that 

 of DC, and the resistances of ADC and AC are equal, and as 

 small as possible. 



