The Magnet with a Switch
Electromagnets and the electric bell
What this lesson covers
Why it matters
Wrap wire around an iron nail, connect a cell — and the nail becomes a magnet. Disconnect — ordinary nail again. A magnet with an ON/OFF switch runs your doorbell right now.
Predict first
An electromagnet stops being a magnet when…
The current MAKES the magnetism. No current, no field — instantly. That obedience is exactly what bells and cranes need.
- the current is switched off — correct
- it gets tired
- never — magnets are forever
What you do
Wind more turns, raise the current, slide in the iron core — build the strongest magnet-with-a-switch you can.
Check yourself
To make an electromagnet stronger,…
Turns × current — plus an iron core — are the three strength knobs.
An electric bell rings again and again because its electromagnet…
Attract hammer → circuit breaks → magnet dies → hammer springs back → circuit remakes… hundreds of times a minute.
Electromagnets in scrapyard cranes are useful because they can…
A permanent magnet would never let go — the switch is the whole job description. (And no, plastic never sticks.)
- add more turns of wire or more current — correct
- paint it red
- use thinner wire only
- switches itself on and off through the vibrating hammer — correct
- stays on continuously
- uses a permanent magnet
- pick up iron AND drop it on command — correct
- attract wood and plastic too
- work without electricity