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Electrical Alarum.

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already described there are three of which mention must be made; the lightning conductor, the alarum, and the relay.

The influence of storm clouds in decomposing the natural electricity of the wire, often produces sufficient tension, not merely to interfere with the transmission of the despatches, but also to produce dangerous discharges. The lightning conductor is designed to remedy these inconveniences.

Represented at M in figs. 385 and 386, it consists of a vertical stand on which are two copper plates, indented like a saw, and arranged so that the teeth are near each other but do not touch. One of these plates is connected with the earth, the other with the line wire. Hence, when, by the inductive action of a storm cloud, electricity accumulates in wires and in the apparatus, it escapes by the points to the plate which is connected with the ground, and thus all danger from a discharge is avoided.

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476. Electrical alarum.-The electrical alarum is intended to warn the receiving station that a despatch is about to be sent. Represented in fig. 391, it consists of a board on which is fixed an electromagnet by means of a piece of brass, E. The current from the line arriving by a binding screw, m, passes to the wire of the electromagnet, thence into the armature, a, into a steel spring, c, which presses against the armature, and ultimately emerges by a second terminal, n.

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Thus, whenever the current of the line wire reaches the electromagnet, the armature, a, is attracted, and a clapper, P, fixed to this armature, strikes against a bell, T, and makes it sound. The moment the clapper strikes, as the armature is no longer in contact with the spring, C, the current is open, the electromagnet no longer attracts, and the armature reverts to its original position by the action of a spring, e, to which it is fixed. The current being closed afresh, a second attraction takes place,

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Fig. 391.

and so on until the telegraph clerk, thus warned, lets the current pass directly into the indicator without passing through the alarum. This he accomplishes by means of an instrument called the shunt.

Relay. In describing the receiver we have assumed that the current of the line coming by the wire, C (fig. 392), entered directly into the electromagnet, and worked the armature, A, producing a despatch; but when the current has to traverse a distance of a few miles, owing to the resistance of the wire and the losses of insulation, its intensity is diminished so greatly that it cannot act upon the electromagnet with sufficient force to print a despatch. Hence it is necessary to have recourse to a relay, that is, to an auxiliary electromagnet, which is still traversed by the current of the line, but which serves to introduce into the communicator the current of a local battery of four or five elements placed at the station, and only used to print the signals transmitted by the wire.

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For this purpose the current from the line entering the relay by the binding screw, L (fig. 392), passes into an electromagnet, E, whence it passes into the earth by the binding screw, T. Now, each time that the current of the line passes into the relay, the electromagnet attracts an armature, A, fixed at the bottom of a vertical lever, p, which oscillates about a horizontal axis.

At each oscillation the top of the lever, p, strikes against a button, n, and at this moment the current of the local battery which enters by the binding screw, c, ascends the column, m, passes into the lever, p, descends by the rod, o, which transmits it to the binding screw, T: thence it enters the electromagnet of the indicator, whence

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Electromagnetic Machines.

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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, þ, 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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Induction by Currents.

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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

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