Which scientist first demonstrated that a current-carrying wire deflects a compass needle?

Hans Christian Ørsted first demonstrated that a current-carrying wire deflects a compass needle.

In 1820, the Danish physicist observed that an electric current in a nearby wire disturbed a compass needle. The experiment provided direct evidence that electricity and magnetism are connected. It helped launch the scientific field now called electromagnetism.

Ørsted’s observation quickly inspired further work. André-Marie Ampère developed mathematical descriptions of forces between currents, while Michael Faraday later discovered electromagnetic induction. James Clerk Maxwell eventually unified electric and magnetic phenomena in a set of equations.

The compass deflects because the current produces a magnetic field around the wire. The field direction reverses when the current reverses. The effect can be small when the current is weak or the compass is far from the conductor, which is why early demonstrations required careful positioning.