384 THE ELECTRIC TELEGRAPH. 



with a finished cable, agrees still better with the calculated 



value. The respective lengths / / 2 , l^ l n , having 



been found to have the average resistance p l5 p 2 , > 3 , . . . . p n 

 per knot respectively, the mean resistance R, per knot, of the 

 whole cable conductor should be 



L being, as before, the total length. 



Lastly, the mean inductive capacity C, per knot, from the 



single reduced average capacities c lt c 9 , <? 3 , c n , 



per knot, of the separate lengths l ly / 2 , 4, /, 



is compared with the reduced capacity, per knot, of the com- 

 plete cable as measured, to see that the inductive capacity of the 1 

 material has remained unaltered in the process of sheathing, 

 and by age, and that the conductor has not, by some acci-' 

 dental application of heat to the cable, become eccentric in 

 its envelope. 



90. Insulating Materials. Efforts have repeatedly been 

 made to insulate cables with india-rubber and other materials, 

 with which it was professed that a conductor covered with 

 the same thickness would suffer less loss of current than 

 with gutta-percha ; but the fate of the attempt has proved 

 in most cases their real value, and gutta-percha is still 

 employed alone in the insulation of large submarine lines. 



Specifically, india-rubber has a greater resistance than 

 gutta-percha, by which the loss of current on a line insulated 

 by the same section of perfect dielectric would certainly be 

 less ; secondly, it has, at the same time, a smaller inductive 

 capacity, by which the retardation of the signals would be 

 less, and the speed of speaking through the cable propor- 

 tionably greater ; and, thirdly, it does not become plastic 

 when moderately heated, and allow the conducting wire to 

 fall eccentric, as is the case with gutta-percha. 



These are the principal advantages which would be gained, 



