Which scientist discovered that a compass needle is deflected by an electric current in 1820?

Hans Christian Ørsted discovered that an electric current deflects a compass needle in 1820.

Ørsted observed that a nearby current-carrying wire affected a magnetic compass needle. The result showed a direct connection between electricity and magnetism, overturning the idea that the two subjects were entirely separate. His report quickly became a foundation for the developing science of electromagnetism.

The deflection depends on the direction of current and the position of the wire. Reversing the current reverses the magnetic-field direction around the wire. The observation later inspired André-Marie Ampère’s mathematical studies of electrodynamic forces and influenced Michael Faraday’s investigations.

Ørsted’s name is also preserved in the oersted, a unit used for magnetic field strength in the centimetre–gram–second system. The SI unit for magnetic field strength is the ampere per metre.