Fig. 60 represents the apparatus devised by Dr. Ritchie as a mode of obtaining rotary motion by the temporary magnetization of an iron bar, which is extensively employed as a contact - breaker. It consists of a circular wooden disc placed between the poles of a horse - shoe magnet, having a deep channel turned in it so as to form a cup. This cup is divided into 2 parts by a wooden bridge, the ends of which come opposite to the poles of the magnet. A brass pillar rises up the centre of the bridge, supporting on its top an iron bar wound with insulated wire, the ends of which come down into the cup, and are of such a length, that when the iron bar is rotated, they will just pass over the bridge without touching it. This bar, or electro - magnet, as it really is, has a pointed pin projecting from its underside, which fits into the brass pillar, allowing the bar to rotate with very little impediment from friction. The 2 semi - cups are filled with mercury, which will stand up above the top of the bridge, the latter thus causing a sort of trough between them.

Fig. 60.

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The 2 wires from the electrodes of a battery are put into the mercury, and the rotating bar is moved round so that it may stand across instead of in the line of the bridge. As soon as this is done, the wires from the iron bar will touch the mercury, and the battery current will circulate round the bar and convert it into an electro - magnet. The N. and S. poles of the horse - shoe magnet will attract dissimilar poles, produced in the iron bar by the action of the battery current, and draw them round until they aire opposite the 2 poles of the horse - shoe magnet. This operation will also carry the wires out of the mercury, communication with the battery will be interrupted, and consequently the electro - magnet will lose all its properties. But the impetus it acquired by its partial rotation will carry it a little beyond the line of the bridge, and this will bring the points of the wires again into the mercury, though not in the same semi - cups as before; the battery current therefore flows through the wire on the iron bar in the opposite direction, consequently the polarity acquired by the bar is opposite to that which it had before.

The end of the electro - magnetized bar that is now N. is thus near the N. pole of the horse - shoe magnet, and these 2 mutually repel each other, and by this force the rotating bar is driven to a position at right angles to the bridge, and where its N. can be attracted by the S. of the horse - shoe magnet. By this alternative magnetization and demagnetization, an attractive and repulsive action is obtained, by means of which a rapid rotation is produced, and a contact made and broken twice in each revolution.

Though convenient for some purposes, this is not suitable for large batteries or coils. Every time the wires leave the mercury, a vivid spark occurs, and the surface of the mercury soon becomes covered with a coating of oxide. This being a non - conductor, prevents the battery current from flowing into the wire, and so interrupts the action.

Fig. 61 shows the general form of the vibrating contact - breaker. It consists of a base - board having an outer brass pillar, a central brass pillar, and an electro - magnet. The electro - magnet is fixed to the board with its poles upwards, and of the ends of the wire wound on it, one is left open, so that the battery may be connected with it, and the other passes under the board to the base of the central pillar. The outer pillar, at its upper part, holds the end of a metallic spring which passes through the ring of the central pillar to the poles of the electro - magnet. Here the end of the spring is armed with an iron plate or clapper, which should stand, when the spring is at rest, about 1/10 in. above the poles of the electro - magnet. A screw passes through the ring at the top of the central pillar, and comes just into contact with the spring. The spring at this point and at the end of the screw is of platinum. When one electrode of a battery is attached to the open end of the wire of the electro - magnet and the other to the outer brass pillar, the circuit is complete.

If the anode of the battery be connected with the electromagnet, the current will enter there, circulate round it, communicate magnetic properties to it, pass under the board to the central pillar, rise up here to the ring, descend through the screw to the spring, and thence by the outer pillar to the cathode of the battery. The electro - magnet will now attract the iron clapper at the end of the spring down to itself, and by this means a separation takes place between the end of the screw and the spring, and the battery circuit is interrupted. The electro - magnet can no longer hold the clapper down; the spring thus liberated rises to the position it formerly occupied, and again comes into contact with the end of the screw that passes through the ring. As soon as this takes place, the current again flows, and the electromagnet draws down the clapper. Thus a rapid vibration is kept up, every oscillation of the spring being associated with making and breaking contact with the battery. When applied to intensity coils, it is usual to employ the iron oundle forming the core of the coil as the electro - magnet, and to place the vibrating spring vertical instead of horizontal.

Fig 61.

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Foucault's contact - breaker consists of a - brass arm, which dips a platinum wire into a cup of mercury, whence it draws the point out, so breaking circuit, in consequence of its other end being attracted towards the core of the coil whenever it is magnetized; the arm is drawn back by a spring when, on the breaking of the circuit, the core ceases to be a magnet.

A common contact - breaker on small coils is constructed of a piece of. thin steel which makes contact with a platinum point, and which is drawn back by the attraction of the core on the passing of a current, and so makes and breaks circuit by vibrating to and fro like the hammer of an electric bell.