HISTORY AND PROGRESS. 



113 



short-circuit to earth, however, he observed a momentary 

 current, the intensity of which was equal to the current first 

 observed, but in the reverse direction. And, lastly, when the 

 connection at A with battery and earth was interrupted, and 

 the end B suddenly put to earth, a current was observed at B, of 

 almost the same intensity as the two preceding currents, and 

 in the same direction as the current due to the battery. 



Both Kramer and Siemens came at once to the conclusion 

 that these phenomena were attributable to static electricity. 

 They compared the subterranean cable to a Leyden jar of 

 very great surface, the interior coating being formed by the 

 metal conductor, the dielectric by the gutta-percha or other 

 insulator, and the outer coating by the metallic covering, or 

 by the dampness occasioned by contact with the earth ; and 

 this view was subsequently confirmed by the experiments of 

 Faraday and Wheatstone. 



74. Siemens and Halske's Submarine Key. Several 

 arrangements have been hit upon for rendering the return 

 currents harmless in the practice of 

 subterranean and submarine tele- 

 graphy. They consist, for the most 

 part, of constructions by which the 

 line is put to earth, for an instant, 

 before bringing the relay in circuit, 

 and by which the signals and pauses 

 are given by reversed currents in- 

 stead of by the occasional contacts 

 of the same pole of a battery as in 

 overland lines. 



That of Siemens and Halske is 

 shown in plan in Fig. 60. 



With the ordinary key, when the 

 current of the battery is sent through 

 the submarine line L, by pressing 

 down the lever K on the front-contact 1 (Fig. 48), and the 

 key then let go till it makes contact with the rest at the 

 back, the return current or discharge passes from the line, 

 over the key, and through the relay R to earth. 



I 



Fig. 60. 



