Electro-magnetism-Wilde's Machine. 755 detailed account of even the most important of these, interesting as they are, is beyond the scope of the present work. We shall merely state a few of the more striking facts connected with these curious machines. In a large machine for producing the electric light, constructed by Holmes, of London, as many as eighty-eight coils of wire are placed round the circumference of a wheel, which is turned by a steam-engine within a series of inducing steel magnets placed in a concentric circle round about. 993. Wilde's Machine.-More interesting is the invention of Mr. Wilde, of Manchester, who conceived the ingenious plan of using the induced current to act upon an inducing electro-magnet, thus strengthening its magnetism and its inducing power. Its principle may be inferred from fig. 289, which represents the upper part of it, the lower portion being almost an exact reproduction of this part on a much larger scale. This part consists of a series of steel T horse-shoe magnets, forming a sort of bridge, within whose arch turns a long BBT coil or armature, known as Siemens' armature, in which the insulated wire is wound lengthwise, instead of crosswise, as in the ordinary form of coil. Very powerful currents are induced in this form of armature by reason of the proximity of the coil to the poles of the magnet. By means of a commutator on the axle of the armature, the current is sent in one uniform direction to the binding screws, T, T', whence it passes to the coils of a huge electro-magnet, over which this primary miniature machine stands. Between the soft-iron poles of this large electro-magnet is turned, by the same motory power as that which turns the upper armature, a much larger Siemens' armature, in which vastly more powerful electric currents are induced. Mr. Wilde has even carried his principle a step farther, and used this second induced current to excite a second electro-magnet still more powerful than the first, and by means of this triple arrangement electric effects of unexampled intensity have been obtained With such a machine, driven by a 15-horse power engine, the armature revolving at the rate of 1500 times per minute, the inventor was enabled to melt a rod of platinum two feet long, a quarter-inch bar 756 Thermo-electricity. of iron fifteen inches long, and seven feet of No. 16 iron wire, and to heat red-hot twenty-one feet of iron wire. The luminous effects were perhaps the most surprising. When the current passed between thick sticks of gas-carbon (1-inch square), placed on the top of a lofty building, the light rivalled that of the sun in splendour; shadows were cast from the flames of street lamps a quarter of a mile distant, and photographs might readily be taken with the power of its chemical or actinic rays. 994. Still further simplifications and improvements of Wilde's principle have been embodied by Siemens, Wheatstone, and Ladd, in elegant and compact machines of such wonderful power, that care must be taken not to work them too long or too violently, otherwise the wire of the inducing coils may get so hot as to produce their own destruction. In all these machines we have exemplified the conversion of mechanical power or energy into electrical energy; rather, in the case of the larger machines, we have the transformation of heatenergy, first into mechanical, and of the latter into electrical energy, together with a fractional restoration into heat energy. Before passing on to describe the most important of all the applications of electricity, viz., the Electric Telegraph, we shall briefly notice one other means of generating electrical currents, whereby heat is converted directly into electricity without any intermediate mechanical transformation. THERMO-ELECTRICITY 995. Is the name by which these heat-born electric currents are designated. сопрет The elementary fact in connection with this subject is, that if any two different metals are included in a closed galvanic circuit, an electric current is set up whenever the junction of the two different metals is heated; and the current flows in a direction which is quite definite for each pair of metals, but is different B for the same metal when differently paired. Thus, for instance, if a piece of iron wire or rod be connected with the copper wires from a galvanometer, G, and we heat with a spirit lamp, or a lucifer match, the junction, B, of the two metals, the Fig. 290. A iron Thermo-electric Order of Metals. 757 galvanometer needle will be deflected so as to indicate the flow of an electric current in the direction shown by the arrow head in the figure 290; but if we apply the heat at the opposite end, A, the current is reversed, so as still to flow from the copper to the iron at the heated junction. Had we, however, connected a platinum wire, instead of an iron one, between the ends, A B, of the copper wires, we should have found that at the heated junction the current would have flowed from the platinum to the copper. Copper is thus said to be thermo-electrically positive with respect to iron, but thermo-electrically negative with reference to platinum. By such experiments, most carefully performed, it has been found that, just as the metals can be arranged in an electro-chemical order (Art. 962) indicating the direction of the current when any two are connected as a galvanic pair, so they may be arranged in a thermoelectric order indicating the direction of the current produced by heating a junction of any two of them. According to Becquerel, this order is Bismuth, platinum, lead, tin, copper, silver, zinc, iron, antimony; The direction of the current at a junction of any pair being the same as the order in which the two are named in the line; and the farther apart they are in the line, the stronger will be the electromotive force of the thermo-electric current produced. 