322 



THE ELECTRIC TELEGRAPH. 



E = E!, it follows, from the above equation, that R = R L 

 + R, which could only occur if either R, the resistance of the 

 element and circuit c E b, were nothing, which is impossible, 

 although it may be made comparatively small ; or if the 



resistance R x were infinite, in which case - = 0, and the 



ixi 

 E 



above expression would become -== 1, that which is equiva- 

 lent to setting the batteries in a single circuit in opposite 

 direction through the galvanometer. 



In practice, it is necessary to consider the resistance of 

 the battery E, and the resistance inserted in the same circuit 

 between the points c b, as separate magnitudes ; because it is 

 not always that we can make the resistance exterior to the 

 battery so great as to be able to neglect entirely the internal 

 resistance without error. 



The circuit as arranged for comparing the electro-motive 

 force of a battery with that of a single cell is as follows : 

 The battery is inserted in the part 

 circuit c E b (Fig. 151), with a coil of 

 wire of known resistance, r ; in the 

 f ex. opposite part-circuit, c E t b, a galvano- 



/ \ scope, G, is inserted, with the con- 



-L El SJ R| _LE s tant cell E x ; and, from the points of 



junction, c b, of the two part-circuits, 

 is the adjustable resistance scale R-j. 



If the resistance of the battery is a?, 

 the resistance of c E b is 

 -^ 



Fig. 151. 



and the currents being balanced, the relation of is 



E i 



JL - ' + * + R . 

 E, R, 



The resistance of an element is generally known approxi- 

 mately, and if the battery is composed of n similar elements, 

 of which each has p units resistance, it is near enough to set 

 j = np, in the above formula ; but it is better, where great 



