Telegraph

Telegraph, in a general sense, means any apparatus for conveying intelligence to a distance by means of signals, but although systems of signalling by flags, guns, semaphores, etc., may properly be designated telegraphy, yet the term is to a large extent restricted to methods depending upon the use of electric currents. The fact that an electric charge communicated to one end of a wire could be made to produce a visible signal at the other snggested the idea of an electric telegraph in 1753, but it was not until the principles of electro-magnetism were to some extent understood that any practical device was produced. The deflection of a magnetised needle by a coil conveying a current was proposed by many, and developed by Cook and Wheatstone. Six needles and as many wires were at one time used, but these were reduced to one; and the single-needle instrument so developed is still employed to a large extent for railway purposes. Another type of instrument, depending upon the temporary attraction of an electromagnet for its armature, was invented by Morse in America in 1835, and this, having received many improvements in detail, is now generally employed for land telegraphs. In 1840 Wheatstone devised his "step-by-step" or ABC instrument (now superseded), which formed the basis of the type-printing instrument used for distributing items of news. A telegraph system consists of a "line" or wire connected at each end to suitable transmitting and receiving apparatus. In the case of land lines, the wire may be suspended in the air from poles or may be drawn into undergronnd pipes. A single wire only is used to connect each pair of stations, the earth serving for a return conductor, and hence it fo11ows that the line must be insulated as perfectly as possible from the earth, The wire - generally of galvanised iron - is therefore supported upon insulators of vitrified and glazed, porcelain, shaped in such a way that some part may be as far as possible protected from wet and dust. Underground wires are of copper, covered with a coating of guttapercha, protected by a lapping of tarred tape, and are drawn into iron pipes placed under the pavements. In a submarine cable a strand of small copper wires is used, and is very carefully covered with several layers of guttapercha. It is mechanically protected by being lapped with jute yarn, and is armoured with spirally-laid galvanised steel wires, the armouring of the parts near the shore being much heavier than that of the deep-sea portion, As a source of electricity, galvanic batteries [BATTERIES] (Daniell's, Fuller's, etc.) are generally used, but in large offices, where the work is heavy, secondary batteries (q.v.) are being introduced, and are found more economical and reliable. The transmitting instrument consists of some form of key (q.v.), which reverses or interrupts the current in accordance with the motions of the operator's hand; and when a message is not being sent, the key must make such contacts that a current from the distant station flows to the receiving instrument. Of the many possible forms of the latter the Morse "Sounder" and "Recorder" are most generally used for land lines. The sounder consists of an electromagnet, with an armature attached to a pivoted brass lever, normally held against a stop by a spring. One end of the magnet winding is connected to "line" through the key, the other to "earth," When a current is received from the line, the armature is attracted, and, by coming into contact with a stop, makes a sharp click; and on the cessation of this current it is pulled away by its spring and, striking another stop, makes another click. If the key at the sending end is only depressed for an instant, these clicks will follow in quick succession; but if the operator holds down the key for a slightly longer time, there will be an interval between the two sounds, The recorder is similarly constructed, but the pivoted lever is prolonged and carries at its end a small wheel, whose edge is kept wet with ink by means of a ro11er. A strip of paper is moved over this wheel (but not in contact with it) by a clockwork mechanism, and when the armature is attracted by the magnet the wheel is lifted and touches the paper. A mark is thus made, and the length of this mark depends upon the length of time during which the armature is attracted - that is to say, upon the time during which the sender depressed his key. If he just closes the circuit and immediately opens it, the result will be a very short mark or "dot," or he may make a longer mark or "dash;" and it is by various combinations of dots and dashes that various letters are indicated. The sounder, of course, makes no visible signal, but the difference between a dot and a dash is easily perceived by the ear, The single-needle instrument is a special form of galvanometer (q.v.) with a vertical needle. If the sending operator, by means of a reversing key, puts the positive terminal of his battery to line, the distant needle will be deflected, say, to the right; if he puts the negative to line, the needle will deflect to the left. If a deflection to one side is regarded as equivalent to a dot, and a reverse deflection to a dash, the system of signals. or "Morse code" mentioned above can be used. This instrument is convenient in many ways, but in regard to speed it is much inferior to the Morse, and in the case of lines joining large towns, speed is a matter of great importance. With the Morse recorder the speed is limited by the quickness with which the operator can move his fingers, and in order to send the gratest possible number of messages over a single line Wheatstone's automatic system is often used. - Here an automatic transmitter is used instead of a key. By means of a special machine holes are punched in a strip of paper, the position of these holes depending upon the signal to be sent, and this strip is drawn by clockwork through the transmitting instrument. Certain pins are arranged to be able to pass through the holes, in the strip, and by so doing make the contacts needed for sending the message. A number of operators may, at the same time, be punching strips which are successively sent through the same instrument, and by this means as many as 400 words a minute may be transmitted over one wire. Much more delicate receiving apparatus is needed for submarine work than for use on land lines. Thomson's mirror galvanometer was originally employed for this purpose, being used in the same way as a sIngle-needle instrument, but it has now been generally superseded by his siphon recorder. In the latter instrument the moving part is a small light coil of fine wire suspended between the poles of a powerful magnet, and connected with the circuit. A current in this coil will cause it to deflect to one side or the other, according to its direction, the arrangement being, in fact, a form of electric motor. [DYNAMO.] This coil is connected by silk threads to a siphon of fine glass tube, which, as the coil deflects, is moved over the surface of a strip of paper propelled by clokwork. One end of the siphon dips into a pot of ink, and, either b electrifying this ink or by causing the paper and siphon to be continuously vibrated, the ink is made to come from the far end of the tube in a succession of fine drops. A line is thus made on the moving paper, which wanters to one side or the other of its middle position when a positive or negative current comes from the cable, and this line is read in the same way as are the deflections of a single-needle instument.