The electric bell consists essentially of an electro-magnet and a vibrating armature piece which oscillates in front of it. If a current enters the terminal A it is conducted through the coils D D of the electro-magnet, which has iron cores, and passes out through the metal of the armature, the contact screw, g, and the terminal B. Such a current will, of course, cause the electromagnet to draw the armature down on to its terminate, the armature itself being attached by a flexible steel spring to the framework, so as to make this movement possible. In the act, however, of drawing down the armature, the contact of the screw g, and a prolongation of the armature spring, is broken, and as this contact forms part of the electric circuit, the current ceases to magnetize the electro-magnet; the armature is, therefore, no longer attracted and springs back by virtue of the steel spring, and contact between it and the screw, g, is restored, which causes the same operation to be repeated. This motion takes place very rapidly to and fro, and is communicated to a light hammer, d, which strikes the bell, b, and produces a continuous and loud ringing sound. The whole apparatus forms a simple method of drawing attention from a distance, and can always be relied upon to work properly.

The only drawback to the bell is that if the screw, g, is set so as to make it as sensitive as possible, a vibration of the support to which the whole apparatus is fixed, due to a passing train or vehicle, might be sufficient to cause the hammer to strike the gong. In railway signal-boxes, where electric bells are much used, it is absolutely necessary that this should not happen, as it might lead to a serious catastrophe. The bells in this case are, therefore, so arranged that on closing the electric circuit the bell-hammer only strikes the gong once, and only when the circuit is broken again outside the bell does it spring back so as to be ready for a second signal.