TBE LOCALISATION OF BREAKS AND FAULTS. 



523 



A make and break key is put in the galvanometer circuit 

 which must only be tapped while adjustment of E, or E^ is 

 being made, in order to prevent any appreciable current leaving 

 or entering the standard cell. 



By putting r=100 at first we ensure that, with a standard 

 cell of a little over a volt, the current from the large battery 

 through the bridge coils does not at any time exceed 14 or 15 

 milliamperes, which is too small to have any heating effect on 

 the coils. A key is put in the large battery circuit, so that 

 the current need only be on for a few seconds while getting 

 balance. 



For example. Suppose a 20 cell battery of Leclanche's ia 

 tested for E.M.F. and resistance, a Clark standard cell of 



Standard Cell. 





-^Hl|l|f|l|!Ws|H 

 b 

 Fig. 294. 



1"44 B.A. volt being used for coraparison. The standard cell 

 must be joined in opposition to the battery, as shown in Fig. 

 294, and keys provided as shown. With r=100, the 

 galvanometer is balanced with R adjusted to 3,278 ohms. 

 Again, with 7j:=200 balance is obtained with Ej = 6,568 ohms. 

 Then resistance of battery is 



6,568- (2 X 3,278) = 12 ohm^ 

 and E.M.F. of battery is 



l-44(l.3:278+^-2)=48-8 volts. 



V 100 / 



When it is desirable to make r any other value than 100, or 

 n any other value than 2, separate resistance boxes may be used 

 as in Fig. 291. 



