Electric Bells

Electric Bells are those that are set ringing by means of an electric current. The ordinary type of trembler-bell now so generally adopted in houses consists of a small electro-magnet that will attract an iron keeper whenever a current is passed round the coils of the magnet. The keeper is so placed that when it is attracted the electric circuit is broken, the current stopped, and the keeper pulled back by a spring. As soon as it is so pulled back the circuit is closed, and the current flows round the magnet, only to be broken when the keeper is drawn up. Thus an intermittent current flows round the coils, and' the keeper oscillates backwards and forwards so long as the battery is kept in circuit. The keeper is attached to the hammer of the bell, and its oscillation causes a series of taps. The battery is put in circuit usually by pressing a small button which by joining two wires connects the battery with the bell. Usually Leclanche cells are used for these bells, for they last very well, and supply a powerful electromotive force if used only occasionally. Electric bells adapt themselves readily as alarums. There are electric fire-alarms, for instance, which start ringing when the temperature of the protected region becomes high. This ringing is effected by an electric circuit being completed at that juncture; two wires may remain separate so long as they are cool, but any undue expansion by heat may cause them to touch each other, and so close an electric circuit. The same principle of forming an electric circuit is used for burglar alarms and similar contrivances.