Hans Christian Ørsted

The compass is a field detector.

Ørsted placed a conducting wire near a magnetic compass. With the circuit open, the compass aligned mainly with Earth's magnetic field. When current was switched on, the needle rotated and tended toward a direction at right angles to the wire.

No mechanical connection linked the wire to the needle. The current had produced a magnetic field strong enough to exert torque on the compass magnet. Reversing current reversed the field direction and therefore reversed the compass deflection.

The field forms loops around the wire.

For a long straight conductor, magnetic field lines are concentric circles centred on the wire. Field strength is greater close to the conductor and weaker farther away. A Hall probe can measure magnitude; a compass reveals direction.

Use the right-hand grip rule with conventional current: point the thumb along the current and the curled fingers show the direction of the magnetic loops. This is a directional rule, not a claim that a physical spiral exists around the wire.

Electricity and magnetism become one subject.

Before this experiment the two phenomena were often studied separately. Ørsted supplied an observable bridge. Ampère quantified forces between currents, Faraday demonstrated induction, and Maxwell later unified the complete field theory.