454 SUBMARINE CABLE LAYING AND REPAIRING. 



on results by other tests. When the fault is large the single- 

 ended break methods are more applicable. The fault must be 

 afiected equally by using the same testing current at both ends, 

 so that its condition is kept as nearly as possible the same for 

 the duration of each pair of tests. This is best effected by 

 using milammeters in the line or bridge fork at each end and 

 regulating the resistance or battery power to produce a certain 

 strength'of current agreed upon. The connections for the test 

 are the same as in Fig. 255, except that the distant end of the 

 cable is free. If milammeters are not available, the station at 

 the low end (nearer to the fault) adds a resistance in the line 

 and adjusts it in relation to his balance until the readings at 

 both ends are the same, using equal battery power and equal 

 bridge ratios at both stations. When the balances are 

 equalised 



X + R=L - X, 



whence j?= — ~^, (1) 



where E is the added resistance and L true CE, of the line. 



When milammeters are used and the observed CR's at each 

 end are A and B respectively we have 



A=x+f=L-y+f, 



B=y+f=-L-x+f, 

 whence B-A = L — 2a; 



and A-B = L-2?/. 



Therefore x = ^'^^~^ (2) 



and T/= ~ ^"^ (3) 



X and y being the distances from either end in units. 



The test should be taken to false zero with zinc to line at 

 both ends. 



Anderson and Kennelly's Earth Overlap to False Zero. — 

 The earth overlap method is much more reliable than the free 

 overlap, because variations in the fault do not so greatly affect 

 the bridge readings. The balance can never exceed the CR of 

 the line however much the fault varies, and at the same time 



