Amongst the many little worries of domestic life is the keeping in order of the various clocks. It ought to be a very simple matter to remember to wind up a clock, but curiously enough almost everybody forgets to do so now and then. We gaze meditatively at the solemn-looking machine ticking away on the mantelpiece, wondering whether we wound it up last week or not; and we wish the wretched thing would go without winding, instead of causing us all this mental effort.
There is usually a way of getting rid of little troubles of this kind, and in this case the remedy is to be found in an electrically-driven clock. The peculiar feature about clocks driven by electricity is that they reverse the order of things in key-wound clocks, the pendulum being made to drive the clockwork instead of the clockwork driving the pendulum. No driving spring is required, and the motive power is supplied by a small electro-magnet.
The actual mechanism varies considerably in different makes of clock. In one of the simplest arrangements there is a pendulum with an armature of soft iron fixed to the extremity of its bob. Below the pendulum is an electro-magnet, and this is supplied with current from a small battery of dry cells. A short piece of metal, called a “pallet,” is attached to the rod of the pendulum by means of a pivot; and as the pendulum swings it trails this pallet backwards and forwards along a horizontal spring. In this spring are cut two small notches, one on each side of the centre of the swing. As long as the pendulum is swinging sufficiently vigorously, the pallet slides over these notches; but when the swing has diminished to a certain point the pallet catches in one or other of the notches. This has the effect of pressing down the spring so that it touches a contact piece just below, and the battery circuit is then completed. The electro-magnet now comes into action and attracts the armature, thus giving the pendulum a pull which sets it swinging vigorously again. The spring is then freed from the pressure of the pallet, and it rises to its original position, so that the circuit is broken. This puts out of action the electro-magnet, and the latter does no further work until the pendulum requires another pull. The movement of the pendulum drives the wheelwork, which is similar to that of an ordinary clock, and the wheelwork moves the hands in the usual way. A clock of this kind will run without attention for several months, and then the battery requires to be renewed. As time-keepers, electrically-driven clocks are quite as good as, and often very much better than key-wound clocks.
Everybody must have noticed that the numerous public clocks in a large town do not often agree exactly with one another, the differences sometimes being quite large; while even in one building, such as a large hotel, the different clocks vary more or less. This state of things is very unsatisfactory, for it is difficult to know which of the clocks is exactly right. Although large clocks are made with the utmost care by skilled workmen, they cannot possibly be made to maintain anything like the accuracy of a high-class chronometer, such as is used by navigators; and the only way to keep a number of such clocks in perfect agreement is to control their movements from one central or master clock. This can be done quite satisfactorily by electricity. The master-clock and the various sub-clocks are connected electrically, so that a current can be sent from the master-clock to all the others. Each sub-clock is fitted with an electro-magnet placed behind the figure XII at the top of the dial. At the instant when the master-clock reaches the hour, the circuit is closed automatically, and the current energizes these magnets. The minute hands of all the sub-clocks are gripped by the action of the magnets, and pulled exactly to the hour; the pulling being backward or forward according to whether the clocks are fast or slow. In this way all the clocks in the system are in exact agreement at each hour. The same result may be attained by adjusting all the sub-clocks so that they gain a little, say a few seconds in the hour. In this case the circuit is closed about half a minute before the hour. As each sub-clock reaches the hour, its electro-magnet comes into action, and holds the hands so that they cannot proceed. When the master-clock arrives at the hour the circuit is broken, the magnets release their captives, and all the clocks move forward together.
It is possible to control sub-clocks so that their pendulums actually beat exactly with the pendulum of the master-clock; but only a small number of clocks can be controlled in this way, and they must be of the best quality. The method is similar to that used for hourly corrections, the main difference being that the circuit is closed by the pendulum of the master-clock at each end of its swing, so that the pendulums of the sub-clocks are accelerated or held back as may be required.
In the correcting systems already described the sub-clocks are complete in themselves, so that they work quite independently, except at the instant of correction. For hotels, schools, and other large buildings requiring clocks at a number of different points, a simpler arrangement is adopted. Only one complete clock is used, this being the master-clock, which may be wound either electrically or by key. The sub-clocks are dummies, having only a dial with its hands, and an electro-magnetic arrangement behind the dial for moving the hands. The sub-clocks are electrically connected with the master-clock, and the mechanism of this clock is arranged to close the circuit automatically every half-minute. Each time this occurs the magnet of each sub-clock moves forward the hands half a minute, and in this way the dummy clocks are made to travel on together by half-minute steps, exactly in unison with the master-clock.