In the clocks actuated by electro-magnetism, the movement of the pendulum is not maintained by repeated impulses of the escape-wheel, as in ordinary clocks, but by magnetic attraction; an electro-magnet being so arranged as to attract the bob of the pendulum in both directions alternately. In Mr. Bain's arrangement, the bob of the pendulum is formed of a hollow coil of covered copper wire, which, on the transmission of an electric current, becomes magnetic, and it is then attracted by several permanent magnets fixed in a hollow horizontal bar, over which the coil of wire moves. The accompanying diagram will serve to explain more clearly the parts of the clock on which the movement of the pendulum depends. The pendulum rod, _B_, is made of wood, and the bob, _A_, consists of a hollow coil of thick copper wire covered with cotton, through which the hollow bar, _C C_, passes. Inside that bar there are several permanent magnets, packed on each side of the ends of the coil of wire, the poles of those on one side being the opposite of those on the other. In the diagram only one magnet on each side is represented, _n_ and _s_, to prevent confusion. The ends of the coil of wire are attached to the pendulum rod, and they are conducted up it so as to form connection with the wires of the voltaic battery, which are connected with gold studs inserted into a horizontal stage fixed to the clock-case. A small movable bridge, formed of wire, and having the ends tipped with gold or platinum, rests upon the stage, and is shifted from side to side by the pendulum. In these movements the gold points touch and slide over the gold studs in the stage, and thereby make and break contact with the voltaic battery, and alternately send and interrupt an electric current through the coil of wire. [Illustration] Suppose, for instance, that the pendulum is about to rise to the right towards _s_, at which time the voltaic circuit is completed. The coil is, therefore, magnetic, and is attracted by the permanent magnet in _C_. As the pendulum approaches the end of its swing, it pushes the movable bridge away from the gold studs on which it rests, and thus breaks connection with the voltaic battery, and the pendulum descends unrestrained by the attractive force of the magnets. As the pendulum descends towards its lowest point, it shifts the bridge on to the metal studs on the other side, which are so disposed as to send a current through the coil in a direction opposite to the former, so that the poles of the voltaic battery are reversed, and the attractive force is exerted in drawing the pendulum towards the left hand. In this manner the power imparted to the coil, as the pendulum vibrates to and fro, produces a continuous repetition of the attraction on each side alternately, and maintains a constant action.