SCIENCE AND PRACTICE. 



321 



Two batteries, E and Ej (Fig. 150), are connected parallel 

 between the points b and c, so that their currents are opposed 

 to each other, and between the same c 



points a conductor whose resistance is 

 R! is inserted. Let the resistance of 

 the circuit c E b be R, that of c E L b 

 be E 2 , and let the intensities of these 

 circuits be i, i lt and ?' 2 , as in the figure, 



then 



1) . . i *, + f t =0 



2) . . E i + E! f , = E 



3) . . E 2 t, + E, , = E 4 



Dividing 2) by 3), 

 R* 



Fig. 150. 



For the determination of which it would be needful, in addi- 

 tion to the values of R, R D and R 2 , to know the values of 

 ^ i lt and i 2 . The same is done, however, by reducing the 

 intensity in one of these circuits to nothing, by which the 

 remaining two become equal to each other. By adjusting 

 one of the resistances, R or R x , we arrive at a point where 

 the intensity a in the circuit c E x b is reduced to nothing, 

 by the currents of its proper battery Ej and of E being 

 balanced. 



Where this is the case, 



t-, =; o 



and equation 1) becomes 



i i l =Q 

 or i = i v 



These values set in (4, we obtain the relation of the two 

 electro-motive forces 



E, E, 



from which we see that with this method it is impossible to 

 compare the electro-motive forces of two batteries when 

 they are equal, and R L a measurable quantity ; because if 



