Discoveries and Inventions of the Nineteenth Century

thick insulated copper wires, are inclosed in iron pipes and laid beneath the pavement, like the gas mains. In the United States, where electric illumination is much used, the conductors have been usually carried overhead like telegraph wires, but not a few fatal accidents have occurred from these conductors falling into the streets. There is no reason to doubt but that in a short time it will be as common for households to draw upon such electric mains for their supply of light and power as it now is to draw gas and water from common mains. The electric supply companies have central stations in suitable positions, where very large and powerful dynamos are regularly driven by steam power. These stations are provided with appliances for measuring the currents and for duly controlling the energy sent out. What will appear very extraordinary when we remember that electricity is in itself unknown, is that the quantity supplied to each house or establishment can be actually measured, and is paid for by meter as in the case of gas. As already said (page 498) electricity can only be measured by its effects, and it is the chemical effect which it is found convenient to use for the purpose we are speaking of. The plan is simply this: two plates of zinc dip into a solution of sulphate of zinc, and from the one to the other there is sent through the solution one-thousandth part of the current to be measured. While the current passes, zinc is deposited on the plate towards which the current goes in the solution, and if this plate is periodically weighed this furnishes the measure of the total current. But how is just one-thousandth of the whole current taken off from the rest and made to circulate through the measuring apparatus? This is very easily done by taking advantage of the law of derived circuits, which for our present purpose may be stated thus: when a current of electricity finds two different circuits along which it can pass, it will divide and circulate through both of them, but the greater part will pass through the circuit of less resistance (if there be any inequality), and by adjusting the resistances of the circuits we can divide the current between the two partial or derived circuits in any required proportions. Electric resistances, it may be mentioned, depend upon the length, section, and nature of the conductor, and are very easily measured and adjusted. While the method just explained serves very well to measure the quantity of electricity that has passed through a conductor in a given period, 537provided that the current has always been in the same direction, it will be sufficiently obvious that it would fail altogether in the case of alternating currents. And, in fact, even in the case supposed this mode of measurement does not take account of the real energy set in motion. A reference to page 498, where the differences of electric currents are mentioned that are commonly spoken of—tension and quantity—will show that electric effects depend upon more than the quantity of electricity passing. Forms of apparatus have been devised for recording the total energy supplied; but their construction and principles are too complex to be here explained. In some cases high tension currents are required, in others it is quantity and not tension that is sought for; and there are ways of transforming the qualities of currents so that the same source shall supply electricity of either class. An example of this may have been noticed in the action of the Ruhmkorff coil, where the mere interruption of the primary or battery circuit, which possesses so little tension that of itself it could not give rise to a spark, nevertheless produces a wave of electricity in the secondary circuit of a tension so high that sparks several feet long may be produced by it. A somewhat recent application of the electric current of the dynamo may be just mentioned here. It is what is known as electrical welding, and depends upon the heat developed by currents being proportioned to the electrical resistance for each part of the circuit. The heat thus generated, where the current passes between two surfaces of metal, even of considerable dimensions, is sufficient to bring them to a semi-fluid condition, so that when simply pressed together they coalesce into one mass. In this way pieces of iron work can be welded together in situations where it would be either inconvenient or impossible to heat them by furnaces. The reader who has followed the last article will probably be prepared to admit that “the magnetic field” is one of the most wonderful things in the whole realm of inorganic nature, as all the powerful effects we have been describing are the results of merely moving wires through it. A wire conveying an electrical current so modifies the space surrounding it, or so acts upon the unknown pervading medium, that conductors moved in it, have other currents generated in them. An intermittent current, like that in the primary circuit of the induction coil, is equivalent to a movement of the magnetic field in regard to the secondary coil, so that the general principle in the coil and the dynamo is fundamentally the same. Quite recently, Professor Elihu Thomson has shown some very novel mechanical effects of repulsions and rotations of conductors placed near the poles of a coil through which rapidly alternating currents are passing. [1890.] We already hear of natural forces which have hitherto in a manner run to waste being now utilised in man’s service by the advantage taken of the capability of a slender wire to convey power. A notable instance is in the case of the famous Falls of Niagara. Here the head of water is used to drive turbines; our readers must not run away with any notion of huge water-wheels being placed below the falls. But from the high level of the water above the falls a tunnel has been