SCIENCE AND PRACTICE. 



381 



and as according to (I. -g- = 

 we have also 



(III. 



The jar U may be formed by a length /^of the same cable 

 as L, in which case the capacities K^/ and E^ of the two 

 lengths are to each other as the lengths. 



which, being substituted in III., 



R 

 r 



IL= lu 



(IV. 



when the length Ig-is some unit used to express the length 



of the cable as a knot, 2026 yards, for instance, the dis- 



-p 

 tance l v of the rupture will be knots from the end. 



In arranging a board for this measurement, a condenser 

 made of alternate plates of gutta-percha and metal, having 

 the same capacity as an unit of length 

 of the cable to be tested, is found 

 more convenient than a piece of 

 cable, which is liable to accident. 



88. Varley's Method of comparing 

 Capacities of Jars. An equally good 

 method of comparing the charge of 

 one cable with that of another, or 

 with that of a condenser of known 

 capacity, has been used with good 

 results by Mr. Varley. He em- 

 ploys a differential galvanometer, 

 whose coils have the initial mag- 

 netic effects m and m, and the 

 resistances g and g . The ends of these coils, joined up in the 



Fig. 176, 



