422 MEASUREMENT OF ELECTROMOTIVE FORCES. 



Without changing the position of the point B, resistances 

 r and r' are added to the segments R and R' respectively, so 

 that the current is null; and then, without its being necessary to 

 determine the resistances R and R' of the circuits containing the 

 electromotive forces, 



E R R +r r 



1017. USE OF GALVANOMETERS OF HIGH RESISTANCE. If 



the resistance r, which closes the circuit of a battery, is very great 

 in comparison with the internal resistance R, the expression 



( R 

 i+- 



reduces sensibly to 



E-Ir. 



In like manner, when we wish to compare two electromotive 

 forces, we have simply 



and if we arrange the experiment so that the resistances or the 

 intensities are the same in both cases, the ratio of the electro- 

 motive forces is equal to the ratio of the intensities observed, or 

 of the resistances introduced into the circuit. 



The sensitiveness of galvanometers of high resistance being, so 

 to say, unlimited, this method is the most exact, the most expe- 

 ditious, and almost the only one in use at present. The couples 



