DUPLEX TELEaEAPHY. 



By E. C. CEACKisrELL, Esq., Superintendent of Electric Telegraphs. 



[Read before the Hoyal Society, Wth May, 1874.] 



Duplex telegraphy, as it is now called — that is, working in two 

 opposite directions on one wire simultaneously — was first tried 

 by Dr. Gintl, the Director- General of Telegraphs in Austria, on 

 a line from Vienna to Prague, as far back as 1853. The relays of 

 the instruments were wound with two wires, so that the trans- 

 mitting station could work the relay at the distant station without 

 affecting the instrument at its own end of the line. This was 

 managed by a battery for the line wire of the ordinary type, and 

 an equating battery for the second relay wire ; but the compen- 

 sating currents could not well be controlled, and the system was 

 not found to be a practical success, although experimentally it 

 worked beautifully. In 1854, Erischen, Siemens, and Halske 

 devised a modification of the duplex working by adopting a some- 

 what complicated system of resistance coils, but discarded the 

 counteracting batteries : these were the first Morse instruments 

 used on the telegraph lines in this Colony between Sydney and 

 South Head, in January, 1858, but were found unworkable, and 

 were reduced to single-acting Morse recorders. Siemens and 

 Halske's arrangement was worked on what is called the differ- 

 ential principle, that is, if two circuits of equal resistance be 

 open to a current, it will equally divide ; but by placing unequal 

 resistance in the two circuits the greater portion of the current 

 will pass through that having the best conductor ; there is then 

 one main circuit through the line, the other a derived circuit 

 through the resistance coils. If the key at the sending station 

 A be pressed, the current divides — one portion proceeds through 

 the line and moves the armature of the relay at the distant station 

 B, the other portion proceeds through the compensating wire 

 and resistance coils to earth ; now these currents being equal at 

 the sending station, they have no tendency to move the tongue 

 of the relay at A, which is ready to receive the current from B. ■ 

 Suppose the resistance at A is less than that of the line, then 

 the current passing through the compensating circuit will be 

 greater than that passing by the line, so that, by pressing the 

 transmitting key, signals will be made at the sending station 

 which would prevent the receipt of perfect signals from the 

 distant station, and would prove that the instrument was not in 



A 



