434 SUBMARINE CABLE LAYING AND REPAIRING. 



method with a differential galvanometer greatly simplifies an 

 otherwise difficult test. 



Mr. Eymer-Jones has prepared tables as an aid in the 

 calculations, and if this mode of solution is preferred, the 

 articles referred to above should be consulted, where the 

 tables and their use are fully explained. 



In these articles Mr. Rymer-Jones points out that 

 the greater the length of cable in circuit or its KR the 

 longer time it takes for the full charge to pass, and if the 

 break is more than 100 miles away the observed charge will 

 be less than it should be unless the galvanometer is perfectly 

 ballistic. A correction is therefore necessary, depending 

 upon the kind of galvanometer used and its periodic time. 

 Curves are given by which the correction can be found when 

 using an ordinary Kelvin astatic, a similar instrument with a 

 much heavier mirror and vane suspension, and a highly sensi- 

 tive Sullivan shore instrument, these being plotted to cover 

 various lengths of cable, assuming the distant end free. These 

 curves are, therefore, applicable in the case of very high resis- 

 tance breaks. 



When the conditions are favourable to a low potential and 

 charge at the end of the cable, as with a long length in circuit 

 or a break of only a few thousand ohms, the retardation is much 

 less. To meet such cases curves are given for the retardation 

 correction with breaks of 5,000, 10,000, 20,000 and 40,000 ohms 

 resistance. 



Wherever possible the test should be made from the end 

 nearest the break so as to deal with as little retardation as 

 possible. 



The following correction is given to take account of the 

 effect of any earth current upon the cable charge. 



Earth Current Correction of Cable Charge. — When an earth 

 current is present it slightly increases or reduces the observed 

 cable charge, according to its direction, and for strict accuracy 

 a correction is necessary. 



In the upper diagram (Fig. 247b) the shaded area represents 

 the correct cable charge due to the testing battery alone. The 

 small black area represents the charge due to the earth current 

 when the latter is in the same direction as the testing current. 

 The observed charge is, therefore, the sum of the two areas, 



