410 SUBMARINE CABLE LAYING AND REPAIRING. 



iiflferenca in the connections is that the shunt box or galvano- 

 meter is connected to cable instead of to earth and the standard 

 cell is not in use. The shunt and high resistance must be the 

 same as in the previous test. 



A given direction of current in a cable may be considered 

 negative or positive according to the way it is looked at, but in 

 a test of this kind, in order to fix ideas, it is convenient to 

 adopt some distinction as to direction and sign. We start by 

 working always with zinc to line at the testing end, whether it 

 is from the standard cell or the testing battery, and this fixes 

 the direction as a basis of comparison for earth currents. A 

 zinc current to line is a negative current to line. Now the 

 earth current is a combination of the natural current from a 

 fault or break and the earth current proper due to difi'erences 

 in the earth potential at distant points. In any case its point 

 of origin is at the break or fault as represented in Fig. 242, or at 



Home End g^^^j^ 



^'• 



Standard Cell Negative E.M.E 



Zinc to Lme at Break 



Fig. 242. — Negative Earth Current. 



the distant end of the line. Consequently it is like a current 

 or potential applied at a distant point of the line. If a zinc or 

 negative current were applied to line at the break, this would 

 be opposite in direction to the negative current applied at 

 the testing end. Hence, if we find the earth current deflec- 

 tion is in the opposite direction to that of the testing current 

 or standard cell, it is considered as negative in sign, and if in 

 the same direction, positive in sign. 



Let the earth current deflection = 8^ divisions. Then by (1) 

 the equivalent voltage (e) of the earth current is : — 



' = T (2) 



For example if the standard cell is 1*46 volts, deflection with 

 constant 224 divisions to left and with earth current 426 divi- 

 sions to right, then the earth current is negative and equal to 



1,460x426 ^„„^ .„, , 



-^—TZTTi = 2,778 millivolts. 



224 