cut which brings the water into pipes 7½ feet in diameter, and these deliver it into three turbines, in passing through which it develops a force of 5,000 horse power, and this force is communicated to a steel shaft 2½ feet in diameter, connected with the revolving parts of the dynamo. Mr. G. Forbes, the engineer, states that the company who have undertaken this enterprise are supplying, 538with a handsome profit to themselves, electrical current or power at ⅛th of a penny per unit, for which English companies charge sixpence. That is, Niagara supplies power at 148th of the price it can be obtained from coal. The fact that mechanical power can be brought from a distance to everyone’s door by a slender wire, and at small cost, suggests the possibility of great social and industrial changes being effected in the future by that one condition. Think of the abolition of factory chimneys and smoke, nay, even of the abolition of the factory system itself, for cheap power transmission seems to promise much in that direction, and there is a shadowing forth of still more in THE NEW ELECTRICITY. The Leyden jar and a few of its most obvious and common effects have been touched upon already, (page 490); but the phenomena which are revealed by a careful study of its charge and discharge show that these are by no means of the simple kind that has generally been supposed. Thus, for instance, if the magnetising effects of what is called current electricity be borne in mind, especially the definiteness of this action as regards the direction of the current (cf. Fig. 257), it would follow that if instead of the iron bar in Fig. 265 we place within the coil some unmagnetised steel needles we should find after passing a current or discharge that these have become converted into permanent magnets, and that their north poles are always towards the left of the supposed current. Years ago experiments were made to ascertain whether the discharges of a Leyden jar repeatedly passed through a coil would magnetise needles in the same way, because it had been assumed that the discharge is simply a current of extremely short duration and of quite definite direction. As far back as 1824 it had, however, been observed that the needles were magnetised sometimes in the wrong direction, yet no attempt was made to explain this—it was sometimes merely mentioned in the books as “anomalous magnetisation.” Dr. Henry of Washington, U.S.A., experimented on the subject, and in 1842 referred this action to a condition of the discharge which had never before been suggested. He says “we must admit the existence of a principal discharge in one direction, and then several reflex actions backward and forward, each more feeble than the preceding, until the equilibrium is obtained.” Some five years afterwards Helmholtz had independently arrived at the same conclusion, and from the fact that when a succession of Leyden jar discharges are sent through the voltameter (Fig. 263) the water is indeed decomposed, but both oxygen and hydrogen are evolved at each electrode. Sir William Thomson (now Lord Kelvin) examined the question from a theoretical point of view, and in a masterly mathematical paper published by him in 1853 not only showed that the discharge must be of an oscillating character, but gave the form of equation by which the rate of oscillation is determined. Faraday proved, as has already been stated, that the matter of the dielectric takes part in such condensing actions as that of the Leyden jar. 539The electrical charge enters into the glass, the particles of which are thrown into a certain state of strain or tension (which Faraday called polarisation), and the discharge of the jar is their release from that tension. So that it appears that whatever electricity may be, it can in some way become bound up with the particles of ordinary matter like glass and other dielectrics, and exert force upon them, which force acts always in two opposite directions. It is the opposition of the form or direction in which the electrical effect is manifested that gave rise to the conception of the two “fluids”—the “positive” and the “negative.” If these “fluids” really existed it would surely have been possible to give to an insulated body an absolute charge of either of them. But this can never be done; if, for instance, you have in the middle of a room a metallic sphere charged with positive electricity, the necessary condition is that on the walls of the apartment or on surrounding objects there is an exactly equivalent quantity or negative electricity. The number of oscillations or alternate momentary currents in a single discharge of a Leyden jar is enormous. Theory shows that under ordinary circumstances they must be enumerated by hundreds of thousands, if not by millions; that is, the apparently instantaneous spark is really made up of say a million surgings to and fro of the electric influence. But theory also shows that the frequency of these oscillations can be controlled or adjusted through an indefinite range. A general notion of the requisite conditions may be obtained by the analogy of sound, and for this we may take the familiar case of the strings of a musical instrument, say the violin, or the harp. Everybody knows that when a stretched string or wire is pulled a little aside it is in a state of lateral strain, striving by its elastic force to return to its position of rest, and if it is suddenly let go it not only rapidly regains that position, but by the inertia of its motion is carried beyond it against its elastic force, which, however, again brings it back, and the movement is continued nearly up to the point at which it was originally released, this swinging movement persisting for an indefinite period, during which the vibrations, which have an ascertainable and perfectly regular frequency, are communicated to the sounding-board of the instrument and from that to the