420 



MEASUREMENT OF ELECTROMOTIVE FORCES. 



Lord Rayleigh and Mrs. Sidgwick* have repeated this deter- 

 mination by the aid of a more complicated arrangement (Fig. 207). 



C and C' are two resistance boxes which close the circuit of 

 an auxiliary battery E', and the plugs are arranged so as always 

 to have a fixed resistance of 10,000 ohms between the poles of 

 the battery. The two keys K' and K" being left open, and the 

 key K closed, the resistance r which compensates the electromotive 

 force of the couple is first tried in the box C. If then the key 

 K is opened and K' and K" closed, the resistance r' is sought 



Fig. 207. 



which balances the electromotive force 

 MABN. We have then 



- R I of the circuit 



or 



r-r 



The resistance R is known in absolute value, and the prin- 

 cipal current I is measured by a balance electro-dynamometer D, 

 analogous to that of Joule (792). 



1015. CLOSED BATTERY. When a polarizable battery of resist- 

 ance R and electromotive force E in open circuit, is closed by 

 a resistance r, it acquires a smaller electromotive force E, and its 

 resistance has in general a different value R. The intensity of the 

 current is defined by the equation 



* Transactions of the Royal Society ', 1884^ p. 411. 



