438 SUBMARINE CABLE LAYING AND KEPAIRING. 



The milammeter may be in the cable or battery circuit. 

 In the formula only the ratio (n) of the currents is required, 

 not the actual values. 



The test is taken and worked out in the following manner : — - 



All bridge balances to true zero. Clamp left-hand key down 

 (zinc to line). Obtain balance B with the greater current (71c) to 

 line. Then put the battery switch over and take balance A 

 with the low current (c) to line. The ratio of the currents to 

 be about 2 to 1. 



Obtain six or more pairs of balances in this way 

 Resistance of cable up to break =iv. 

 Resistance of break with current (c) =/. 

 The ratio of the two currents = n. 



Generally, the earth current is negative, that is, opposite in 

 direction to the testing current. The balance A is then greater 

 than B, and we have 



x-.J = A--' (1) 



c 



o:+(f-d) = B-- (2) 



^ ^ nc 



where d is the fall in the resistance of the break due to the 

 larger current. By subtraction of (2) from (1) we obtain 



^ = (A-B-fO-^. 

 c n—l 



Let — ^ = P 



n-1 -^' 



then, by substituting this value of - in (I), 



x = A-(A-B)F- if- dP)- ... (3) 



It is not usual for the balance A to be less than B, but this 

 sometimes happens (the earth current being positive), and we 

 then have 



^+/-^^+-^ (4) 



a>+if-d) = B + ^,; (5) 



whence, by similar process to the above, we obtain 



Jc = A + (B-A)F-(/-clF), ... (6) 



