Fig. 24.—Mechanism of Electric Bell.
Fig. 25.—Diagram showing principle of Bell-push.
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.