How did a twitching compass needle reveal that electricity and magnetism are linked?

In 1820 the Danish physicist Hans Christian Ørsted found that a compass needle swings away from magnetic north when an electric current runs through a wire nearby. That simple twitch showed that an electric current creates a magnetic field, confirming a direct relationship between electricity and magnetism.

The popular story says he stumbled on it during a lecture, by chance. That is a myth. Ørsted had been hunting for a link since 1818, and his early results were so confusing that he was unsure what he had. He had good reasons to look: he had met a physicist who believed the two forces were connected, and he held a Kantian conviction about the unity of nature. Volta's voltaic pile of 1800 had inspired him to start experimenting.

What did he find? At first he thought the effect spread out from all sides of the wire like light or heat. Three months later he looked harder and saw the truth: a current produces a circular magnetic field as it flows through a wire. The needle did not point toward the wire; it swung sideways, to lie along a circle around it.

News spread fast. The Royal Society awarded him its Copley Medal in 1820, and in September François Arago demonstrated the discovery to the French Academy. Ampère began developing a mathematical theory. A compass needle had just shown that electricity could push on a magnet, and the obvious question was whether a magnet could, in turn, produce electricity.