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. [Illustration: 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. Such pendulums do not act very well, because it is difficult to keep metallic surfaces like _Q_ clean, and therefore misses often occur. Besides, the strength of the current varies with the goodness of the contact and with other things. What is now preferred is to make an arrangement by which an electric current winds the clock up every minute or so. By this means the impulse which drives the clock is not a varying electric one, but is a steady weight. The most successful clocks have been made on these principles. The advantage of electricity is, that by means of the current that actuates the clock, or winds it up, you can at regular intervals set the hands in motion of a great number of clocks. So that only one going clock with a pendulum is needed. The other clocks distributed over the building have only faces and hands, and a very few simple wheels, to which a slight push is given by an electro-magnet, say, every minute or so. The system is therefore well adapted for offices and hotels.