When you push the button of an electric door bell or calling bell it keeps on ringing as long as the button remains pressed.
Do you know how does it function? An electric bell is a simple device based on the magnetic effects of electric current. It is used in offices, houses, industries and for fire alarms.
It consists of a U-shaped electromagnet and a soft-iron armature. The armature has a small hammer for striking the gong. This hammer hits the gong repeatedly and produces sound. The gong is made of a metal. For operating the bell, a push button is pressed. In an electric bell, the button is a switch that connects the supply of electricity to the bell.
When the button of the bell is pressed, the current flows through electromagnet winding, armature, contact spring and the contact screw. The flow of the current magnetizes the soft-iron core of the electromagnet. This attracts the armature, causing the attached hammer to strike the metal gong and thereby produce sound.
As the armature moves forward due to magnetic attraction the contact spring moves away from the contact screw. This breaks the circuit and the current stops flowing. As a result, the soft-iron core loses its magnetism. It, therefore, no longer attracts the armature which, then, is pulled back by the contact spring to its original position. As soon as the armature comes to its original position the electric circuit is again completed and the soft iron becomes magnetized. It again attracts the armature and thereby the hammer strikes against the gong and produces sound. As long as the push button remains pressed, the circuit is alternately broken and completed causing the hammer to strike the gong. Thus an electric bell keeps ringing.
If a steel core is used instead of a soft-iron, then the steel core will become a permanent magnet due to passage of electric current through the winding. Consequently, the armature will stay attracted even when the contact spring moves away from the contact screw, so the hammer will strike the gong only once.