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.