The Decisive Twitch

In 1820, Danish physicist Hans Christian Oersted was running an electric current through a wire during a lecture when a nearby compass needle twitched every time he switched the current on and off. He had long suspected a link between electricity and magnetism — and here was the proof: electricity was creating magnetism.

The rule is elegant: a moving electric charge creates a magnetic field around itself. Coil the wire into loops and the fields combine into a stronger magnet. Wrap hundreds of coils around an iron core and you get an electromagnet — a magnet you can switch on and off.