478 suBMARI^^; cable laying and repairing. 



If ordinary shunts are used, therefore, the multiplying power 

 to be used in the case of this particular galvanometer is 



G + K + S 

 S ' 

 where K has the above value. 



The most reliable methods in use to-day are null methods in 

 which equal quantities of charge are balanced, but even in these 

 tests a correction factor has to be applied to obtain the true 

 capacity. The sources of error are chiefly dissimilarity in 

 dielectrics and insulation resistance in the cable and standard 

 condenser. The condenser dielectric is mica or oiled paper, 

 while that of the cable is gutta-percha, and the rate of elec- 

 trification or absorption is different in these materials, which 

 leads to variation in the determination of capacity depending 

 upon the duration of charge. 



The time of charge is proportional to the insulation resis- 

 tance and the capacity, and, £s condenter insulation is always 

 high and that of cables often low, there is more often than not 

 a lower insulation resistance per unit of capacity in the cable 

 than in the condenser leading to the rate of charge being 

 quicker in the cable, and resulting in the apparent capacity 

 coming out too high. The charging current must, neverthe- 

 less, be kept on a suflBcient time to obtain the full capacity of 

 the cable, and this tends still further to increase the apparent 

 capacity. There is also the disparity in capacity between 

 cables and condensers, the latter necessarily being in small 

 units, owing to their weight and cost. 



The true capacity of a cable is derived from the original tests 

 on 2 mile drum lengths of core in the factory immersed in the 

 usual way at a temperature of 75°F. The sum of the capa- 

 cities of the several lengths of core composing the finished 

 cable is taken to represent its true capacity. The comparison 

 of the capacity of a piece of cable core with that of a condenser 

 of known capacity is the first step, and, accepting the capacity 

 of the condenser as the true standard, it is evident that the 

 capacity of the cable can most truly be measured by comparison 

 with that of the condenser when the piece of cable under measure- 

 ment difFtrs least in its electrical properties from those of the 

 condenser, and this condition we have iji the short length of 



