What it shows
An electromagnet pulls a springy steel strip towards it, which breaks its own contact, loses the magnetism and springs back to close the contact again. The strip vibrates to and fro, striking a tin lid to ring like a doorbell.
The science
Current in the coil wrapped round the nail makes it a magnet strong enough to attract the steel strip. The circuit runs from the battery through the contact screw, through the strip, through the coil and back. When the strip is pulled towards the nail it leaves the contact screw, the circuit breaks, the nail stops being a magnet, and the strip springs back to the screw. That remakes the circuit and the cycle repeats many times a second, so the hammer on the strip hits the gong repeatedly. This make-and-break action is the heart of every electric bell.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 10 cm iron nail
- About 5 m of thin enamelled copper wire (28 to 30 SWG)
- A short strip of springy steel such as a broken hacksaw blade or a metal ruler, with the teeth taped
- A drawing pin or bolt as the contact screw
- A tin lid or small steel bowl as the gong
- A 2 x AA battery holder with cells (3 V) and a push switch
- A wooden base, screws, hot glue and sandpaper
Safety
For this project: An adult must handle the hacksaw blade (tape the teeth) and screws. Run the bell in short bursts; the coil warms up.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
9 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Wind the enamelled wire tightly around the nail in neat layers, leaving 15 cm free at each end, and sand the enamel off the free ends.
Screw or glue the nail horizontally onto the base so its head faces where the strip will stand.
Fix one end of the steel strip upright in a slot in the base about 3 mm from the nail head, with a small bolt or bead glued on as the hammer.
Mount the contact screw so its tip just touches the back of the strip near its top when the strip is at rest.
Fix the gong so the hammer hits it when the strip is pulled towards the nail.
Wire the battery red lead through the push switch to the contact screw; connect the strip base to one coil wire; connect the other coil wire to the battery black lead.
Press the switch: the strip should snap towards the nail, break contact and spring back, buzzing against the gong.
If nothing happens, check the enamel is sanded off and the strip really touches the screw at rest.
If it sticks to the nail, increase the gap slightly or use fewer turns; adjust until it rings steadily.
What you should see
Pressing the switch makes the strip vibrate and ring the gong continuously until the switch is released.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the strip spring back after being attracted?
Moving away from the screw breaks the circuit, so the nail loses its magnetism and the spring returns it.
2Why use an iron nail and not a steel bolt?
Soft iron loses its magnetism quickly when the current stops; hardened steel would stay magnetised.
3What happens to the ringing if you add more turns of wire?
The electromagnet becomes stronger, pulling harder, so the gap can be larger.
4Why must the enamel be removed from the wire ends?
Enamel is an insulator; without removing it no current flows into the coil.
Take it further
Replace the push switch with the reed switch door sensor project so the bell rings when a door opens.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each