760 The Invention of the Telegraph. station. The zinc pole of the battery was connected with the return or common wire, and the copper pole was joined to a plate of metal or a trough of mercury, extending beneath all the keys; thus, on depressing any key, a current passed through the corresponding wire and deflected at the distant station, a needle which bore the same letter as the transmitting key. Any message might thus be spelled out and readily interpreted at any distance; the sole fault was the clumsiness and costliness of the transmitting system of wires. In 1837 a remarkable simplification was introduced and patented by Messrs. Cooke and Wheatstone, to the latter of whom the world is indebted for many other novelties of high value and great ingenuity, among which are the stereoscope, the concertina, and symphonion, which delight the eye and the ear. The novelty of this invention consisted (i.) in using the two directions, to which the needle might be swung by changing the current, as separate symbols, and so reducing the number of indicating needles required; (ii.) in using the current to ring an alarum bell at the distant station before sending a message; and (iii.) in connecting two sets of batteries and instruments at the two communicating stations, so as to make the power of communication reciprocal. To recount the many trials and experiments of these and other investigators, by which improvements were one by one effected, and even to name the contrivances suggested for speedy telegraphic purposes, would occupy a volume in itself. We shall therefore merely sketch the general features of the final results to which all these investigations, so enthusiastically prosecuted, have led. 1000. Every telegraphic system must of necessity comprise (a) the current generator, or battery, (b) the conducting or circuit wire, by which the current passes to the distant station; (c) the transmitting apparatus by which the signals are to be sent; and (d) the receiving or indicating apparatus by which the signals may be interpreted. With regard to the first of these, the battery, there is nothing special required farther than a steady-going battery of moderate strength or electro-motive force. Daniell's or Leclanche's battery is particularly suitable for telegraphic purposes. The conducting wire used for land-telegraphs is generally galvanized or zinc-coated iron wire, carried on high poles, and insulated either by means of glass or porcelain insulators of a cup-shape. The Common Needle Telegraph. 761 Underground wires coated with gutta percha, or other insulator, were first employed and are still employed in large towns, where air-lines would have their insulation endangered. A very valuable discovery was made by Steinheil in 1837; when experimenting whether the iron rails of a railway would serve the purpose of a return wire to complete the circuit, he found that the earth itself might be made to serve the purpose, and the expense of wire was thus at once reduced to one half. The figure (293) will show the arrangement usually adopted for this purpose. It represents two stations connected by a line of telegraph. C Z is the battery at the one end, C' z' at the other; G, Gʻ are the galvanometers, to be presently referred to, by means of which signals can be exchanged ; H and H' are two handles, by means of which either the zinc pole or line wire H P Fig. 293. the copper pole of the battery can be connected with the galvano. meter wire and the line wire, while the copper or the zinc pole is at the same moment connected with the wire, which leads to a large metal plate buried in the earth. By turning the handle, H, for instance, to the left hand, the wire from the copper pole of the battery is connected with the end of the galvanometer wire, the other end of which is connected with the line wire, while at the same time the zinc wire of the battery is put in metallic communication with the earth-plate, P'. Practically the effect is the same as if the current flowed from C through G along the line wire to the distant G', thence to the distant earth-plate P', thence through the earth to P, and back to the battery at Z. By turning the handle to the right, a current flows through the whole circuit, as it may be called, in the reverse direction, and the distant galvanometer, G, turns to the opposite hand. The real theoretical explanation of the case now adopted, is to 762 Single Needle Telegraph. say that the earth drains off the opposite electricitics, which would otherwise accumulate at the terminals, the effect being thus precisely the same as if both plates were connected directly, the current being in reality one uninterrupted succession of discharges of the positive and negative electricities, produced at the copper and zinc poles of the battery. 1001. The sending apparatus, or transmitting key, by which the signals are given, is in its simplest form merely a contrivance for making and breaking contact between the battery and the line; and, in the older form of the instrument, it is simply a commutator for changing the connections between the battery poles and the line wire. They will be described incidentally in explaining the indicating contrivances. 