Principle of electromagnetic induction states that when electric current is flow through a wire, it produce electromagnetic field which is behave same as magnet.

Working of Electromagnetic Electrical Door Bell

         When the switch is pushed 'on', current flows through the coil, and the coil becomes an electromagnet. Consequently, it attracts the metal strip and moves the clanger to hit the bell. However, it also breaks the circuit, which causes the coil to no longer be a magnet, and causes the clanger to move back. The circuit will then be reestablished, and the bell will keep ringing until the switch is released. 

        An electric bell essentially consists of an electro-magnet and a vibrating armature piece that oscillates in front of it. If a current enters the terminal A, it will be conducted through the coils D of the electro-magnet, which has iron cores, and pass 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 is attached by a flexible steel spring to the framework to make this movement possible. In the act of drawing down the armature, however, the contact of the screw with the armature spring is broken, and since this contact forms part of the electric circuit, the current will cease to magnetize the electro-magnet; the armature will, therefore, no longer be attracted and will spring back by virtue of the steel spring, and contact between it and the screw, g, will be restored. Consequently, the same operation will be repeated. Motion takes place very rapidly to and fro, and is communicated to a light hammer, d, that 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 on to work.