By the end of the 18th century, scientists had noticed many electrical phenomena and many magnetic phenomena, but most believed that these were distinct forces. Then in July 1820, Danish natural philosopher Hans Christian Oersted published a pamphlet that showed clearly that they were in fact closely related

Oersted made the discovery for which he is famous in 1820. At the time, although most scientists thought electricity and magnetism were not related, there were some reasons to think there might be a connection. For instance, it had long been known that a compass, when struck by lightning, could reverse polarity. Oersted had previously noted a similarity between thermal radiation and light, though he did not determine that both are electromagnetic waves. He seems to have believed that electricity and magnetism were forces radiated by all substances, and these forces might somehow interfere with each other.

During a lecture demonstration, on April 21, 1820, while setting up his apparatus, Oersted noticed that when he turned on an electric current by connecting the wire to both ends of the battery, a compass needle held nearby deflected away from magnetic north, where it normally pointed. The compass needle moved only slightly, so slightly that the audience didn’t even notice. But it was clear to Oersted that something significant was happening.

Some people have suggested that this was a totally accidental discovery, but accounts differ on whether the demonstration was designed to look for a connection between electricity and magnetism, or was intended to demonstrate something else entirely. Certainly Oersted was well prepared to observe such an effect, with the compass needle and the battery (or “galvanic apparatus,” as he called it) on hand.

Whether completely accidental or at least somewhat expected, Oersted was intrigued by his observation. He didn’t immediately find a mathematical explanation, but he thought it over for the next three months, and then continued to experiment, until he was quite certain that an electric current could produce a magnetic field (which he called an “electric conflict”).

On July 21, 1820, Oersted published his results in a pamphlet, which was circulated privately to physicists and scientific societies. His results were mainly qualitative, but the effect was clear–an electric current generates a magnetic force.





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