428 SUBMAKINE CABLE LAYING AND EEPAIEING. 



tion. Plug 3 In only. Battery switch to B putting zinc to line. 

 Only the galvanometer Gj is now in circuit, and a very low shunt 

 is used to obtain a readable deflection on the scale. The 

 galvanometer short-circuit key to be changed over to Gi as 

 shown in dotted lines in diagram. Current passes through Gi 

 and plug 3 to cable. The shunt is adjusted to produce a con- 

 venient deflection, which is noted. The correction for earth cur- 

 rent in then taken with the same shunt in the following way : 

 The earthing key is closed for 2 seconds, thus discharging the 

 cable outside the galvanometer, and while it is closed the 

 battery switch is put to the earth position E. Then the earth- 

 ing key is opened and the earth-current deflection noted. The 

 cable deflection must then be corrected in the usual way by 

 deducting the earth-current deflection if in the same direction, 

 or adding it if in the opposite direction. 



The constant through the variable resistance is then observed 

 by plugging over from 3 to 2 and putting battery switch 

 to B. The variable resistance is adjusted until the cable 

 deflection (corrected as above for earth current) is reproduced. 

 The resistance so adjusted is then equal to the total 

 resistance in circuit, namely, battery, shunted galvanometer, 

 cable and break. To obtain the latter two quantities alone, the 

 battery and shunted galvanometer resistances if appreciable, 

 must be deducted. Let R stand for the resistance of cable and 

 break alone (a:-\-f) after the above deductions are made. 



(4) Condenser Charge. — The cable charge must now be com- 

 pared with that of a standard condenser of known capacity under 

 the same conditions. Plugs in 3 and 4 only. Battery switch to 

 C. The resistance Ri in circuit with the battery is made equal 

 to the joint resistance of Gj^ and the shunt used when the cable 

 charge was taken, and current now flows through this resistance 

 in series with the cable. The throw is taken by the condenser 

 key, any convenient shunt being used ; and since the condenser 

 is charged to the same potential as the cable we have 



Apparent capacity of cable = -rK, 



where D and d are the cable and condenser charge throws 

 multiplied by the multiplying power of the respective shunts 

 used with them, and K the capacity of the condenser. 



