462 SUBBIAKINE CABLE LAYING AND REPAIEING. 



but low-3hunted dead-beat reflecting galvanometers may be 

 used, such as Sullivan's, their respective deflections per unit, 

 current being determined as explained on p. 534. A short- 

 circuit key is not necessary with these galvanometers when 

 shunted with 1 ohm or thereabouts, as the charge and dis- 

 charge is sufiiclently bye-passed by the shunt. 



The diagram shows the battery current put to line by means 

 of a switch or key and the commutator plug in 1. The 

 milamoaeters are deflected according to the currents passing, 

 the current at station B being less than that at A by reason of 

 leakage to earth through the fault. 



These deflections are observed at both ends at the same 

 instant of time by arrangement. Then station A should earth 

 the line at the battery switch or key and see if there is any 

 earth current present strong enough to affect his milammeter. 



C^bon Zinc STATION B 



|-|l|l|l|lll|l|l 1 STATION A 



1 



L-x 



-©^ 



Battery 

 Switch 

 or Key 



1 



Fig. 257. — Kempe's Loss of Current Test. 



If there is, the cable deflection must be corrected accordingly, 

 that is, by subtracting the earth current deflection if it is in 

 the same direction as the cable deflection, and vice versa. All 

 deflections are taken from scale zero. Station A then plugs 

 over to 2 and switches battery on through a known resistance, 

 adjusting the latter until his cable deflection (corrected for earth 

 current as explained) is exactly reproduced on the milammeter. 

 We then have 



c / 1 



f+L- 



(1) 



and U = x+Ii^^±-^ (2) 



f + L-x 



Substituting - for its value in (1), 



Ii^x + -(L-x); (3) 



whence x = --, (4) 



