428 Mr. 0. Heaviside on Duplex Telegraphy. 



differential method. It is represented in its simplest form in 

 Plate VII. fig*. 1. The relay at each station is wound with two 

 coils of equal length ; and the connexions with the battery and 

 the line are made in the same manner as if each station were 

 taking a test of the resistance of the line with the differential 

 galvanometer. The resistance ?*, then, at station A equals the 

 whole resistance outside station A ; and r 1 at station B equals the 

 whole resistance outside station B. Then, when the battery E 

 is in circuit, as its current divides equally between the two coils 

 of the instrument^, the latter is unaffected; but that half of 

 the current which passes to the line necessarily influences the 

 instrument tf tf at the other station, since the whole of it passes 

 through one coil, and then divides between the other coil and 

 the battery E ; . Thus each station does not work its own relay, 

 but only that of the opposite station, and the conditions of du- 

 plex working are satisfied. 



It is upon this system that nearly the whole of the existing 

 methods of duplex telegraphy are founded. As the object is to 

 prevent out-going currents from working the sending-station's 

 instrument, it is plain that there may be many modifications 

 having for object the easier production of balances under differ- 

 ent circumstances — as by varying the distance of one or both 

 coils from the armature instead of altering the resistance r (fig. 1) . 

 There are also a few small points to be attended to before this 

 system can be considered perfect. First, it is necessary for the 

 external resistance to be as constant as possible, in order that the 

 currents sent by a station (say A) may never affect its own in- 

 strument. But this external resistance includes B's apparatus ; 

 and B's battery is sometimes in and sometimes out of circuit. 

 A variation in the external resistance will therefore be caused 

 unless the transmitting apparatus is so arranged that a resistance 

 equal to that of the battery is substituted for it when the latter 

 is not in circuit. Again, there should be no interval of time 

 during which neither the battery nor this equivalent resistance 

 is in circuit. These things can generally be arranged with 

 little difficulty. Thus, taking the case of the simplest trans- 

 mitting instrument (the common Morse key), consisting of 

 merely a lever with a front and back contact, the equivalent 

 resistance may be connected with the back, and the battery-pole 

 with the front contact ; and the interval of disconnexion may be 

 avoided by the use of suitable springs, or other means, by which 

 the front contact is made just before (or practically at the same 

 time as) the back contact is broken, and viee versa. There will 

 then be only a very much smaller interval of time daring which 

 the received currents can pass both through the battery and its 

 equivalent resistance. The. application of this to more compli- 



