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SCIENCE AND PRACTICE. 379 



in resistance, or 



in knots. 



86. Rupture of the Conductor while the insulation remains 

 good. Another kind of fault, and one of a serious nature, 

 when it occurs, which is happily seldom, is when the copper 

 conductor, from its inability to withstand the elongation to 

 which some point of the cable is exposed, snaps asunder, and 

 the electric continuity is lost. This fault is of more danger 

 while paying out than before. The only way to determine 

 the distance of the rupture, in such a case, is by comparing 

 the static capacity of the cable from the end to the fault with 

 the average capacity of the core, per knot. 



There are two very excellent methods of doing this ; the 

 one is by Mr. De Sauty, the other by Mr. Varley. 



87. De Sauty' 's Method of comparing the Capacities of Leyden 

 jars ly Bridge System. This is one of the most elegant of the 



Fig. 175. 



many applications of the null-methods, its purpose being to 

 compare the capacity of a cable with that of a jar of unit 

 surface ; in determining the distance of a rupture of the con- 

 ductor it is invaluable. 



The method in question depends upon the same principles as 

 Wheat stone's bridge, the only difference being that capacities 

 are dealt with in one half of the bridge instead of resistances. 

 At the point of junction of two resistances, r and R (Fig. 



