THE LOCALISATION OP BREAKS AND FAULTS. 477 



The potential P is proportional to the total number of 

 divisions on the slides and therefore no observation of this is 

 necessary. The volts per division, multiplied by the total 

 number of divisions, gives this potential in volts as follows : — 



Potential P=c?^^ volts. 



Capacity Tests. — To measure the electrostatic capacity of a 

 cable It is compared with or balanced against a standard con- 

 denser of known capacity. 



The simple method of comparison by discharge throws is 

 applicable on laid cables up to about 200 miles in length, but 

 no reliance can be placed upon comparisons by quick charge or 

 discharge on laid cables of 1,000 miles or over. 



When comparing capacities by discharge throws it is neces- 

 sary to shunt the galvanometer for the larger capacity, and 

 sometimes to have different shunts for the cable and condenser 

 throws. A universal shunt should always be used for this pur- 

 pose, because it is Independent of the galvanometer resistance 

 as explained on p. 386. If a universal shunt box is not 

 available the ordinary slides will do equally well, connecting 

 the galvanometer between the terminals and 101 and the 

 wires to and from the shunted galvanometer to and index. 

 If slides are nob available and an ordinary resistance box has to 

 be used, the correction for inductance in the galvanometer must 

 be applied, which is determined as follows : A convenient throw 

 (d) with a given condenser is obtained on the galvanometer 

 unshunted. Substitute for the first condenser one of twice the 

 capacity, or put one of equal capacity in parallel with the first 

 so that double the capacity is in the inductive circuit. Obtain 

 throw ((/j) with shunt applied equal to the resistance of the gal- 

 vanometer. Then we have 



S ^ 



andasS = G, ^^I^ . d^ = 2d, 



where K is the apparent increase of galvanometer resistance 

 due to Inductance. From this we get 



K = 2G 



m 



