GIG TJfE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



of currents in an insulated and submerged line, so as to take 

 account of obstacles, and to overcome them by appropriate means. 

 Several physicists, among whom we may name Faraday, Wheatstone, 

 Thomson, and Siemens, applied themselves to the task, and all 

 contributed to the solution of this important problem. 



It was recognized that a cable submerged in sea-water, is 

 transformed, when an electric current traverses it, into a con- 

 denser analogous to the Leyden jar. The electrical charge of 

 the line wire within acts upon the outside conductors, the metallic 

 guard, and the sea-water, across the insulating coveting composed 

 as we have seen of gutta-percha. The induced currents which 

 arise in this way under the influence of the current thrown into 

 the line by the sending apparatus continue for a certain time 

 after the current is stopped, so that a despatch of a new current 

 is impossible till after this interval ; otherwise the currents would 

 act as if the line were traversed by a continuous flow of electricity, 

 and signalling would become impossible. It was also proved that the 

 conductibility of the gutta-percha is not zero, and the current 

 is weakened by the loss which takes place across the insulating 

 covering. 



\Vhen once these causes were recognized, it became possible to 

 counteract their effects. For galvanic electromotors, for the battery, 

 magneto-electric induction apparatus were first substituted, as they 

 produce currents of greater intensity, propagating themselves with 

 greater rapidity than ordinary currents. Methods beside this have 

 been devised for neutralizing the induced currents ; one due to 

 Whitehouse, consists in throwing alternately into the cable two 

 currents in opposite directions, and the induced currents resulting from 

 them are then themselves opposite, and destroy or neutralize each 

 other. Varley interposes, between the manipulator and the 

 line, a condenser of very large surface (40,000 square feet), 

 which, according to M. du Montcel, works in the following way in 

 neutralizing the induced currents : " At the moment of contact in the 

 manipulator (which is a simple key reverser), an electric current is 

 sent across, the cable to act on the indicator, and this current is 

 positive or negative according to which of the two keys of the 

 manipulator has been depressed. But 'as soon as this key rises, a 

 communication is established between the condenser and the earth, and 



