PLATE X.
By permission of
Vickers Limited.
WHERE ELECTRICAL MACHINERY IS MADE.
When an outbreak of fire is discovered it is of the utmost importance that the nearest fire-station should be notified instantly, for fire spreads with such rapidity that a delay of even a few minutes in getting the fire-engines to the spot may result in the total destruction of a building which otherwise might have been saved. In almost all large towns some system of public fire alarms is now in use. The signal boxes are placed in conspicuous positions in the streets, and sometimes also in very large buildings. The alarm is generally given by the starting of a clockwork mechanism which automatically makes and breaks a circuit a certain number of times. When this occurs an alarm bell rings at the fire-station, and the number of strokes on the bell, which depends upon the number of times the alarm mechanism makes and breaks the circuit, tells the attendant from which box the alarm has been given. One well-known form of call box has a glass front, and the breaking of the glass automatically closes the circuit. In other forms turning a handle or pulling a knob serves the same purpose.
It is often required to maintain a room at one particular temperature, and electricity may be employed to give an alarm whenever the temperature rises above or falls below a certain point. One arrangement for this purpose consists of an ordinary thermometer having the top of the mercury tube fitted with an air-tight stopper, through which a wire is passed down into the tube as far as the mark indicating the temperature at which the alarm is desired to sound. Another wire is connected with the mercury in the bulb, and the free ends of both wires are taken to a suitable battery, a continuous-ringing bell being inserted in the circuit at some convenient point. If a rise in temperature takes place the mercury expands and moves up the tube, and at the critical temperature it touches the wire, thus completing the circuit and sounding the alarm. This arrangement only announces a rise in temperature, but by making the thermometer tube in the shape of a letter U an alarm may be given also when the temperature falls below a certain degree. A device known as a “thermostat” is also used for the same purpose. This consists of two thin strips of unlike metals, such as brass and steel, riveted together and suspended between two contact pieces. The two metals expand and contract at different rates, so that an increase in temperature makes the compound strip bend in one direction, and a decrease in temperature makes it bend in the opposite direction. When the temperature rises or falls beyond a certain limit the strip bends so far as to touch one or other of the contact pieces, and the alarm is then given. Either of the preceding arrangements can be used also as an automatic fire alarm, or if desired matters may be arranged so that the closing of the circuit, instead of ringing a bell, turns on or off a lamp, or adjusts a stove, and in this way automatically keeps the room at a constant temperature.
Electric alarms operated by ball floats are used to some extent for announcing the rise or fall beyond a pre-arranged limit of water or other liquids, and there is a very ingenious electrical device by which the level of the water in a tank or reservoir can be ascertained at any time by indicators placed in convenient positions any distance away.
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.