510 SUBMARINE CABLE LAYING AND REPAIRING. 



tance added to the line, the direction of testing current being 

 that which is in opposition to the earth current. On reversing 



Hl|lll|l|l|^^ 



Fig. 283. — Black's Reversals by Milammeter. 



the testing current, a resistance, R, is inserted in the line and 

 adjusted to balance the bridge or reproduce the deflection 

 without altering d. Then 



x=d- - . 

 2 



Elimincdion of Earth Current Effect hij Mance's Method. — 

 The earth current eff"ect may also be eliminated in a CR test 

 of a good cable by the Mance method. The test is taken on 

 the bridge to scale zero with two different strengths of current 

 to line in the proportion approximately of 2 to 1 . The method 

 and formula is the same as previously described for localising 

 breaks and faults except that as no fault is present /=0 and 

 the formula is simply : — 



L=A-(A-B)P, 



when the earth current is negative and A (the balance with 

 the lower current) is greater than B (the balance with the 

 higher currrent), or 



L=A + (B-A)P, 



when the earth current is positive and A is less than B. 



The values of the co-efl&cient P for different ratios {n) of test- 

 ing currents are given in Table III., p. 439. 



The distant end of the cable is earthed in the usual way, and 

 the test is correct when taken from one end only. The con- 

 nections are as in Fig. 284. 



The following example will show how the earth current 

 effect is eliminated. Of course, if the earth current opposes 



