436 SUBMARINE CABLE LAYING AND REPAIPaNG. 



The other letters have the same meanings as before. 



To determine the earth curent charge it is convenient to find 

 the area of the triangle upon the line r^+.v and of the triangle 

 upon Ti} the difference then gives the area upon x alone, equiva- 

 lent to the earth current charge. 



Let the ratio 



f-, 



Then the above diflference, in relation to the quantity Q, can 

 easily be shown to be equal to 



t"-+(^y^ 



If r^ is low compared to R, the fraction may be taken 



as unity, and we have : — 



Earth current correction =-p, ( 2mn+n^ ). (11) 



The ratio — is found by observation of the condenser throws by 



earth current and testing battery respectively. These throws 

 are taken by the condenser key with battery switch to C. 



For earth current E.M.F. throw, plug in 3 only. For battery 

 E.M.F, throw, plug in 4 only. 



The ratio of the throws, multiplied by the multiplying power 

 of the respective shunts (if any), gives the ratio between e and 

 E. The earth current correction is really a ratio, and should be 



deducted from the ratio p- in formula (9) if the earth current is 



in the same direction as the testing current, and added thereto 

 if in the opposite direction. Then the corrected distance to 

 the break is given by using the new value of n so obtained in 

 formula (10). 



This correction is not of importance where the break is very 

 high in resistance compared with the cable conductor, but if 

 only 5,000 or 10,000 ohms a correction is necessary and espe- 

 cially for breaks of lower resistance. The longer the portion of 

 cable in circuit the more important the correction. 



