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it emerges by the wire, Z, to return to the local battery from which it started. Then when the current of the line is open, the electromagnet of the relay does not act, and the lever, p, drawn by a spring, r, leaves the button, n, as shown in the drawing, and the local current no longer passes. Thus the relay transmits to the indicator exactly the same phases of passage and intermittence as those effected by the key in the station which sends the despatch.

477. Electromagnetic machines.-Many physicists have attempted to utilise the attractive force of electromagnets as a motive power. M. Jacobi, of St. Petersburg, appears to have been the first to construct a machine of this kind, with which, in 1838, he moved on the Neva a small boat containing twelve persons. Since that time the construction of these machines has been materially modified; but in all the expense of zinc and acids which they use far exceeds that of steam engines of the same force. Until some cheaper source of electricity shall have been discovered there is no expectation that they can be applied at all advantageously.

Fig. 393 represents an electromagnetic machine constructed by Froment. It consists of four electromagnets acting in two couples, on two pieces of soft iron, P, only one of which is seen in the drawing. This piece, attracted by the electromagnets, EF, transmits the motion by means of a connecting rod to a crank fixed at the end of a horizontal axis. To this is fixed a fly-wheel like that of a steam engine, which is intended to regulate the rotatory motion. On this axis also is a piece of metal, n, of a greater diameter, the action of which will be described presently.

The current of the battery, entering at A, passes into a cast-iron base, B, then by various metallic connections it reaches the metal piece, n. Thence the current ought to pass alternately to the first couple of electromagnets, EF, and then to the second, ef. In order to understand how this attraction in the path of the current is effected, let us refer to fig. 394 on the right of the picture, which represents a section of the piece, n, and its accessories. On this piece is a projection, e, which is called a cam, and which, during a complete turn, successively touches two springs, a and b; these are intended to transmit to the electromagnets the current, the direction of which is indicated by the unbarbed arrows; the barbed arrows do not show the direction of the current but the direction of the motion of the various pieces of the machine.

These details being known, it will be seen that the current passes

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-478] . Induction by Currents. alternately into the two springs, a and b, and from thence into the two systems of electromagnets, EF and ef: the piece P is first of all attracted, then a similar one, which is placed at the other end of the axis of the fly-wheel. There is thus produced a continuous circulating motion, which is transmitted by an endless band to a system of wheel work, which works two lifting pumps.

CHAPTER XII.

INDUCTION BY CURRENTS.

478. Induction by currents.—We have already seen (398) that by the term induction is meant the action which electrified bodies exert at a distance on bodies in the natural state. Hitherto we have only had to deal with electrostatical induction ; we shall now see that dynamical electricity produces analogous effects.

Faraday discovered this class of phenomena in 1832, and he gave the name of currents of induction, or induced currents, to instantaneous currents developed in metallic conductors under the influence of metallic conductors traversed by electric currents, or by the

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influence of powerful magnets, or even by the magnetic action of the earth ; and the currents which give rise to them he has called inducing currents.

The inductive action of currents at the moment of opening or closing may be shown by means of a bobbin with two wires. This consists (fig. 394) of a cylinder of wood or of cardboard, on which a quantity of stout silk-covered copper wire is coiled; on this is coiled a considerably greater length of fine copper wire, also insulated by being covered with silk. This latter coil, which is called the secondary coil, is connected by its ends with two binding screws, a, b, from which wires pass to a galvanometer, while the thicker wire, the primary coil, is connected by its extremities with two binding screws, c and d. One of these, d, being connected with one pole of a battery, when a wire from the other pole is connected with c, the current passes in the primary coil, and in this alone. The following phenomena are then observed :

i. At the moment at which the thick wire is traversed by the current the galvanometer, by the deflection of the needle, indicates the existence in the secondary coil of a current inverse to that in the primary coil, that is, in the contrary direction; this is only instantaneous, for the needle immediately reverts to zero, and remains so as long as the inducing current passes through cd.

ii. At the moment at which the current is opened, that is, when the wire, cd, ceases to be traversed by a current, there is again produced in the wire, ab, an induced current instantaneous like the first, but direci, that is, in the same direction as the inducing current.

479. Induction by magnets and by the action of the earth.It has been seen that the influence of a current magnetises a steel bar; in like manner a magnet can produce induced electrical currents in metallic circuits. Faraday has shown this by means of a coil with a single wire of 200 to 300 yards in length. The two extremities of the wire being connected with the galvanometer, as shown in fig. 395, a strongly magnetised bar is suddenly inserted in the bobbin, and the following phenomena are observed :

i. At the moment at which the magnet is introduced, the galvanometer indicates in the wire the existence of a current, the direction of which is opposed to that which circulates round the magnet, considering the latter as a solenoid on Ampère's theory (465).

ii. The needle soon returns to zero, and remains there as long as the magnet is in the coil ; when it is withdrawn, the needle of the galvanometer, which has returned to zero, indicates the existence of a direct current.

The inductive action of magnets may also be illustrated by the following experiment: a bar of soft iron is placed in the above bobbin and a strong magnet suddenly brought in contact with it; 480] Properties of Induced Currents. '503 the needle of the galvanometer is deflected, but returns to zero when the magnet is stationary, and is deflected in the opposite direction when it is removed. The induction is here produced by the magnetisation of the soft iron bar in the interior of the bobbin under the influence of the magnet.

Faraday discovered that terrestrial magnetism can develop induced currents in metallic bodies in motion ; that it acts like a powerful magnet placed in the interior of the earth in the direction of the dipping needle, or, according to the theory of Ampère, like a

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series of electrical currents directed from east to west parallel to the magnetic equator. He first proved this by placing a long helix of copper wire covered with silk in the plane of the magnetic meridian parallel to the dipping needle ; by turning this helix through a semicircle round an axis, in its middle, perpendicular to its length, he observed that at each turn a galvanometer connected with the two ends of the helix was deflected.

480. Properties of induced currents.-Notwithstanding their instantaneous character, it appears mainly from the experiments of Faraday their discoverer, that induced currents have all the properties of ordinary currents. They produce violent physiological luminous, calorific, and chemical effects, and finally give rise to new induced currents. They also deflect the magnetic needle, and

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