412 SUBMARINE CABLE LAYING AND REPAIEING. 



In the formulse which follow always let balance with low 

 current (c) be called A ohms, and balance with high current 

 (nc) be called B ohms. After each balance, note the mil- 

 ammeter reading before changing the current. Take several 

 pairs this way, noting the milammeter reading for each. 

 Usually the balance, B, with the high current, is less than A 

 with the low current. When this is the case, the formula for 

 distance of break in ohms is 



a;=A-(A-B)P + ^M (3) 



Where P and M are coefficients given in Table I., e the earth 

 current potential in millivolts, and nc the high current in milli- 

 amperes to line. The last term in the formula is the correc- 

 tion for earth current, and in this case is additive. This 

 correction is always additive when A exceeds B, that is, when 

 the earth current is negative or against the direction of a zinc 

 to line current applied at the testing end. This may be 

 remembered by the double letter A — direction Against Add. 



It sometimes happens that the balance B with the high 

 current is greater than A. This may be expected when the 

 earth current is positive, that is, in the same direction as the 

 standard cell current, zinc to line. 



The formula in this case is 



a^ = A + (B-A)P-^M (4) 



no 



The correction is here subtractive, because the earth current 

 is in the same direction as a zinc to line current at the testing 

 end. This may be remembered by the double letter S — 

 direction Same Subtract. 



The milammeter may be connected in the battery circuit 

 instead of direct to cable to obtain bigger deflections, but in 

 this position with an even bridge it indicates double the current 

 going to line. This does not affect the ratio (n) of the 

 currents, but to obtain the actual value of the larger current (nc)- 

 to line the readings must be divided by two. Of course 

 there is no division by two if the milammeter is connected 

 direct to cable. The nc reading must be converted into milli- 

 amperes before being applied to the formula, and, therefore, if 

 the Johnson and Phillips milammeter is used with the G or 