1002. Fig. 294 will give an idea of the principle of the single needle telegraph of Cooke and Wheatstone, which is still extensively used in this country, though not very line much anywhere else. It consists of an upright galvanometer with an astatic pair of needles, the motions of the outside one to right or left constituting the signals, one end of the galvanometer wire leading to the connection, N, the other to the line wire. R is the commutator, or current change, which has two insulated metal plates in per manent connection with the two poles of the exciting battery, C Z, and so arranged that a slight motion to one side, say the left, of the handle, H, puts one of these plates and the corresponding pole of the battery in connection with the earth through P, while it puts the other and its battery pole simultaneously in connection with the line through N. R Earth 000007 H Fig. 294. Thus a negative electric wave passes along the line, and deflects both the local needle and the distant needle to the same side. A slight motion to the right reverses the connection, and deflects both needles to the opposite hand. Out of a combination of these two motions to right and left, an alphabet is agreed upon, those letters getting the simplest signals which occur most frequently in ordinary language. Thus, for instance, as E and T are most frequently used, one swing of the needle to the left stands for the letter E, and one to the right for letter T : A is one left, one right; B is one right and three left; C right lest, right left, and so on. The Morse Telegraph. 763 The actual form of the commutator in use is cylindrical, but the form shown in the figure is identical in principle, and is frequently used for commutating purposes on the Continent. 1003. Of this single needle, or of any needle telegraph, the great disadvantage in practical use is the transient nature of the signals; and it is now completely superseded by the admirable self-recording instrument invented by Professor Morse, of America, about the year 1837. Without giving the mechanical minutiæ, the adjoining figure (fig. 295) will enable us to explain the general features of the Morse system. The signalling apparatus is a mere make and break brass key, B, which is connected permanently with the line wire, and which may be connected with the zinc or copper pole of the local battery, according to whichever end is depressed. When not in action, it is held by a spring in the position represented in the figure, so that the line-wire is always in connection with the earth-plate. The Morse, M, seen at the other end of the line, consists of an electro-magnet, M, one end of whose coil is connected to the line, the other to the "earth :" a brass armature, A S, with an iron piece opposite the magnet, and movable about a hinge at A, carries a pointed style or pencil, s, at the other end. It is obvious, then, that so long as the key, B, at the other end is depressed, M is magnetized, and the armature, A S, attracted; thus, if the pointer, S, press on a strip of paper, B, coiled on a bobbin, R, and unwound when desired by means of clockwork in K, it is evident that a momentary magnetization of M, caused by a momentary depression of the key, B, will mark merely a dot, while a longer-continued depression will mark a line on the paper. In this way a succession of dots and dashes may be transmitted with great rapidity; and, an alphabet being agreed on, composed of combinations of dots and dashes, Professor 764 Telegraphic Alphabet-The Relay. Morse has in this way solved the problem of conveying a permanent message to any distance by means of the electric current. The Morse telcgraphic alphabet, which is now adopted by all nations, represents the most frequently occurring letters by the simplest symbols; a great number of arbitrary signals and abbreviations being adopted by experienced practical telegraphists. It is as follows: Besides these, there is an ingenious system of dot-and-dash symbols for numbers, as well as for the names of all different telegraphic stations. 1004. The Relay, represented in fig. 296, is a remarkably ingenious though simple contrivance, by which the feeble electric current which survives the leakages of a long journey, and is unable to impress its message directly, can summon to its assistance a local line Earth It battery at the distant station, and so accomplish its mission. consists of a very fine coil of silk-covered wire, R, through which the line-current passes to the earth. This current would be too feeble to act upon the Morse directly, but affects the sensitive Relay so as to attract a light brass armature, A T, with the axle, X, of which the copper pole of a local battery is connected, while the zinc pole is connected to the coil of the Morse instrument, M; a metal pin in |