506 SUBMARINE CABLE LAYING AND REPAIRING. 



with the testing current. If the earth current opposes the 

 testing current it appears like a resistance added to the line. 

 The bridge result, consequently, comes out too high. If the 

 testing current is reversed the earth current, being in the same 

 direction, assists the testing current, and the line resistance 

 appears less than it really is. The bridge balance is then too 

 low. Also the actual current in the line is higher when the 

 earth current assists the testing current than when it opposes 

 it, although the testing battery power is the same in either 

 case. A mllammeter should be connected between bridge and 

 cable, and the currents to line observed with every balance. 



Let c be the smaller current to line (when the testing and 

 earth currents are in opposition), 

 nc the larger current to line (when the currents are in 



in the same direction), 

 A the balance with the smaller current c to line, 

 B „ „ larger ,, nc „ 



n the ratio between the two currents to line, 

 e the difference of potential giving rise to the earth 



current, 

 X the OR of the cable. 



The balance A will be obtained either by zinc or carbon 

 testing current to line according to the direction of the earth 

 current. The thing to remember is that A is the balance with 

 the lesser current to line, when the earth current opposes the 

 testing current, and B with the greater current to line when 

 the earth current assists the testing current. The A balance 

 will be higher than B. 



The apparent resistance due to earth current is equal to — 



- when the earth current opposes the testing current, 



(1) 



7t+ 1 



