In factories and other large buildings a watchman is frequently employed to make a certain number of rounds every night. Being human, a night-watchman would much rather sit and snooze over his fire than tramp round a dark and silent factory on a cold winter night; and in order to make sure that he pays regular visits to every point electricity is called in to keep an eye on him. A good eight-day clock is fitted with a second dial which is rotated by the clockwork mechanism, and a sheet of paper, which can be renewed when required, is placed over this dial. On the paper are marked divisions representing hours and minutes, and other divisions representing the various places the watchman is required to visit. A press-button is fixed at each point to be visited, and connected by wires with the clock and with a battery. As the watchman reaches each point on his rounds he presses the button, which is usually locked up so that no one else can interfere with it, and the current passes round an electro-magnet inside the clock case. The magnet then attracts an armature which operates a sort of fine-pointed hammer, and a perforation is made in the paper, thus recording the exact time at which the watchman visited that particular place. The current for ordinary electric bells is generally supplied by Leclanché cells, which require little attention, and keep in good working order for a very long time. As we saw in Chapter IV., these bells soon polarize if used continuously, but as in bell work they are required to give current for short periods only, with fairly long intervals of rest, no trouble is caused on this account. These cells cannot be used for burglar or other alarms worked on the closed-circuit principle, and in such cases some form of Daniell cell is usually employed. CHAPTER XV ELECTRIC CLOCKS 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.