Ørsted Discovers Electromagnetism (1820): Compass Meets Current

In April 1820 the Danish physicist Hans Christian Ørsted noticed that an electric current flowing through a wire made a nearby compass needle swing aside. His short Latin report, published on 21 July 1820, was the first clear proof that electricity and magnetism are connected. Within months scientists across Europe had built on it, laying the foundations of electromagnetism, electric motors and the telegraph.

Background: two separate forces

For centuries electricity and magnetism were treated as different phenomena. Some observers had noticed that lightning could magnetise steel or reverse compass needles, but experiments with static electricity showed no steady link. The invention of the voltaic pile in 1800 made a continuous current available for the first time.

Hans Christian Ørsted (1777–1851), professor of physics at the University of Copenhagen, was influenced by the German philosophy of nature, which held that the forces of nature were different expressions of a single underlying power. He had suggested in print years earlier that electricity and magnetism might be related, but he had not found the effect.

The discovery

In April 1820, while preparing or giving an evening lecture, Ørsted placed a wire connected to a battery close to and parallel with a compass needle. When the circuit was closed, the needle moved. The effect was weak at first, because his battery was small, and the audience was not greatly impressed.

Ørsted returned to the problem in July with a more powerful battery. He found that the needle turned to stand across the wire, and that its direction reversed if the current was reversed or if the needle was placed above instead of below the wire. The effect passed through glass, wood, water and other non-magnetic materials. He concluded that the magnetic effect of the current circled around the wire.

Why the result was surprising

Scientists of the time expected forces to act in straight lines, pulling or pushing directly between two objects, as gravity and static electricity seemed to do. Ørsted’s needle did something different. It was neither attracted to nor repelled by the wire but turned sideways, as if pushed around it. Ørsted described the effect as a “conflict” of electricity that acted in circles around the conductor.

In modern terms, a current produces a magnetic field whose lines form closed loops around the wire. A compass needle lines up with these loops. The idea that forces could act in circles was hard to accept at first, and making sense of it took the work of Ampère, Faraday and, later, Maxwell.

Publication and response

Ørsted described his results in a four-page Latin pamphlet, Experimenta circa effectum conflictus electrici in acum magneticam (Experiments on the Effect of the Electric Conflict on the Magnetic Needle), dated 21 July 1820. He sent it to leading scientists and journals, and translations soon appeared in French, German, English and Italian.

The response was swift. François Arago demonstrated the experiment to the French Academy of Sciences in Paris in September 1820. Within weeks André-Marie Ampère showed that two parallel currents attract or repel each other, and Jean-Baptiste Biot and Félix Savart measured how the force weakens with distance. In 1821 Michael Faraday made a wire rotate around a magnet, the first electric motor principle, and in 1824 William Sturgeon built the first electromagnet.

Key people

• Hans Christian Ørsted (1777–1851): Danish physicist and chemist who made the discovery.
• François Arago (1786–1853): French scientist who brought the experiment to Paris.
• André-Marie Ampère (1775–1836): French physicist who founded the mathematical theory of electrodynamics.
• Michael Faraday (1791–1867): English scientist who extended the work, leading to electromagnetic induction.

Myths and facts

Myth: Ørsted discovered electromagnetism by pure chance. The famous story of a lucky accident in front of students comes partly from later recollections by others. Ørsted had been looking for a connection for years, though the exact form of the effect surprised him.

Myth: the discovery immediately led to practical machines. It took decades. The first practical telegraphs appeared in the 1830s, and large electric motors and generators came later still.

Legacy

Ørsted’s discovery began a chain of research that ended in James Clerk Maxwell’s unified theory of electricity, magnetism and light (Maxwell’s electromagnetic theory). Practical results included the galvanometer for measuring current, the electromagnet and the electric telegraph. Early needle telegraphs, such as the Cooke and Wheatstone telegraph, worked on exactly Ørsted’s principle, using currents to swing magnetic needles that pointed to letters.

Ørsted also made the first, impure, preparation of aluminium in 1825 and helped found the Polytechnic Institute in Copenhagen in 1829, now the Technical University of Denmark. The oersted, a unit of magnetic field strength, honours him.

Timeline

• 1777, 14 August: Hans Christian Ørsted is born in Rudkøbing, Denmark.
• 1800: Volta announces the voltaic pile.
• 1820, April: Ørsted first sees a current deflect a compass needle.
• 1820, 21 July: Ørsted’s Latin report is published.
• 1820, September: Arago and Ampère take up the discovery in Paris.
• 1821: Faraday demonstrates electromagnetic rotation.
• 1824: William Sturgeon builds the first electromagnet.
• 1851, 9 March: Ørsted dies in Copenhagen.

Why it matters

Ørsted’s compass needle revealed that electricity and magnetism are two sides of one force. That insight underpins electric motors, generators, transformers and the electrical technology of the modern world.