THE LOCALISATION OF BREAKS AND FAULTS. 495 



This shows considerable disparity between the product of 

 insulatioa resistance and capacity in each case, that of the 

 cable being much lower than the condenser, and hence the- 

 apparent capacity might be expected to come out too high. 



It was not possible to equalise these by shunting the con- 

 denser, as the resistance of 11 megohms necessary for the shunt 

 was not available. 



Cable being free at the distant enc?, the mean reading on the 

 slides for both currents after a charge of 15 seconds was 1,030. 

 Hence the 



Apparent capacity of cable = 6o( y^gQ "■••) = '^^'-^ microfarads. 



The leakage correction for cable (K) is found from 



1 ^ 



log K = — =0-01446=log of 1 035. 



^ 2-3026X46U ^ 



522 



True capacity of cablt= =504 microfarads. 



^ ^ 1-035 



In Mr, J. Elton Young's valuable work " Electrical Testing 

 for Telegraph Engineers" (Appendix X.), a correction for 

 leakage in the condenser is given, on the suggestion of the late 

 Mr. W. J. Murphy. 



The complete correction for insulation resistance in cable and 

 condenser is then 



k 

 True capacity = Apparent capacity X-^ 



where, in the case of the condenser, 

 log k-- 



2-3026/fl 



The Silvertown testing key designed by Mr, Rymer-Jones, 

 illustrated in Fig. 275, is largely used in capacity and insulation 

 tests. There are three ebonite pillars, two of which support 

 horizontal contact levers movable about the point of support 

 and provided with ebonite handles. The levers can be moved 

 independently, but one of the handles carries a distance piece 

 which prevents them both touching the back contact bar at 

 the same time. The back bar is fixed on the third pillar, and 

 is curved or channel shaped, so as to present two contact sur- 

 faces — one for each lever. The contact surfaces have springs- 



