134 THE ELECTRIC TELEGRAPH. 



pole N' south ; but as both poles were previously north by 

 the influence of the permanent magnet N s, the effect of the 

 current is to strengthen the north magnetism of N, and at the 

 same time to weaken only that of N'. The tongue c is, there- 

 fore, attracted to the pole N with double force, and remains 

 on that side after the cessation of the current, attracted by 

 the pole N, whose distance from c is then less than that of N'. 

 The platinum contact of c remains against D, and closes the 

 local circuit until the key at the transmitting station is let 

 go, and the cessation of the current in the primary wire of 

 the induction apparatus induces a negative current in the 

 secondary coil, line, and relay, which has the reverse eifect 

 of the last current, strengthening the north magnetism of N', 

 and correspondingly weaking that of N. The pole N' there- 

 upon attracts c against the insulated point D', where it 

 rests until another positive current passes and throws it off 

 again. 



The Morse recording instrument is of the usual construc- 

 tion of Digney, or Siemens and Halske, described above. 



83. A plan of this admirable system is shown in Fig. 74, 

 arranged for two stations, c and c are the induction coils, 

 of which e e are the soft iron cores ; the limits of the primary 

 and secondary coils are shown by concentric rings. K is a 

 transmitting key, which closes two working contacts in front ; 

 R the polarised relay ; B the local battery ; and i the receiving 

 instrument. 



At each of the stations the middle contact of the key is 

 connected to line, and also to one end of the primary wire of 

 the induction apparatus. The battery is included in two 

 circuits : first, between the remaining end of the primary coil 

 and the second contact b of the key ; and, secondly, in the 

 ordinary local circuit of the relay and recording instrument. 

 One end of the secondary wire of the induction-coil is to 

 earth, the other connected with the first contact a of the 

 key, the back or reposing contact c leading through relay to 

 earth. 



The key differs slightly from that used in the ordinary 

 Morse circuits, having, as we have seen, two working con- 



