in such a manner that the relay tongue is caused to vibrate between the contact points, when the current through the line coils is insufficient to maintain it on either of the contacts.
The principle of the relay may be understood by referring to fig. 3. It will be seen that the ends of the local windings are joined to terminals B and C and their centre to terminal A, which is joined through an adjustable resistance Y to the relay tongue. This resistance is for regulating the local current and keeping it below the value of the steady current through the line coils. Terminal B is connected to earth through a condenser K, while C has a resistance coil X in its earth lead.
Assuming that there is no current in the line coils and that the relay tongue has just reached the marking contact, there will then be a momentary rush of current through the winding AB to charge the condenser K, in a direction to keep the tongue to the marking side, thus preventing any tendency of the tongue to rebound. This charging current dies away rapidly, however, and directly its strength falls below the steady current flowing through the winding AC, the preponderance of the latter causes the tongue to move toward the spacing contact. Immediately the tongue leaves the marking contact, the condenser K discharges through both windings BA and AC in such a direction as to accelerate the movement of the tongue, so that its transit time from one contact to the other is thereby lessened.
Fig. 3.
When the tongue reaches the opposite contact the condenser K is again charged, but this time from the other pole of the battery; a similar cycle of effects therefore takes place on that side and the tongue moves in the reverse direction. In this manner the relay tongue is kept vibrating, at a speed depending on the values given to the condenser and resistance. In practice the adjustments are such that the rate of vibration of the tongue under the control of the local current is approximately equal to the rate at which the transmitter at the distant station sends reversals at working speed. When this obtains, the signals passing through the line coils merely determine the length of time that the tongue remains in contact with either stop, its movement therefrom being effected by the local current through the local windings as soon as the strength of the line current falls below that of the local current in the coil AC. It is this effect combined with the action of the condenser in lessening the time of transit of the tongue, that enables a higher speed of working to be attained on long and difficult circuits than if ordinary polarized relays were used.
In the original Gulstad relay the line coils were not differentially wound; it could be used, therefore, only on Bridge duplex or simplex circuits. To utilize the advantages of the Gulstad principal on differential duplex circuits, the British Post Office has modified the Post Office standard relay, by adding extra windings and terminals. This modified instrument is known as a “G” relay and is equally suitable for differential or bridge duplex working. The internal and external connexions of this relay are shown in fig. 4, in which the dotted lines indicate the extra coils. For the correct reception of the incoming signals a Wheatstone receiver is connected to the relay tongue.
References.—E. Lack, “The Gulstad Relay,” I.P.O.E.E. Journal (vol. vii., p. 183); Electrical Review (June 1898 and Aug. 1902); Herberts Telegraphy (latest ed.) ; E. Lack, “Post Office Standard Relay ‘G’,” I.P.O.E.E. Journal (vol. x., p. 34).
Tele-photographic Systems.—In 1909 T. Thorne Baker read a paper before the Royal Institution in London describing his “telectrograph” process of transmitting pictures over long distances. The method, which was used on a large scale by the Daily Mirror between London and Paris, is based upon the Bakewell copying telegraph.
Fig. 4.
Synchronously rotating metallic drums, driven by electric motors, are employed one at each end of the telegraph line over which it is desired to transmit, say, a picture. A half-tone photograph of the picture is first printed upon thin sheet lead and subjected to a process which breaks up the photograph into a number of dotted lines printed in fish glue. This record is fixed round the transmitting drum, which is traversed spirally by an iridium stylus. The contact of the latter with the lead is interrupted every time one of the fish glue dotted lines comes beneath it, for duration depending upon the width of the line. The lead sheet is connected to the line, so that the transmitting instrument sends a series of electric currents whose periods of duration are determined by the width of the lines composing the photograph. At the receiving station, the rotating drum carries a piece of absorbent paper impregnated with a colourless solution, which turns black or brown when decomposed by an electric current. Every brief current through the paper causes a mark to appear, having a width depending on the duration of the current. The arriving currents are therefore arranged to pass through a platinum stylus under which the receiving drum rotates, then through the moistened paper resulting in the production of a number of marks on the paper due to chemical decomposition. These marks gradually combine to produce the picture at the transmitting station.
References.—T. Thorne Baker, “Telegraphy of Photographs, Wireless and by Wire,” Royal Institution Proc. 1908-10, vol. xix.
Foss and Petersen Method.—In this system a high frequency generator capable of producing sparks is used at the receiving station. The sparks so produced are capable of puncturing a paper wrapped round a metal drum which rotates in unison with a similar drum at the sending station.
The line wire is arranged so that when the sending end is connected to earth the generator is partly short-circuited, thus suppressing the sparking. At the sending end the shunting of the generator is effected by means of a contact pin passing over a cylinder on which the writing or illustrations are inscribed in insulating ink so that the shunt circuit is cut out each time the pin passes part of the writing (see Patent Specification No. 105,914, 1917). (W. No.)
United States
Technical developments made after 1910 practically revolutionized telegraphy as practised in the United States. These include printing-telegraph arrangements applied to telephone as well as to telegraph circuits, simultaneous telephone and telegraph operation for long small-gauge cable circuits, and the use of alternating currents with resonant circuits in the so-called carrier systems for multiplexing wire conductors.
Radical changes were also made in the arrangements for and the methods of handling telegrams in large offices. Belt conveyers, typewriters, pneumatic tubes, automatic time-stamps and other labour-saving devices came to be used to a large extent. About 75% of all telegrams handled by the Western Union Telegraph Co. over trunk circuits in 1921 were transmitted and
