492 SUBBIAKINE CABLE LAYING AND REPAIRING. 



Leakage Correction for Capacity Tests. — After correcting 

 for diflference in absorption in the dielectrics of the cable and 

 condenser, there is still an important correction due to 

 difference in insulation per unit of capacity to be taken account 

 of before the true capacity of the cable can be said to be de- 

 termined with any degree of accuracy. Where the insulation 

 resistance per microfarad capacity is the same in the cable and 

 the condenser with which it is compared there is what might 

 ba termed leakage equilibrium and no correction is necessary. 

 But this condition is never or hardly ever met with. Conden- 

 sers are usually high in insulation per unit capacity and cables 

 often low, which causes the apparent capacity to be too high, 

 but the opposite condition is sometimes met with, causing the 

 apparent capacity to be too low. 



Let F be the capacity of the cable, 

 / „ „ „ condenser, 



r „ insulation resistance of the cable, 

 R „ „ „ „ condenser, 



Then if the insulation resistances are in inverse proportion to 

 the capacities — that is, if 



F_R 



7 r' 

 or, what is the same thing, if 



there is leakage equilibrium between cable and condenser, and 

 no correction for differences in insulation is necessary. 



This is such a desirable state of things and saves so much 

 time if the correction can be dispensed with, that it is worth 

 while to artificially create leakage equilibrium if practicable. 

 Thus, if the "Pr of the cable is lower than the fR of the con- 

 denser equalisation may be effected by shunting the condenser. 



For example, a cable of approximately 280 microfarads and 

 0"63 megohm insulation was compared with a condenser of 99 

 microfarads and 10 megohms insulation. Here the insulation 

 per unit capacity in each case was 



(Cable Fr) 280 x 0-63 = 176 megohms per microfarad. 

 (Condenser/R) 99x10 = 990 „ „ „ 



To make the insulation per microfarad of the condenser equal 



