are worked in one operation, and give about 82 kilogrammes of alloy. Each kilogramme of aluminium in the alloy represents the work of 44 horse-power during an hour. M. Minet obtains free aluminium by electrolysing fused cryolite, mixed with 60 per cent. of sodium chloride, between two carbon electrodes. The mixture is contained in a rectangular iron trough connected with the negative electrode; the trough thus transmits about 5 per cent. of the current, and so becomes coated with a thin layer of aluminium, which protects the iron. The aluminium liberated falls in drops into a crucible placed underneath the negative electrode. The aluminium thus obtained contains only 2 to 5 thousandths of iron. CHAPTER XXXVII. ELECTRIC TELEGRAPH. 444. General Conditions.-The applications which form the subject of the preceding chapters utilise the energy of the electric current, and the question of greatest importance is that of efficiency. Electric telegraphy belongs to that category of applications which only require a current giving a very small amount of useful work-in most cases only as much as suffices to call into action, by a sort of detent, some other mechanical energy, such as that of a weight or of a spring. The characteristic properties on which this class of applications depends are the facility with which the time and place at which electrical effects occur can be controlled, and the rapidity with which an electrical action at one point is followed by a resulting effect at a distant point. P OR L L FIG. 357. R'O 445. Electrical Communication between Two Points.-Practice has shown that in order to put two stations, A and B, into electrical connection only one wire is needed, provided that at the station A that pole of the battery which is not joined up with the line wire, and at the station B the end of the wire itself, are connected with large copper plates buried in the ground. The action is as if the earth itself played the part of a return wire. This arrangement has a twofold advantage; in the first place it is more economical, as the wire is the most costly part of the installation, and in the second place there is only half the resistance in the circuit, since the resistance of a good earth-connection is almost inappreciable. The same wire can be used for sending in either direction. Each station must have its battery P, its transmitter T, and its receiver R (Fig. 357): S and S are the copper earth-plates. The receiver may be placed either at R or at R'. In the second case the two receivers always work at the same time, which enables the signals sent to be verified at the sending station. 446. Duplex Method. It may even be arranged that the two stations communicate simultaneously by the same wire. One of the arrangements for this purpose is that represented in Fig. 358. The receiver is placed on the bridge of a Wheatstone's balance, the two sides a and a' of which are equal, while the two other sides are the line and a resistance p put to earth. If the two resistances are equal, it is readily seen that a current from A does not pass through the receiver at A as the bridge is in equilibrium, but that it acts on the receiver at B, and conversely. Three cases are possible. In the first case a single key is depressed, which is the case just described. Secondly, the two keys. are depressed simultaneously; the two receivers then give the same signal under the influence of the same current if the two bat teries have their opposite poles to the line, and under the influence of a local current if they are arranged in the same direction-that is, so that the same poles are joined to the line, their electromotive forces acting then in opposition to each other. Lastly, one of the keys is depressed, while the other is free and is not connected either with the battery or the earth: the current then gives the desired signal at the receiving station, but it acts also on the local receiver, though only for so short a time that no great inconvenience is caused; this difficulty may, moreover, be obviated by a somewhat more complicated arrangement, which we need not now describe. By means of condensers C, joined to the resistance p, the capacities of the real line and the artificial line may be balanced in the same way as their resistances are. 447. Lines with Condensers.-It is often desirable to isolate the line wire completely by interposing condensers C at the ends § 449.] Air Lines. 517 (Fig. 359). When the key is depressed the local battery charges the first coating of the condenser, and the potential of the second becoming raised or lowered to a corresponding degree, sends to the line a flux of the same kind as that which the battery would have given. This flux charges the first coating of the second condenser, which by the same action causes a flux, always of the same sign, through the instrument of the receiving station. The advantage of this arrangement is that it protects the line against the action of earth FIG. 360. FIG. 359. currents, which sometimes interfere with the working. 448. Air Lines.-The wire ordinarily used for air lines is galvanised iron 4 millimetres in diameter. Its resistance is 10 ohms, and its electrostatic capacity 0.01 microfarad per kilometre. The use of copper had been given up, owing to its small tenacity and its high price; but it is coming into use again in the form of a special kind of bronze, of which wires can be made which are almost as strong as iron, and have almost as high a conductivity as pure copper. As the wire is surrounded by a medium which is absolutely devoid of conduc- line is decreased. FIG. 361. 449. Submarine and Subterranean Wires.-In lines laid in the ground or under water, where the surrounding medium is a conductor, an insulating envelope is needed. The conductor or core consists of a strand of copper wires cc (Fig. 361), surrounded by several layers of gutta percha, then by a layer of jute, and lastly by a protecting sheath of steel wire covered with tarred hemp. The whole thus forms a condenser having a considerable capacity. The cable from Ireland to Newfoundland has a capacity per kilometre of 0.22 microfarad, with a resistance of 1.62 ohm. 450. Signalling Instruments-Alarum.-We shall only de scribe a few of the ordinary pieces of apparatus, and shall restrict ourselves to the indication of their most important features. A piece of soft iron L (Fig. 362), is held in front of an electromagnet by means of a spring, D. When at rest it touches a spring, r. The circuit comprising the wire of the electromagnet is thus closed between the terminals A and E, and when a current passes the electromagnet is thereby excited and attracts the iron, L, so that it now does not touch the spring, r; the current is thereby broken, the iron, L, again touches r, and so |