ABSTRACT

I propose in this paper to give a short account of the theory of duplex telegraphy by the principal methods, and to describe two other methods, which are, I believe, entirely original.

To begin at the beginning. Prior to 1853, it is said to have been the current belief of those best qualified to judge, that to send two messages in opposite directions at the same time on a

;Mr. 0. Heaviside on Duplex Telegraphy. 427 single wire was an impossibility; for it was argued that the two messages, meeting, would get mixed up and neutralize each other JUOre or less, leaving only a few stray dots and dashes as survi~ vors (after the manner of the Kilkenny cats, who devoured one another and only left their tails behind). However, Dr. Gintl effectually silenced this powerful argument by going and doing it, . In order to be able to receive messages from another station, it is necessary for the receiving instrument to be in circuit with the line; and in order to send to another station, the battery 'iUust be in circuit. Hence, in order to receive and send at the same time, both the sending and receiving apparatus must be in circuit together. This can be arranged by making one continuous circuit between the two earths, and including the line and all the

~pparatus at each station. But if nothing further were done, the 1·eceiving instruments would be worked both by the received and sent currents; and if both stations worked at once, inextricable confusion would be the only result. Now, evidently, if the effect of the sent currents on the sending-station's instrument can be neutralized, the" feat" is accomplished. There are many ways of doing this. Dr. Gintl surmounted the difficulty in what was, to say the least, a very ingenious manner, although, from a modern point of view, it was decidedly clumsy. He made his key, while being depressed to send a current to the line through his own relay, at the same time close a local circuit, including a coil of wire outside the principal coils of the relay, in such a manner that the current in this local circuit (which contained an independent battery) circulated round the cores of the electromagnets in the opposite direction to the current going out to the line; and by placing a rheostat in this local circuit he was able to vary the strength of the local current, so that the effect of the out~going current on the relay was exactly neutralized. The relay then responded only to currents coming from the opposite station, which, of course, passed through the inner coils alone. Did both stations depress their keys simultaneously, the current in the batteries, inner coils, and the line was that due to both batteries ; but in each relay as much of this current as was due to the corresponding battery was neutralized by the local current. The line-current might even be nothing, which would happen if each station bad equal batteries and the same poles to earth. Then the relays would be worked entirely by the local current.