Run electricity through a wire and the wire becomes slightly magnetic. Coil that wire around a nail so the little pushes stack up, and the nail becomes a proper magnet — until you cut the power, when it instantly forgets.
Most people think an electromagnet's power comes mainly from the battery's size. In fact strength scales with ampere-turns, turns multiplied by current, and sliding an iron core inside multiplies the field a hundredfold by aligning its own domains.
What's actually happening
In 1820, Hans Christian Ørsted noticed a compass needle twitch every time he switched a nearby current on — the first hint that electricity and magnetism are one phenomenon. Every wire carrying current wraps itself in a circular magnetic field. One straight wire's field is feeble; the trick is to cheat by geometry.
Coil the wire into loops and every loop's field points the same way through the middle — they stack. Ten turns, ten times the field; the strength scales with turns × current, the "ampere-turns". Slide an iron nail into the coil and it amplifies things a hundredfold more: iron is full of tiny pre-magnetised domains that snap into alignment with the coil's field and add their own. A nail, a metre of wire, and a battery make a magnet that out-pulls anything on your fridge.
The superpower isn't strength — it's the switch. A permanent magnet can never be turned off; an electromagnet obeys a button, can reverse its poles by reversing the current, and can vary smoothly from nothing to maximum. That controllability is the beating heart of nearly everything electric that moves: motors are electromagnets timed to keep chasing each other, relays are switch-flipping magnets, scrapyard cranes lift and release cars on command, and a loudspeaker is an electromagnet wiggling a paper cone to the shape of your music.
An electromagnet's real superpower isn't strength but the off switch — controllable magnetism is the beating heart of every motor, relay and speaker.
- 1Wrap a metre of thin insulated wire around a large iron nail, 20+ neat turns, leaving two free ends.
- 2Hold the bare wire ends against a AA battery's terminals (briefly — it warms up) and dip the nail into a tray of paperclips.
- 3Lift, count your catch, then let go of one wire end mid-air. Every clip drops at once. Add ten more turns and beat your record.
Common questions
Strength scales with ampere-turns — the number of coil turns multiplied by the current. Add more turns or more current, and slide an iron core inside, which amplifies the field a hundredfold or more by aligning its own magnetic domains.
A permanent magnet can never be turned off. An electromagnet obeys a switch, can reverse its poles by reversing the current, and can vary smoothly from nothing to maximum — the control behind motors, relays and scrapyard cranes.
Hans Christian Ørsted, in 1820, when he noticed a compass needle twitch every time he switched a nearby current on — the first hint that electricity and magnetism are one phenomenon.