the line at a time, although the sending may be from any station to any other station as desired. The economic advantages of being able to send two messages simultaneously over one line have led to much invention and development, particularly of duplex systems in which the two sendings are in opposite direc tions, one from each end of the circuit. The first duplex arrange ment was devised by Dr. Gintl of Vienna in 1853. Outstanding developments were made by Frischen of Hanover, who introduced the artificial line in 1854, and by Stearns of Boston, Mass., who improved Frischen's artificial line in 1868 and invented the bridge duplex. Frischen invented the double current duplex in 1863, and this later improvement was taken up and fur ther developed in England by Varley, Heaviside and others, be tween 1863 and 1872. An essential feature of any duplex system is the provision at each station of a relay or other receiving instru ment, so arranged as to be unaffected by any signals sent out from In the preceding paragraph it was assumed that the transmit ting apparatus at the distant station was applying no current to the line. The effect of such a current application, which occurs whenever a signal is being sent from the distant station, will always be to increase or decrease the strength of the current in the line coil of the relay at the station first considered, without, however, making any great change in the current in the artificial line coil of that instrument. The relay is then operated because of the greater current in one coil than in the other, and thus responds to the signals being received from the distant station. With apparatus arranged in this way at each end of a ,line, both stations may send to each other at the same time without inter ference. Because each relay is operated by the difference between the currents passing through its two coils, this method of working is known as the differential duplex.
Bridge Duplex.—The second method of duplex working in com mon use employs the principle of the Wheatstone bridge. (See ELECTRTCITY.) In this plan, fig. 13, the duplicate pulses of cur rent for outgoing signals pass to the line and the artificial line, respectively, through equal bridge arms, and have no effect upon the relay or other receiving instrument forming the cross-wire at the extremities of those arms. The relay is, however, responsive to the unbalancing effect of any application of current at the dis that station, but fully responsive to all signals received from the distant station.
Artificial Line.—Practically all modern duplex schemes employ the principle of having the signalling pulses of current issue from the transmitting apparatus in duplicate, one pulse going over the line to the distant station and a second identical pulse traversing a branch circuit, known as an artificial line, which is associated with the duplex equipment. To maintain substantial identity be tween the characteristics of the two sets of pulses, it is important that the artificial line shall closely resemble the real line in the values of, and connections between, its electrical resistance and capacity. With all outgoing signals represented by duplicate pulses of current, the relay or other receiving instrument may be made irresponsive to such signals in either of two ways: Differential Duplex.—In the first method, fig. 12, the electro magnet of the relay is wound with two separate but equal coils • tant station, and thus responds to all signals received from that station.
Quadruplex.—It is possible to operate both differential and bridge duplexes by either single current or double current methods, but in modern practice the double current scheme is generally preferred. By combining a modified single current system with a
double current duplex, Edison produced in 1874 the quadruplex. With this scheme four messages, two in each direction, may be simultaneously transmitted over a one-line wire. The two relays at each terminal of the line are arranged to be unresponsive to outgoing signals by either the differential or bridge methods. In receiving signals one of the relays responds only to reversals of current effected by the distant double current transmitter. The other relay responds only to increases and decreases in current strength, regardless of the direction in which the current is flow ing; these variations in the applied power are controlled by a single current key or equivalent means, at the distant station. Fig. 14 is a diagram of a modern quadruplex circuit.
Between the years 1840 and 1855 a series of instruments was worked out by Wheatstone in England, Froment and Breguet in France, Siemens in Germany and House in America, with the ob ject of producing a letter-printing instrument, and although these early instruments never came into very extensive use the prin ciples involved are embodied in the modern systems.
The chief advantages of automatic operation of telegraphs are : increased accuracy and greater output with attendant labour saving and cost reduction. The modern automatic systems fall generally into three classifications: (I) signals are recorded in code form and must be transcribed; (2) a facsimile of the original is reproduced; (3) the message is received in printed form.
Recording Systems.—In general, the recording systems employ an automatic transmitter, controlled by a perforated tape, which sends to the line impulses corresponding to a dot-dash code.
The syphon rests against a moving tape, drawing a continuous line which is undulated above or below the centre by deflections of the galvanometer when impulses of dot or dash polarity are received. (See fig. 15.) Various chemical recorders have been proposed by Dyer, Bain, Edison and others and are used to a limited extent. They con sist of a metallic wire resting upon a moving, chemically treated tape. The wire and tape form a part of the circuit and when current passes through them an electrolytic decomposi tion of the wire occurs which causes discolouration of the tape, fig. 16. Signals are usually re corded as long and short lines. The modern transmitters used with these systems are motor driven mechanical devices arranged so that the perforations in the tape control the motion of levers which actuate contacts, send ing the proper impulses to the line. The older types simply per mitted a brush to make contact through the perforations with a metallic bed or roller. The perforated tape used for transmission in the above systems is prepared in various types of perforating machines, the more modern of which are provided with keyboards like a typewriter, and are so arranged that the depression of a key will perforate the proper combination of dots and dashes for that character. (See Plate I. fig. 5.) The first suggestion for a per forator was made by Bain in 1846.