Question

Explain the construction and working of an electric bell with a labelled diagram. Why is its armature made of soft iron and not steel?

Answer 1

1.

Construction:

• Electromagnet (M): It consists of a horseshoe-shaped soft iron core wound with an insulated copper coil (CC) in opposite directions on its two arms.
• Armature (A): A soft iron strip positioned near the pole pieces of the electromagnet that is free to move.
• Spring Strip (SS): A flexible metallic spring strip attached to the armature (A) that maintains electrical contact with the adjusting screw when the bell is at rest.
• Adjusting Screw (S'): A contact screw that touches the spring strip (SS) to complete the circuit loop.
• Hammer (H) and Gong (G): A metal hammer (H) is fixed to the moving armature to repeatedly strike the large circular metal gong (G).
• Circuit Elements: A DC source (Battery) and a push switch (Key K) connected in series with the electromagnet coil, spring strip, and contact screw.

Working:

When the key (K) is closed, the circuit completes, energising the electromagnet (M). This magnetic field pulls the soft iron armature (A), causing the hammer (H) to strike the gong. The movement causes the spring (SS) to break contact with the screw (S'), interrupting the current. The electromagnet loses magnetism, and the spring returns the armature to its original position, closing the circuit again, allowing the cycle to repeat rapidly for continuous ringing.

2. The armature is made of soft iron rather than steel because soft iron is a ferromagnetic material that quickly magnetises and demagnetises. It loses its magnetism instantly when the current is switched off, ensuring the hammer strikes only when the key is pressed and allowing the mechanism to function rapidly. Steel retains magnetism, which would prevent the mechanism from working properly.