the Electromotive Forces of Voltaic Cells. 



515 



as will cause the current in E G to vanish, and the current- 

 strength J in T must then be observed If J = current- 

 strength in T, the electromotive force of the battery E is 

 E=WJ." 



The deci-ampere balance, or, when a sufficient number of 

 battery-cells is available, the centi-ampere balance, answers 

 well for the current measurements here required. An arrange- 

 ment of the circuit which is convenient for most purposes is 

 shown in the diagram (fig. 2) ; but it may be remarked that 



STANDARD RESISTANCE 

 WWWWWvVNAAAAA/WVWVVVVVV, 



-CELL TESTED 



OR QUADT ELECTR. 





III 



Fig. 2. 



the reversing-keys there shown may be replaced by ordinary 

 make-break keys. Referring to the diagram, a battery of a suf- 

 ficient number of cells is joined in circuit through a reversing- 

 key with a rheostat, a deci-ampere balance, and a standard 

 resistance. The poles of the cell to be tested are connected in 

 circuit with a key and a sensitive mirror-galvanometer to the 

 two ends of the standard resistance in such a way that both 

 the battery and the cell to be tested tend to send a current in 

 the same direction through that resistance. Care should be 

 taken that the circuit of the cell to be tested is well insulated, 

 and that both it and the standard resistance are free from 



