DUPLEX TELEGRAPHY.

Globe, Volume I, Issue 41, 17 July 1874, Page 2

DUPLEX TELEGRAPHY.

At a recent meeting of the Royal Society of New South Wales. Mr E. C. Cracknell read a paper on the “ Duplex telegraphic instrument.” We learn that duplex telegraphy is now in operation through the Cook’s Straits cable : “ Duplex telegraphy, as it is now called—that is, working in two opposite directions on one wire simultaneously was first tried by Dr Qinth, 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 transmitting station could work the relay at the distant station without affecting the instrument at its own end of the line. This was managed by the battery for the line wire of the ordinary type, and an equating battery for the second relay wire, but the compensating currents could not well be controlled, and the system was not found to be a practical success, although experimentally it worked beautifully. In 1855, Frischen Siemens, and Halske devised a modification of the duplex working by adopting a somewhat complicated system of resistance coils, but discarded the counter acting batteries. These were the first Morse instruments used on the telegraph lines in this colony between Sydney and the South Head in January, 1868, but were found unworkable, and were reduced to single acting Morse recorders. Siemens and Halskc’s arrangement was worked on what is called the differential 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 more 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 tha distant station, and would prove that that instrument was not in proper adjustment. To overcome this difficulty the resistance must be gradually increased until the two currents are equalised, so that the closing of the key has no effect on the armature of the relay, and no signals are made except at the distant receiving station. It is therefore evident that when station A is sending to station B, A’s relay is unaffected, while the relay at B will be made to work and the instrument record A’s signals, and vice versa. Stern’s duplex system, lately perfected in America, and now daily attracting the attention of telegraph engineers in most parts of the world, is on a principle which is dependent on producing an equality of tentions which are sometimes called potentials; but to be more explicit to those who have not studied the science of electricity, it is, according to Jenkin, the difference of electrical condition in virtue of which work is done by moving from a point at higher tention to that of a lower, or perhaps Mr Piece has made it clearer by calling it an analogous term of pressure, as applied to fluids and gases. With this system the relay coils are also wound with double wire. It has proved a great success, partly by. improvements on old principles, but more particularly in consequence of the introduction of more perfect appliances since Gintel’s and Sicmen’s and Halske’s inventions. The arrangement before you this evening is perhaps the most simple duplex working apparatusyct devised. The two instruments are the ordinary Morse recorders, with relays, in everyday use on the lines in this colony. Tha relays are wound with one continuous wire, and the only additions are the two vertical water columns for producing the necessary artificial resistance for dividing the currents, the actual line resistance being equal to 150 miles of wire. With this plan only one battery is employed for the line circuit, which is continuous, that is, in one direction, and the adjustment requires very little attention. It must not be understood that two distinct currents of electricity pass in opposite directions through one line of wire at the same time, but the problem has been solved by increasing the amount of current on the main circuit when signals are sent simultaneously. Although duplex telegraphy may now be considered beyond a doubt as to its practibility, we are not likely to remain satisfied with its success for any lengthened period. Mr Mcyar’s ingenious invention of the multiplex telegraph, exhibited in the late Vienna Exhibition, bids fair to eclipse all the telegraph instruments now in use for rapidity of signalling through one wire. This instrument has already been tried between Paris and Lyons, with four transmitters on one line, and 100 to 120 messages of average length are sent through per hour.”