476 



SUBMARINE CABLE LAYING AND REPAIRING. 



tions in every case coming out very close to the actual posi- 

 tions. Tlie test has also been tried on low resistance faults, 

 and localisations of such faults on quite long cables have come 

 out within a fraction of a mile. 



In place of the condenser throw method for measuring the two 

 potentials on shore the author has suggested a null method 

 with the slides, by which only two balances to zero are neces- 

 sary. The connections are as in Figs. 261 and 262, in which 

 it will be seen that the disposition is equivalent to a bridge 



Pr—-~, 



R 



Cable 1 



T 



J Standa rd Cell Fault 



jQDg TEarth 



Battery Plug 

 or Key 



Fig. 262. — Connections for Measurement of Potentials at Shore 

 Station by Slides in Clark's Test. 



arrangement. The slides are adjusted until the potential is 

 equal to p and the galvanometer balanced. The switch or com- 

 mutator is then moved over to the standard cell and the slides 

 readjusted until the galvanometer is again balanced. The 

 slide readings can then be obtained directly in volts. 



For example, eay the slide reading is S divisions with a stan- 

 dard cell of e volts, and Sj divisions with the potential ^j. This 

 potential is then found by 



Standard cell S divisions —e volts, 



Potential^ Sj divisions =-q^ volts, 



o 



