THE LOCALISATION OF BREAKS AND FAULTS. 427 



When this balance is secured the galvanometer will not be 

 aflfected by the leakage current through the cable dielectric but 

 only by the charge throw, which will then be proportional 

 to the quantity of electricity entering the cable. It is well 

 known that in low insulation cables the charge throw may be 

 4 or 5 times higher than the discharge — that is, when it is 

 mixed up with leakage current. The throw is of two parts 

 namely the quantity required to charge the cable and the 

 leakage through the dielectric, which requires a continual flow 

 of a certain quantity of electricity per second to maintain the 

 charge at the full potential. By means of the differential 

 balance in this test the steady leakage current passing to cable 

 does not show on the galvanometer, so that on charging the 

 cable the throw is not interfered with and is truly proportional 

 to the quantity of charge entering the cable. The balance is 

 only for this purpose, not for measuring resistance, and there- 

 fore to assist in obtaining it, the coils of the galvanometer on 

 either side (which in the Kelvin astatic are on hinges) may be 

 moved towards or away from the suspended needles to assist in 

 balancing. The shunts also are adjusted as well as the variable 

 resistance, the only point being that the shunt on Gj must 

 be of such value that a conveniently large deflection is 

 obtained when the charge throw ia observed after balancing. 

 The resistance adjusted to balance in this case does not come 

 into the calculation. 



(2) Cable Charge. — These conditions of balance being obtained 

 the cable is discharged by closing the earthing key which at 

 the same instant disconnects the battery. After 2 seconds the 

 key is opened, thus putting battery again to line. The galvano- 

 meter, although balanced to steady currents, will indicate the 

 throw due to the charge entering the cable. Several throws 

 are taken in this way, carefully adjusting the variable resis- 

 tance each time if necessary to maintain the steady balance. 



Let the throw so obtained be D divisions — that is, D repre- 

 sents the actual deflection on Gi multiplied by the multiplying 

 power of the shunt used. 



The battery is then taken off" and line earthed by putting 

 the switch to E. 



(3) Lme and Break Resistance by Fieproduced Deflection. — The 

 total resistance of the circuit is now measured by direct deflec- 



