HISTORY AND PROGRESS. 159 



having equal and opposite magnetic effects on the needle 

 or tongue of the relay, will produce no effect at A. ; but it 

 will deflect the tongue of B'a relay by passing through the 

 coil r t . The same arrangements being made at station B, 

 when the key K L is pressed down also, it is evident that 

 the deflection of the armature of relay B will not be dis- 

 turbed, because the magnetic effect of the home circuit is 

 neutralised, as in the case of A. 



But the current from A can now no longer pass so directly 

 to earth, in consequence of the interruption at the back 

 contact of the key iq. It has, however, two paths open to 

 it : the one through E D and other through p l and R X to earth. 

 During the manipulation of the key in Gintl's apparatus, 

 the circuit is interrupted during the instant which elapses 

 between the breaking of one contact and the making of the 

 other by the key. With the method before us this cannot 

 be the case ; the current passes from the line L L , through both 

 the coils r L and j^ of the relay, and n lt to earth. The current 

 encounters, therefore, twice as much resistance that is to 

 say, that of the line, &c., and that of RJ also, which are equal, 

 and has, in consequence, only half the intensity it formerly 

 had. The effect on the relay remains, however, the same, 

 because the current has to pass through both the coils r x and 

 p l9 which being wound in opposite spirals, work now in the 

 same sense and with double force upon the armature. At A 

 the relay is also deflected, since the balance between the 

 currents in r and p has been destroyed by the opposing 

 current from B, which passes, as in the case of the current 

 arriving at station JB, through both the coils r and j? of the 

 relay and the resistance R. When at this moment the key 

 K is let go back on to its reposing contact, the arriving 

 current is shunted from p and R to the back contact of the 

 key and short-circuit w. Only half the resistance now 

 opposes the current, whose intensity is, therefore, doubled, 

 but to balance this, as before, only half the relay is traversed 

 by the current. 



One of the greatest benefits to be derived from this method 

 of telegraphing in opposite directions is a system of repetition 



