THE LOCALISATION OF BREAKS AND FAULTS. 



429 



The condenser charge should be repeated several times and 

 if there is much variation in the throws, owing to the testing 

 current affecting the break, the resistance R may be substituted 

 for the cable by plugging over from 3 to 2. This resistance, 

 as determined above, was equal to the cable CR and break 

 (•'*''+/), and consequently, if the testing battery current is 

 put through this in series with Rj^, there will be the same 

 potential at the junction as existed before at the junction of 

 R]^ and the cable. The condenser throw {d) is then taken 

 again under these steady conditions. 



The result worked out as above does not give the true 

 capacity of the cable because it is calculated from the charge 

 on a falling potential. The true capacity is the apparent 

 multiplied by the ratio of the initial to the mean potential. 



H'l'H'l^ 



Fig. 247. — Showing Quantity of Charge Measured (Shaded) in Rymer- 

 Jones Break Test. 



In the diagram Fig. 247, let E=the potential at the begin- 

 ning of the cable when charged, and e=the potential at the 

 break. 



The break resistance is set out on the horizontal line as it 

 actually exists in series with the cable. The final potential on 

 the cable is 



= E(l-ir), 



(1> 



where ^ + / = R and n=-. 



XV 



