Experiments on the Effect of a Current of Electricity on the Magnetic Needle
A current-carrying wire turns a compass needle — electricity and magnetism are one.
A teacher switched on a battery, and the compass on the bench twitched. With that twitch, electricity and magnetism stopped being two things.
The big idea
A compass needle points north because the Earth is a faint magnet. Oersted found that an electric current is a magnet too: send a current down a wire, hold a compass near it, and the needle turns — not toward the wire, but to lie crosswise around it, as if the wire were the centre of invisible magnetic rings. Switch the current off and the needle drifts back north. Reverse the current and the needle turns the other way. Electricity in motion makes magnetism — the two were secretly one force all along.
How it came about
It happened in Copenhagen in the spring of 1820, reportedly during a lecture, when Oersted noticed a compass needle stir as he closed a circuit. But it was not really luck. Oersted belonged to a school of thought — the Romantic “philosophy of nature” — that insisted all the forces of nature are deep down the same, and he had been hunting for a tie between electricity and magnetism for years. Others, who were sure the two had nothing to do with each other, would not have looked.
He spent months making sure, then wrote it up in a terse four-page pamphlet in Latin and mailed it to scientists across Europe in July 1820. The reaction was explosive. In Paris, within a week of hearing the news, André-Marie Ampère was building on it; by autumn the basic laws of the new science of electromagnetism were being written.
Why it mattered
For centuries electricity and magnetism had been studied in separate rooms. Oersted knocked down the wall between them in a single afternoon's demonstration. That union is the seed of the entire electrical world: because a current can make a magnetic force, you can build a motor that spins, an electromagnet that lifts, a loudspeaker that pushes air, a telegraph that clicks. Eleven years later Michael Faraday found the mirror image — that a moving magnet makes a current — and the two halves together became the electric age. His name now marks a unit of magnetism: the oersted.
An everyday picture
Picture water draining from a bath: it doesn't rush straight at the plughole, it spirals around it in rings. Oersted's wire is the drain, and the magnetism it makes wraps around it in just that circular way — which is why the needle lines up sideways instead of pointing at the wire. The compass is a tiny boat that turns to sit along the swirl. Move to the other side of the wire, or reverse the flow, and the swirl runs the opposite way, so the boat turns around.
Where it sits
Oersted's experiment needed something only twenty years old: Volta's pile (1800), the first source of steady current. Once that current was available, the discovery was waiting to be made. It set off a chain — Ampère's electrodynamics, Faraday's induction (1831), and finally Maxwell's equations (1865), which fused electricity, magnetism and light into one theory. Almost every electrical machine you use is a great-grandchild of the moment a Danish compass needle refused to point north.
The first experiments respecting the subject which I mean at present to explain, were made by me last winter, while lecturing on electricity, galvanism, and magnetism, in the University.
It is sufficiently evident from the preceding facts that the electric conflict is not confined to the conductor, but dispersed pretty widely in the circumjacent space.
From the preceding facts we may likewise infer that this conflict performs circles; for without this condition it seems impossible that the one part of the uniting wire, when placed below the magnetic pole, should drive it towards the east, and when placed above it towards the west; for it is the nature of a circle that the motions in opposite parts should have an opposite direction.