996. By connecting a number of thermoelectric pairs, as in fig. 291, we may form a thermo-electric battery, which will give a constant current so long as we keep the front face of junctions at a higher temperature than the back face, the resulting current being from the last iron to the first copper. Many attempts have been made to replace the galvanic battery by such a cleanly source of electricity as this arrangement would be; but the currents are so much less energetic than those resulting from chemical action, that no important practical results have as yet followed in this direction. Fig. 291. 997. The Thermopile.-The most important use to which the thermo-battery has been put, is the detection of minute differences of temperature, for which purpose it was first used by Melloni in his experiments on radiant heat. A large number of antimony-bis. 758 A T B The Electric Telegraph. muth, or copper-iron, pairs are arranged in a compact form as in fig. 292, each pair and layer being carefully insulated by varnished paper. The whole is enclosed in a tube, à B (fig. 292), one face of the junctions being exposed at each end of the tube. Such an instrument is extremely sensitive to difference of temperature at the two faces; connected with a sensitive astatic galvanometer, or the still more sensitive reflecting galvanometer of Sir William Thomson (Art. 983), it will indicate the approach of the hand to either face by an instant deflection of the needle; and a momentary touch with the warm finger may swing the needle round half the face of the dial or more. If a piece of ice, or a tumbler of cold water, be brought near the face, there will be a turning of the needle in the opposite direction. Thus the mere direction of the needle indicates whether any face is being heated or chilled only it must be remembered that the heating of one face has the same effect as the chilling of the opposite face. Fig. 292. Very delicately constructed thermopiles have been used to detect the heat transmitted from the moon and the stars to this earth ; degrees of temperature far too minute for the indication of any ordinary thermometer.* THE ELECTRIC TELEGRAPH. 998. Having described various methods by which the electric current may be produced, it still remains for us to give a general idea of the principles of the most wonderful, and by far the most useful, of the manifold applications of this unseen power, viz., the electric telegraph. The veriest child of the present day is familiar with the name of the electric telegraph, which sounded so strange and mysterious to all but the philosophic few within a comparatively recent period. To enumerate all the uses of this admirable invention, would be to catalogue the multitudinous pursuits of mankind; for there is no art or industry, no profession, or situation in modern life, in which the assistance of this wingless messenger is not summoned to perform for man the duty of the fabled Mercury. Information of importance to individuals, or to the public, is sent * One of these instruments exhibited by Faraday at the Royal Institution was sc delicate that the warmth of the body of a fly in walking over it was sufficient to produce a visible change. Construction of the Telegraph. 759 from any part of the world to any other, and questions are asked and answers returned within a few minutes. When the signs of coming storms, now much better understood than formerly, are anywhere observed, a telegraphic notice, which travels a thousand times faster than any storm, can be sent to places in the direction of the storm's progress, so that useful precautions may be taken. In sea-ports, ship-captains, so warned, can delay sailing until the danger has passed. If telegraphs on railways did not constantly send information along the lines of the arrival and departure of trains, not half the present traffic on the lines could be safely carried on. When, at the Observatory of Greenwich, the clock marks the instant of noon, or any other time, the fact, through a telegraph connected with the clock, is declared at many other important stations, as Liverpool, York, Glasgow, Edinburgh, where by the dropping of a ball obeying the telegraph, in a conspicuous place, or the firing of a gun, the information is widely spread. A gun fired on the Castle Hill of Edinburgh by an electric wire, nearly a mile in length, stretched high in the air from the Observatory, is heard for many miles around. Shipmasters about to sail, hearing such a report, can set their chronometers exactly to Greenwich time; and clocks and watches over the country, which maintain order in the whole business of society, may thus be regulated. The Construction of the Telegraph. 999. The invention of the galvanic battery, the discovery of the deflection of a magnetic needle by the current, and the discovery of the magnetization of soft iron by the current, were the three great steps in the history of the electric telegraph. As early as 1830 it had been suggested that Oersted's discovery might be employed for the transmission of signals to a distance; and a model telegraph, consisting of some thirty pairs of conducting wires and as many indicating needles, was exhibited by Professor Ritchie at the Royal Institution of Great Britain. Much about the same time a similar form of telegraph was proposed by Schilling, in Prussia; but the great complexity and costliness of such a system rendered these inventions practically useless. The first simplification was effected by making a single return wire serve as a common completer of the circuit for all the thirty wires and needles; and one form of instrument consisted of a set of keys like those of a pianoforte, each key corresponding to and connected with one of the wires, with its needle placed at the distant |