The invention of Electro-Magnetic Clocks closely followed the introduction of the electric telegraph; and Professor Wheatstone, to whom the world is principally indebted, in conjunction with Mr. Cooke, for the perfection and application of the needle telegraphic instrument, claims also to be the original inventor of Electro-Magnetic Clocks. His claim is, however, disputed by Mr. Bain, who asserts that he was the first who conceived the idea of applying the power of electro-magnets to the regulation and movements of clocks, and it must be admitted that he brought the invention into a working state.

In the first stage of the invention, the object attempted to be attained was to regulate several clocks, once an hour—or oftener, if required—so that they might all indicate precisely the same time. For this purpose Mr. Bain took for a standard time-keeper a clock of the best possible construction, placed in circumstances favourable to maintaining accuracy. The minute-hand of his clock, the instant that it pointed to the hour, made connection with a voltaic battery that brought into action a series of electro-magnets attached to the clocks to be regulated; one of them being fixed on the top of each clock. Its momentary action was made to collapse a pair of clippers, which in closing seized the minute-hand of the clock to which it was attached, and brought it to the hour point. Thus all the clocks in the series could be regulated every hour, for the collapse of the clippers pushed the hand forward if it were too late, or thrust it back if it had gained. Mr. Bain contemplated the application of this contrivance to all the public clocks of a town, by having wires laid down in the streets to connect them in one voltaic circuit. Such a plan would, however, have involved greater expense and trouble in its accomplishment than the object seemed to merit; but the regulation of any number of clocks in a large establishment might have been practicable by that means. We are not aware, however, that this mode of regulating clocks by electricity was ever adopted, and it has since been superseded by an arrangement made by Mr. Shepherd, junior, to be presently noticed.

Improving on this first application of electro-magnetism to the regulation of clocks, Mr. Bain afterwards employed the power to keep the clocks in action, so that each clock might be propelled by magnets alone, without any weight, and without the ordinary train of wheels.

Every one acquainted with the mechanism of a clock is aware that the weight communicates motion to a train of wheels, and that the movement is regulated by the vibration of a pendulum, which is acted on by the last wheel of the train. That wheel, called the escapement, is so formed, that each tooth catches in succession into a detent fixed on the pendulum near the point of suspension, which allows one tooth to pass at each double vibration. The pendulum, therefore, governs the movement of the train of wheels by checking the escapement, and allowing the teeth to pass one by one; and as pendulums of given lengths vibrate in given times, if their actions be not interfered with, the clocks will keep regular time. But the pressure of the escape-wheel against the detent, and the consequent friction, prevent the pendulum from acting freely. In the best made clocks there are special contrivances to detach the pendulum as much as possible from the wheels, and likewise to compensate for variations in the length of the pendulum by change of temperature.

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.

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.

The only wheels required in a clock of this kind are those which turn the hands; and the motion is communicated from the pendulum to the seconds wheel by means of a small attached lever, working on a ratchet wheel. The minute and the hour hands derive their movements from the seconds wheel in the usual manner.

The voltaic battery employed to work Mr. Bain's clocks consists of a pair of large copper and zinc plates buried in the moist earth, which excite a sufficient amount of electricity to maintain the motion of the pendulum. A battery of this kind will remain in action a long time, and will serve to keep a clock going for several months. It is, indeed, a near approach to the attainment of perpetual motion, since nothing but the wearing away of the materials, or the accumulation of dust on the connecting points, seems to prevent the realization of that mechanical chimera.

There is a disadvantage attending the arrangement of Mr. Bain's clocks, arising from the attachment of the pendulum to the wheels; and as the moving force is derived directly from voltaic electricity, any variation in the power of the battery causes variation in the lengths of the vibrations, and produces irregularity. For the purpose of remedying these defects, Mr. Shepherd, junior, has adopted an arrangement which detaches the pendulum from the clock movement, and makes its vibrations altogether independent of the varying force of voltaic batteries.

In Mr. Shepherd's arrangement, the impulse of the pendulum is given by successive blows from a spring, which is drawn back and then liberated at each vibration. The hands of the clock are also moved by electro-magnets, by which means the impelling forces and the resistances encountered by the pendulum are always constant. By making the pendulum thus independent of the works, and employing it merely to make and break contract at regular intervals, any number of clocks in the same establishment may be set in motion, and kept exactly together, by a single pendulum.

The large clock over the principal entrance to the Great Exhibition was on this construction. It would have been impossible, with any approach to regularity, to have moved hands of that size, exposed as they were to the wind, unless the pendulum had been independent of such resistances.

Electro-Magnetic Clocks have not yet come into general use, partly owing to imperfections in the battery connections, which occasionally put a stop to their movements, but principally on account of the high prices charged by the patentees. As no trains of wheels are requisite in an Electro-Magnetic Clock, it might be manufactured very cheaply; and when the price is reduced to its proper standard, and the trifling practical defects are remedied, these clocks may possibly supersede others.