I do not propose here to describe the striking mechanism of clocks. There are several different ways of arranging it. They are rather complicated to follow out, but they all resolve themselves into a few simple principles. As the hour hand revolves it carries a cam so arranged as to be deeper cut away for the twelfth hour, less for the eleventh, and so on. When the minute hand comes to the hour it releases the striking mechanism, which, urged by a weight, begins to revolve, and, driving an arm carrying a pin, raises a hammer, which goes on striking away as the arm revolves. This would continue for ever if it were not that at the same moment an arm is liberated which falls against the cam. At each stroke the arm is (by the striking apparatus) raised a bit back into position. When it comes back into position it stops the striking. It thus acts as a counter, or reckoner of the blows given, stopping the movement when the clock has struck sufficiently. If the counting mechanism fails to act, we have the phenomenon which occasionally occurs of a “Grandfather” clock striking the whole of the hours for the week without stopping.
A chiming clock is simpler still. For here we have a barrel covered with pins, like the barrel in a musical box. As the pins go round they raise hammers which fall against bells. The barrel is wound up and driven by a spring or weight. When the clock comes to the hour, the barrel is released, and rotating, plays the tune.
If you want to make a clock wake you up in the morning it can be done by making the striking arrangement hammer away with no counting mechanism to stop it until the weight has run down. If, not content with that, you want the sheets pulled off the bed or the bed tilted up, or a can of water emptied over the person who will not rise, a mechanical device known as a relay must be used. It is very simple. What is wanted is that, after the lapse of a time which a clock must measure, a considerable force must be exerted to pull off the bedclothes. It would be absurd to make the clock exercise this pull. It is obviously better to attach the clothes by a hook to a rope which passes over a pulley, and from which hangs a weight. A pin secures the weight from falling, the pin being withdrawn by the clock. The work is thus done by the weight when released by the clock.
In like manner, if you have a telegraph designed to print messages at a distance, you do not send along the wires the whole force necessary for doing the printing. You only send impulses, which, like triggers, release the forces by which the letters are to be stamped.
Electric clocks of many kinds have been invented. The principle of an electric escapement is similar to that of an ordinary escapement.
Fig. 68.
The reader no doubt knows that, when a circuit of wire is joined or completed leading to a source of electricity, electricity flows through the wire.
If the wire is wound round a piece of iron, then, whenever the circuit is joined, a current is set in motion, and the iron becomes an electro-magnet. When the circuit is severed the iron ceases to be a magnet.
If put at a proper position it would at each time an iron pendulum approached give it a small impulse provided that at that instant the current is turned on. This can easily be made to be done by the pendulum itself. For just as the pendulum is coming back to the central position a flipper P attached to the rod can be caused to make contact with a piece of metal fixed on its path. Then the electro-magnet, becoming magnetised, exerts a pull on the iron pendulum. On the return beat of the pendulum the other side of the flipper R strikes the obstruction. But if that side R is covered with ebonite or some non-conducting material no current will be set in motion, and the electro-magnet will not (as it would otherwise do) retard the pendulum. Such a pendulum has therefore an impulse given to it every second beat.