Hans Christian Ørsted

21 April 1820: a lecture demonstration in Copenhagen

During a University of Copenhagen lecture demonstration on 21 April 1820, Hans Christian Ørsted connected a wire across a galvanic battery and saw a nearby compass needle deflect from magnetic north. The movement was slight — reportedly too slight for the audience to notice.

The difference was simply whether the circuit was open or closed: only the completed circuit moved the needle. Ørsted read that as evidence that a closed electric circuit could act on a magnet.

Three months before he published

He did not announce anything that day. He reconsidered the observation and then ran stronger, more systematic experiments in July, with Johan Georg Forchhammer Esmark helping repeat and extend the work on a more powerful galvanic apparatus.

The published experiment used 20 copper troughs. With that apparatus the needle could turn about 45 degrees when the wire was no more than three-quarters of an inch from it.

Ørsted issued the result on 21 July 1820 as a short Latin pamphlet, sent directly to scientists and learned societies.

How firm the April date is

The American Physical Society supports 21 April for the first observation. Ørsted's own 1820 account says only that his first experiments fell during the preceding winter's lectures, and University of Copenhagen material places the demonstration more generally in spring 1820.

How accidental it was is also disputed. The safest statement is that the needle's particular motion was unexpected, though Ørsted had long been looking for a connection between natural forces.

Nothing was publicly transformed on 21 April: the July pamphlet, not the April lecture, put the result in front of European science.