In Hans Christian Ørsted’s 1820 experiment, a compass needle deflected when an electric current flowed through a nearby wire.
Ørsted noticed the effect during a lecture demonstration in Copenhagen. With the circuit open, the compass pointed toward magnetic north as usual. When current passed through a wire placed above the compass, the needle moved away from that direction. Reversing the current reversed the direction of the deflection.
This observation provided the first clear evidence that electricity and magnetism are linked. The wire did not simply attract the compass like a bar magnet; the needle responded according to the direction of the current and its position around the wire.
Ørsted’s discovery quickly inspired work by André-Marie Ampère and others. It helped establish the field of electromagnetism and eventually contributed to technologies including electric motors, generators, and transformers. The SI unit of magnetic field strength, the oersted, is named after him.