With a labelled diagram, explain the construction and working of an electric bell.

1. 1 · State the principle. An electric bell works on the magnetic effect of current: an electromagnet is magnetic only while current flows.
2. 2 · Name the parts.
   • U-shaped electromagnet with a soft iron core
   • Soft iron armature on a spring, with a hammer
   • Gong, contact screw, push button and battery
3. 3 · Draw the diagram. [Sketch: horseshoe electromagnet, armature and spring, hammer near the gong, contact screw, push button, battery; all labelled]
4. 4 · Press the button. Current flows through the coils. The core is magnetised and pulls the armature. The hammer strikes the gong.
5. 5 · The circuit breaks. The armature leaves the contact screw, so the current stops and the core loses its magnetism.
6. 6 · The cycle repeats. The spring pulls the armature back, the circuit is completed again, and the hammer strikes again and again.
7. 7 · Close with a use. The doorbell at home and the school bell work this way.

Scoring tip: The key mark is for explaining how the circuit breaks at the contact screw. Without it, the bell would ring only once.