The most familiar of all electrically worked appliances is probably the electric bell, which in some form or other is in use practically all over the world. Electric bells are operated by means of a current of electricity sent through the coils of an electro-magnet, and one of the very simplest forms is that known as the single-stroke bell. In this bell an armature or piece of soft iron is placed across, but at a little distance from, the poles of an electro-magnet, and to this piece of iron is fixed a lever terminating in a sort of knob which lies close to a bell or gong. When a current is sent round the electro-magnet the armature is attracted, so that the lever moves forward and strikes a sharp blow upon the gong. Before the gong can be sounded a second time the current must be interrupted in order to make the magnet release the armature, so that the lever may fall back to its original position. Thus the bell gives only one ring each time the circuit is closed. Bells of this kind may be used for signalling in exactly the same way as the Morse sounder, and sometimes they are made with two gongs of different tones, which are arranged so as to be sounded alternately.
For most purposes however another form called the trembler bell is much more convenient. Fig. 24 is a rough diagram of the usual arrangement of the essential parts of a trembler bell. When the circuit is closed by pressing the bell-push, a current flows from the battery to the electro-magnet EE, by way of terminal T. The electro-magnet then attracts the soft iron armature A, thus causing the hammer H to strike the gong. But immediately the armature is pulled away from the terminal T¹ the circuit is broken and the magnet loses its attraction for the armature, which is moved back again into contact with T¹ by the spring S. The circuit is thus again closed, the armature is again attracted, and the hammer strikes the gong a second time. This process goes on over and over again at a great speed as long as the bell-push is kept pressed down, resulting in an extremely rapid succession of strokes upon the gong. It will be noticed that the working of this bell is very similar to that of the automatic contact-breaker used for induction coils (Chapter VIII.). For household purposes this form of bell has completely driven out the once popular wire-pulled bell. Bell-pushes are made in a number of shapes and forms, and Fig. 25 will make clear the working principle of the familiar form which greets us from almost every doorway with the invitation, “Press.” In private offices and elsewhere the rather aggressive sound of an ordinary trembler bell is apt to become a nuisance, and in such cases a modified form which gives a quiet buzzing sound is often employed.
It is frequently necessary to have an electric bell which, when once started, will continue ringing until it is stopped. Such bells are used for fire and burglar alarms and for many other similar purposes, and they are called continuous-ringing bells as distinguished from the ordinary trembler bells. In one common form of continuous-ringing bell two separate batteries are used, one to start the bell and the other to keep it ringing. When a momentary current from the first battery is sent over the bell lines the armature is attracted by the electro-magnet, and its movement allows a lever to fall upon a metal contact piece. This closes the circuit of the second battery, which keeps the bell ringing until the lever is replaced by pulling a cord or pressing a knob. Continuous-ringing bells are often fitted to alarm clocks. The alarm is set in the usual way, and at the appointed hour the bell begins to ring, and goes on ringing until its owner, able to stand the noise no longer, gets out of bed to stop it.
There is another form of electric bell which has been devised to do away with the annoyance of bells suddenly ceasing to work on account of the failure of the battery. In this form the battery is entirely dispensed with, and the current for ringing the bell is taken from a very small dynamo fitted with a permanent steel horse-shoe magnet. The armature is connected to a little handle, and current is generated by twisting the handle rapidly to and fro between the thumb and finger. A special form of bell is required for this arrangement, which is not in general use.
In the days of wire-pulled bells it was necessary to have quite a battery of bells of different tones for different rooms, but a single electric bell can be rung from bell-pushes placed in any part of a house or hotel. An indicator is used to show which push has been pressed, and, this like the bell itself, depends upon the attraction of an armature by an electro-magnet. Before reaching the bell the wire from each bell-push passes round a separate small electro-magnet, which is thus magnetized by the current at the same time that the bell is rung. In the simplest form of indicator the attraction of the magnet causes a little flag to swing backwards and forwards over its number. Another form is the drop indicator, in which the movement of the armature when attracted by the magnet allows a little flag to drop, thus exposing the number of the room from which the bell was rung. The dropped flag has to be replaced, either by means of a knob fixed to a rod which pushes the flag up again, or by pressing a push which sends the current through another little electro-magnet so arranged as to re-set the flag.
The electric current is used to operate an almost endless variety of automatic alarms for special purposes. Houses may be thoroughly protected from undesired nocturnal visitors by means of a carefully arranged system of burglar alarms. Doors and windows are fitted with spring contacts so that the slightest opening of them closes a battery circuit and causes an alarm to sound, and even if the burglar succeeds in getting inside without moving a door or window, say by cutting out a pane of glass, his troubles are not by any means at an end. Other contacts are concealed under the doormats, and under the carpets in passages and stairways, so that the burglar is practically certain to tread on one or other of them and so rouse the house. A window may be further guarded by a blind contact. The blind is left down, and is secured at the bottom to a hook, and the slightest pressure upon it, such as would be given by a burglar trying to get through the window, sets off the alarm. Safes also may be protected in similar ways, and a camera and flashlight apparatus may be provided, so that when the burglar closes the circuit by tampering with the safe he takes his own photograph.
The modern professional burglar is a bit of a scientist in his way, and he is wily enough to find and cut the wires leading to the contacts, so that he can open a door or window at his leisure without setting off the alarm. In order to circumvent this little game, burglar alarms are often arranged on the closed-circuit principle, so that the alarm is sounded by the breaking of the circuit. A burglar who deftly cut the wires of an alarm worked on this principle would not be particularly pleased with the results of his handiwork. The bells of burglar alarms may be arranged to ring in a bedroom or in the street, and in the United States, where burglar and in fact all electric alarms are in more general use than in England, large houses are sometimes connected to a police station, so that the alarm is given there by bell or otherwise.
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.