Electric bell (Construction and working mechanism)

An electric bell is a device in which to and fro movement of a smaller hammer on a gong produces the sound of a bell.
It works on the principle of magnetic effect of current, i.e. electromagnetism.
It is an important application of an electromagnet which is used in offices, houses, schools, alarm equipment etc.

Construction and connections:

It consists of a U shaped electromagnet in front of which is a soft iron strip or armature supported by a spring.
The spring is connected with an adjustable screw.
An iron hammer is attached to the armature or iron strip which can move to and fro depending on the movement of the iron strip.
The armature carries at its back a strip of flexible steel which in normal position remains in contact with a screw.
A metallic gong is fixed to the base board and hammer (striker) may strike on it.
One end of the coil of the electromagnet is connected to terminal T₁ and the other end is connected with the spring which through the adjustable screw is connected to the terminal T₂.
When the terminals are connected to a battery and the switch is pressed, the current flows through contact points via electromagnet.

Working mechanism:

When a push button fitted in battery is pressed, the current flows through the contact points via electromagnet.
The U shaped electromagnet is magnetized and it attracts the armature towards it.
The hammer fitted with strip gives a strike on the gong producing sound.
The moment this happens, contact of the attached spring with screw is broken, i.e. the circuit is broken.
As a result, current stops flowing and electromagnet is demagnetized.
Due to this action of the spring, armature comes to its previous position, i.e. comes in contact with the screw and the circuit is again completed.
Each approach of the armature or strip causes the hammer to strike the gong.
Thus, same process is repeated many times and the bell goes on ringing.

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