THE LOCALISATION OF BREAKS AND FAULTS. 531 



For instance, suppose a 12 c^ll battery was to be comparad 

 on the slides with a standard cell of 1*4 volts. The approximate 

 position of the sliding contacts to balance would be 



10,000 X —^,=780 

 12x1-5 



(reckoning each cell of the battery 1-5 volts approximately). 

 Then the pointer should be set to about 750 from zero (7 on 

 the double index dial and 50 on the single dial) and moved up 

 -gradually to balance as described. This will prevent the larger 

 battery working through the standard cell. In practised hands 

 there is no fear of adversely affecting the standard cell. There 

 is good protection in the key which normally keeps the circuit 

 open, and it is only closed for the very brief time of tapping to 

 make a momentary contact. 



Those who are new to the test should put a resistance of 

 1,000 ohms in series with the standard cell, and keep it in cir- 

 cuit until near the balance, when it can be cut out by a short- 

 circaiting plug. 



The E.M.F. of Clark standard cells falls with increase of 

 temperature : the formula for variation of voltage being given 

 •by Lord Raleigh {Phil. Trans., 1885, p. 799) as : — 

 E, = 1-4339 [1-0-00077(^-15)]. 



Cidmium standard cells are less affected by variation in tem- 

 perature the coefficient per degree Centigrade being only 

 0-00004. These cells have a voltage of 1-0195 at 15°C. 



Measurement of E.M.F.'s by Deflection.— The E.M.F. of a 

 cell or battery may be determined by comparison of direct 

 deflections with a standard cell. A reflecting galvanometer 

 and high resistance are employed, and it saves calculation to 

 adjust a shunt on the galvanometer to make the standard cell 

 deflection some multiple of the voltage. The connections shown 

 in Fig. 300 form a convenient arrangement with a Sullivan 

 subdivided universil shunt and resistance of 100,000 ohms. 

 If the E.M.F. of the battery to be tested is within 6 volts, the 

 galvanometer shunt is adjusted so that the standard cell 

 deflection is 100 times its voltage. That is, 143-5 divisions 

 for a standard cell of 1-435 volts. Each 100 divisions on the 

 scale then represents 1 volt, and using the whole scale of 600 



M m2 



