.S7A' WILLIAM S/KMKXS, J-'.K.S. 3! 



ruble, who assert that the maximum result is obtained by a 

 conductor of comparatively small diameter. 



The results obtained by means of these formulas are, however, 

 modified by disturbing causes, which have to be taken into 

 account by the electrical engineer. Among these, the conducting 

 power of the gutta-percha itself is the most important. It appears, 

 from certain experiments made at Birkenhead by Messrs. Newall 

 and Co. upon one-half of the Atlantic cable, that when the entire 

 cable is formed into an electric circuit only about one-third part 

 of the current will follow the wire throughout its length, and the 

 remaining two-thirds will pass through the gutta-percha covering 

 to the earth. The relative amount of leakage through the covering 

 increases in an extraordinarily rapid ratio with an increase of 

 temperature ; and it must be deemed a most fortunate circum- 

 stance that the temperature of the great oceans is probably not 

 above 40 Fahr. at the bottom, being the temperature of maximum 

 density of water. Messrs. Buff and Beetz have found that glass 

 also becomes conductive of electricity, when but moderately 

 heated ; and they attribute the effect to electrolysis, or decompo- 

 sition of the alkali it contains. In the case of gutta-percha, it 

 arises possibly from the decomposition of water of hydration or of 

 some vegetable constituent of that substance. A careful experi- 

 mental inquiry into this question, including some other deterio- 

 rating effects upon gutta-percha, would be of great practical 

 importance ; and it is to be hoped that the Gutta-percha Com- 

 mittee, lately appointed by this Society, will furnish some valuable 

 information. 



The effect of leakage through the coating is retardation, in the 

 direct proportion of the surface of the conductor, and the inverse 

 ratio of the thickness of the coating ; but the coefficient varies 

 according to the temperature, and quality of the material. There 

 are some other disturbing causes, of comparatively less importance, 

 namely, voltaic induction and magnetisation of the iron sheathing 

 by the line-wire current. The voltaic induction, or tendency of 

 one current to produce a current in the opposite direction in 

 another conductor parallel to itself, is of importance only in the 

 case of compound cables, and may even be turned to advantage 

 if the return current is laid through one of the parallel wires 

 instead of the earth. By the same expedient, magnetisation 



