518 



PROFESSOR W. E. AYRTON, MR. T. MATHER AND MR. F. E. SMITH: 



The current, whose value is to be determined by the current weigher, is passed 

 through a standard resistance R (figs. 22, 23) and adjusted in strength until the P.D. 

 between the terminals of R balances the E.M.F. of the cell S. A double commutator 



Fig. 22. Diagram of circuit. 



C, fig. 23, with copper contacts to reduce thermal E.M.F.'s, reverses the current in 

 the standard resistance R and simultaneously reverses the connections to the 

 standard cell S. The standard resistance is described on p. 520 ; it is provided 

 with current and potential leads and is immersed in a tank of insulating oil. To 



Slider 



LJ 



u 



Fig. 23. Detailed diagram of circuit. 



A represents an ammeter. 



B 

 C 



E 

 G 



M 



a battery. 



a double commutator. 



an earthed point of battery. 



a galvanometer. 



a main commutator. 



N represents a variable resistance. 

 P ,, a multiple commutator and plug board. 

 R a standard resistance. 

 S and S' represent standard cells. 

 T represents a turning head for enabling either 

 S or S' to be used. 



