456 SUBMARINE CABLE LAYING AND REPAIEING. 



both testing stations flows through the fault in the same direc- 

 tion (although it flows through the cable in opposite direc- 

 tions). To eliminate the earth current efiect the observations 

 are taken to false zero. The time for earthing and testing 

 being agreed upon,?the first pair of tests is taken and compared. 

 The end nearer the fault will test lower than the other. Now 

 add a resistance (R) to the lower end equal to the difierence 

 between the two results. On trying a second pair it will 

 generally be found that the end previously lower will now be the 

 higher, showing that the fault has a fair exposure as its resist- 

 ance increases with decrease of current. The observer will 

 know that this promises well for the test. Now reduce E. by 

 the difference between the last pair and test again. This time 

 the balances probably come out alike but if not, further 

 re-adjustments of E must be mad e in the same way until equal 

 pairs are obtained. Then 



R + ic= L — it? ; 



T — "R 



whence :7j=±L^ohins (4) 



Where x is the distance of fault from the near end 

 L ,, true CR of line, 



E ,, is the correct added resistance to equalise 

 the balances. 



In the manipulation of this test it is best that the end 

 which tests last in one pair should test first in the next. 



The Author has found the following formula useful in giving 

 a very near approximation to the correct resistance to add 

 after a pair of trial balances have been taken. Balances are 

 taken at both ends when the added resistance is too great and 

 again when io is too little. Then the correct resistance to add 

 will be 



E = R, + '^#-^lJ:iyohm8. 



Where (k is the difference between the observed CE's at both 

 ends when the added resistance is too high (Ri) 



and do is the difierence between the observed CR's at both 

 ends when the added resistance is too low (Ro). 

 After the first pair has been taken the added resistance is 



made rather more (say 10% more) than the diff"erence between 



