In Ørsted’s 1820 experiment, what did a current-carrying wire deflect near a compass?
Answer
The compass needle
In Hans Christian Ørsted’s 1820 experiment, a current-carrying wire deflected a nearby compass needle.
Ørsted placed a straight conducting wire near a compass and noticed that the needle moved when electric current flowed. Reversing the current reversed the direction of the deflection. The observation demonstrated a relationship between electricity and magnetism: an electric current produces a magnetic effect around its conductor.
The result helped launch the science of electromagnetism. André-Marie Ampère quickly investigated the phenomenon mathematically, while later work by Faraday and Maxwell developed a broader theory linking electric and magnetic fields.
The compass needle did not move because the wire became a permanent magnet in the ordinary sense. The magnetic field existed while current flowed and changed direction when the current direction changed.