A wobbling compass needle revealed electricity and magnetism are linked

During an 1820 lecture demonstration, Hans Christian Ørsted noticed a compass needle swing whenever he switched a nearby electric current on and off. The accidental observation was the first proof that electricity and magnetism are connected, launching the field of electromagnetism.

— Hans Christian Ørsted, Experiments on the Effect of a Current of Electricity on the Magnetic Needle — Self-published pamphlet, 1820