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 

 more 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 

 must 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 

 apparatus at each station. But if nothing further were done, the 

 receiving 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 bat- 

 teries ; 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 had equal batteries and the same poles to earth. 

 Then the relays would be worked entirely by the local current. 

 But local circuits are nuisances, and it is not to be wondered 

 at that this method of GintPs never came into practical use. 

 But the possibility of the "feat" having been once demon- 

 strated, it was not long before another and much superior me- 

 thod was introduced. It was discovered about the same time in 

 1854 by Frischen and Siemens-Iialske, and may be called the 



