Model Electric Bell

This demonstration models the principle of an electric bell using an electromagnet and a vibrating steel strip. When current flows through the coil, the strip is attracted, breaking the circuit. The strip then springs back, re-closing the circuit, and the cycle repeats to produce continuous vibration.

1. Wind about twenty turns of PVC-covered copper wire around one arm of an iron C-core.
2. Connect one end of the coil to a low-voltage DC supply (about 1 V).
3. Fix a hacksaw blade or a strip of hard steel under another supply terminal so that it projects outward. Attach a small mass (e.g., Plasticine) to the free end to slow vibrations.
4. Position the C-core just beneath the projecting blade, close enough that the blade can be pulled downward but not touching.
5. Connect the other end of the coil to the remaining terminal of the supply using a short wire. Tape the bare end of this short wire along the blade so that the coil’s free end touches it lightly.
6. Switch on the supply. The coil becomes an electromagnet, pulling the blade downward. This breaks contact, switching off the coil. The blade springs back up, restoring contact and re-energizing the coil. The cycle repeats, causing vibrations.
7. Observe the vibrating blade as an analogue of how an electric bell repeatedly strikes.

Large model electric bell demonstration - Physics with Simon Poliakoff:

📄 A model electric bell - IOP: https://spark.iop.org/model-electric-bell

• Purchase an electric bell demonstration kit.
• Add a small metal striker to the end of the vibrating blade to hit a bell and make the demonstration more realistic.
• Vary the number of coil turns to show how electromagnet strength affects the vibration speed.
• Try different blade materials (steel vs aluminum) to show that only ferromagnetic materials work effectively.
• Adjust the added mass to change the vibration frequency.

• Use a low-voltage DC supply (1–2 V) to keep the current safe.
• Ensure the hacksaw blade has no sharp teeth exposed, or replace with a smooth steel strip.
• Do not allow wires to short-circuit across supply terminals.
• Keep the electromagnet connections secure to avoid sparking.

• Why does the blade repeatedly move up and down? (The coil magnetizes only when current flows; motion breaks and remakes the circuit, producing vibration.)
• How does this relate to the ringing of a real electric bell? (In a bell, the vibrating arm repeatedly strikes a bell dome while current cycles on and off.)
• What role does the added Plasticine mass play? (It slows down vibrations, making the effect easier to observe.)
• What would happen if the blade were made of copper or plastic instead of steel? (They would not be attracted to the electromagnet, so the device would not work.)
• How would increasing the number of turns on the coil affect the electromagnet’s pull? (It would strengthen the field and increase the attraction to the blade.)