THE LOCALISATION OF BREAKS AND FAULTS. 479 



core. In the first place, there is no retardation of charge or 

 discharge, these being instantaneous, as in the condenser, and 

 in the next place, the insulation resistance more closely approxi- 

 mates to that of the condenser. 



Besides measuring the capacity of the drum lengths, the 

 capacities of the completed sections of sheathed cables are tested 

 in the factory before shipment. These sections differ according 

 to the type of cable, and vary from 200 to 250 nauts of deep- 

 sea cable down to 50 or 100 nauts of intermediate, or 1 to 

 10 nauts of shore end. 



Being coiled in tank and iron sheathed, there is considerable 

 inductance present under test, which almost invariably has the 

 effect of making the apparent capacity less than the sum of the 

 capacities of the core lengths composing them. This error does 

 not exist in laid cables, and can be completely overcome in 

 coiled cables by testing to both ends of the conductor joined 

 together, as in Fig. 277. Mr. Arthur Dearlove has obtained by 

 the ordinary comparison of throws practically the correct value 

 of the capacity of a coiled cable of 1,000 nauts length by this 

 device. {The Electrician, July 24, 1896, p. 414.) The amount 

 of inductance present in this length was evidenced by the fact 

 that the working speed through it was found by Mr. Dearlove 

 to be only one-fourth of its actual speed when laid. This 

 method of neutralising inductance should always be adopted 

 ■when taking capacities or insulation resistances of cables in 

 tank, whether afloat or ashore. 



It is very necessary to be able to correct observed capacities 

 on laid cables or to apply correct methods, so that when wanted 

 for localisations the true capacity up to a sealed or partially 

 sealed break can be ascertained with some degree of certainty. 



De Sauty's Capacity Test. — This method was described as 

 far back as 1871 in Latimer Clark and Robert Sabine's work 

 "Electrical Tables and Formulae," and has been a most useful 

 test on cables up to about 400 miles in length. The cable and 

 condenser are connected bridge fashion with the simple resis- 

 tance ratio arms a and b as in Fig. 263. These arms, which 

 may be in the form of slides, are adjusted until there is no 

 throw on the galvanometer when the cable and condenser are 

 giqaultaneously charged and discharged through the respective 



