The Needle That Almost No One Saw Move

The Mechanism

The Danish physicist *Hans Christian Ørsted* (born Rudkøbing, on the Danish island of Langeland, 14 August 1777; died Copenhagen, 9 March 1851) was the son of a pharmacist who had read enough on his own to learn Latin and Greek by the age of twelve, took his Ph.D. in philosophy from the University of Copenhagen at age 22 in 1799 with a dissertation on *Kant's metaphysics of natural science*, and was appointed Professor of Physics at the University of Copenhagen in 1806. From the very beginning of his scientific career he was a committed *Naturphilosoph* — a follower of the German Romantic-era philosophical school led by *Friedrich Schelling* and *Johann Wilhelm Ritter* which held, on Kantian metaphysical grounds, that *all natural forces are manifestations of a single underlying force* and that the apparently distinct forms — gravity, electricity, magnetism, heat, chemical affinity, light — must be discoverable as the same force operating under different conditions. By 1812 Ørsted had become specifically convinced, in print, that *electricity and magnetism must be unifiable*: in *Ansicht der chemischen Naturgesetze* (Berlin, 1812; English ed. *View of the Chemical Laws of Nature*) he wrote that *one should try whether electricity, in its most latent form, has any action on the magnet as such*. He had been searching for an empirical demonstration of the electric–magnetic link, on philosophical grounds, since at least 1812 and probably since 1808. The standard 19th-century textbook account of his discovery — that on a single April 1820 evening in Copenhagen, in the middle of a public lecture on the unrelated topics of heat and electric currents, he accidentally noticed his compass needle deflect when he closed an electrical circuit and was surprised — is a myth that Ørsted *himself* tried, repeatedly, to correct in print. The *Annalen der Physik* editor Ludwig Wilhelm Gilbert popularised the "happy accident" version in his German translation and commentary of the discovery pamphlet in late 1820; Ørsted wrote a public letter in *Bibliothek for Physik, Medicin og Oeconomie* (1821) explicitly stating that the experiment had been *prepared in advance, on the basis of his decade-long theoretical conviction*, that he had been *looking* for the effect rather than stumbling on it, and that he had carried out the experiment in front of his class because he *wanted to test* the prediction, not because he was demonstrating something unrelated. What is true is that the effect, when it appeared, was *small*. The actual experiment was performed on the evening of *21 April 1820* in a lecture room at the University of Copenhagen, in front of an audience of his advanced-physics students (estimates of the audience size vary from twelve to twenty). The apparatus was: a *voltaic pile* assembled the night before (a stack of alternating zinc and copper discs separated by saltwater-soaked cardboard, generating a few volts and a steady direct current of perhaps an ampere), connected by two short copper wires to a *thin platinum wire* approximately the thickness of a sewing needle stretched horizontally across the front of the lecture bench, with the wire passing *over* a *small magnetic compass* mounted on a wooden stand on the bench, the compass enclosed in a glass case to prevent draughts from interfering with the needle. The wire was oriented along the magnetic-north–magnetic-south direction, parallel to the resting position of the compass needle. When Ørsted closed the circuit by touching the two ends of the platinum wire to the terminals of the voltaic pile, the platinum wire grew warm (the experimental purpose, as far as the audience knew, was to demonstrate the joule-heating effect of a current). The compass needle moved. The motion was small — a deflection of perhaps fifteen degrees — and almost no one in the lecture room appears to have noticed it. Ørsted himself was, by his own account in *Bibliothek for Physik*, *struck* by the motion in the moment he saw it. The current direction was reversed; the needle deflected the other way. The wire was moved to a position *under* rather than *over* the needle; the needle deflected in the opposite direction. The effect was reproducible. The current was creating a circulating magnetic field around the wire. *Electricity and magnetism were the same force.* Ørsted spent the following three months refining the experiment in private — different wire materials, different current strengths, different compass geometries — and on *21 July 1820* he printed a four-page Latin pamphlet titled *Experimenta circa effectum conflictus electrici in acum magneticam* (*Experiments on the effect of an electric conflict on the magnetic needle*) and sent copies, by post, to the leading European physicists and scientific societies. The pamphlet reached François Arago at the *Académie des Sciences* in Paris in late August. Arago demonstrated the effect to the Académie on *4 September 1820*. *André-Marie Ampère*, the senior French theoretical physicist, was in the audience. Within a week of the Arago demonstration Ampère had constructed the *quantitative mathematical theory of the magnetic field produced by an electric current* — the result now known as *Ampère's circuital law*, the second of *Maxwell's four equations of electromagnetism*. Ampère presented his first memoir on the new theory to the Académie on *18 September 1820* and a fuller memoir on *2 October 1820*. The development of electromagnetism that began with the small motion of Ørsted's compass needle on the evening of 21 April 1820 continued: *Michael Faraday*'s 1831 discovery of *electromagnetic induction* (the converse effect: a changing magnetic field produces an electric current); *James Clerk Maxwell*'s 1864-1865 unified theory of the electromagnetic field, which mathematically encoded Ampère's law and Faraday's law (and two additional laws of his own discovery) into the four coupled partial differential equations now known as *Maxwell's equations*; and *Maxwell*'s 1862-1865 derivation, from his own equations, that the equations supported wave solutions propagating at the measured speed of light — the prediction that *light itself is an electromagnetic wave*, the first unification in physics on the scale that would eventually become the Standard Model. Every electric motor, every transformer, every electrical generator, every dynamo, every radio transmitter, every WiFi router, every MRI scanner, every particle accelerator, every cell phone, every electromagnetic technology in industrial civilization traces its lineage back to the small motion of a magnetic compass needle in a Copenhagen lecture room on the evening of 21 April 1820. Ørsted himself was awarded the Royal Society's Copley Medal in 1820, was elected to virtually every European scientific society in his remaining lifetime, founded the Society for the Dissemination of Natural Science in Denmark in 1824 and the Polytechnic Institute of Copenhagen in 1829, and on 9 March 1851 — fifty years to within a few months after he had first begun searching, on Kantian metaphysical grounds, for the empirical link between electricity and magnetism — died in Copenhagen at age 73. The SI unit of magnetic-field strength (in the older CGS-Gaussian system) is the *oersted* (Oe). The motion of his needle, on the evening of 21 April 1820, was the first piece of experimental evidence that the universe contains *fewer separate forces than it appears to*.