CHAP, vi.] ELECTRICITY AND MAGNETISM. 323 



of the current equally will be proportional to the 

 electromotive-forces. 



(b) If the poles of a standard battery are joined by a 

 long thin wire, the potential will fall uniformly 

 from the + to the pole. Hence, by making 

 contacts at one pole and at a point any desired 

 distance along the wire, any desired proportional 

 part of the whole electromotive - force can be 

 taken. This proportional part may be balanced 

 against the electromotive - force of any other 

 battery, or used to compare the difference be- 

 tween the electromotive -forces of two different 

 cells. 



(c) The electromotive - force of a battery may be 

 measured directly as a difference of potentials by 

 a quadrant electrometer. In this case the circuit 

 is never closed, and no current flows. 



361. Measurement of Internal Resistance of 

 Battery. This may be done in three ways. 



(a) Note by a tangent galvanometer the strength of 

 the current, first, when the resistance of the 

 external circuit is small ; and secondly, when a 

 larger known external resistance is introduced. 

 From this the proportion between the internal 

 resistance and the introduced external resistance 

 can be calculated. 



(b) (Method of Opposition). Take two similar cells 

 and join them in opposition to one another, so 

 that they send no current of their own. Then 

 measure their united resistance just as the resist- 

 ance of a wire is measured. The resistance of 

 one cell will be half that of the two. 



(c) (Mance's Method). Place the cell itself in one 

 arm of the Wheatstone's bridge, and put a key 

 where the battery usually is, adjust the resistances 

 till the permanent galvanometer deflection is the 

 same whether the key be depressed or not. 



