528 



SUBMARINE CABLE LAYING AND KEPAIRING. 



In the Kelvin-Varley slides there are 100 coils in CD and 

 101 in AB, the extra coil being, as before, to make up for the 

 shunted pair of coils, which are equal in resistance to one. 



It is possible, therefore, to divide the potential into 

 100 X 100=10,000 equal parts. The resistance of the coils on 

 AB is 1,000 ohms each, making 100,000 ohms for the whole. 



The Vernier set of coils CD consists of 100 coils of 20 ohms 

 each, making 2,000 ohms, equal to two coils on AB. 



Measurement of Battery E.M.F. by Slides — There are two 

 ways of measuring the E.M.F. of a battery by the slides as 

 follows : — 



1. By connecting an independent main battery to the slides 

 and balancing with standard cell, then putting the battery. 



100 Coils of. 

 20 Ohms each 



Fig. 298.— Battery E.M.F. by Slides. 



whose E.M.F. is to be measured, in the place of the standard 

 cell, balancing again and comparing the two readings on the 

 slides. The connections are as in Fig. 298. 



The fall of potential over the resistance of the slides, pro- 

 duced by the independent battery, is shown in Fig. 299. The 

 E.M.F. of this battery must be greater than that of the battery 

 to be tested. 



One balance is obtained with the standard cell connected 

 as shown in the figure ; then the cell ia removed, the battery 

 to be tested connected up in its place, and a second balance 

 obtained on the slides. The double pointers are moved first to 



