Main article: History of electromagnetic theory
One of the first drawings of a magnetic field, by Ren? Descartes, 1644. It illustrated his theory that magnetism was caused by the circulation of tiny helical particles, "threaded parts", through threaded pores in magnets.Although magnets and magnetism were known much earlier, the study of the magnetic field began in 1269 when French scholar Petrus Peregrinus mapped out the magnetic field on the surface of a spherical magnet using iron needles.[nb 2] Noting that the resulting field lines crossed at two points he named those points 'poles' in analogy to Earth's poles. Almost three centuries later, William Gilbert of Colchester replicated Petrus Peregrinus' work and was the first to state explicitly that Earth is a magnet.[1] Published in 1600, Gilbert's work, De Magnete, helped to establish magnetism as a science.
In 1750, John Michell stated that magnetic poles attract and repel in accordance with an inverse square law.[2] Charles-Augustin de Coulomb experimentally verified this in 1785 and stated explicitly that the North and South poles cannot be separated.[3] Building on this force between poles, Sim?on-Denis Poisson (1781?1840) created the first successful model of the magnetic field which he presented in 1824.[4] In this model, a magnetic H-field is produced by 'magnetic poles' and magnetism is due to small pairs of north/south magnetic poles.
Three discoveries challenged this foundation of magnetism, though. First, in 1819, Hans Christian Oersted discovered that an electric current generates a magnetic field encircling it. Then in 1820, Andr?-Marie Amp?re showed that parallel wires having currents in the same direction attract one another. Finally, Jean-Baptiste Biot and F?lix Savart discovered the Biot?Savart law in 1820 which correctly predicts the magnetic field around any current-carrying wire.
Extending these experiments, Amp?re published his own successful model of magnetism in 1825. In it, he showed the equivalence of electrical currents to magnets[5] and proposed that magnetism is due to perpetually flowing loops of current instead of the dipoles of magnetic charge in Poisson's model.[nb 3] This has the additional benefit of explaining why magnetic charge can not be isolated. Further, Amp?re derived both Amp?re's force law describing the force between two currents and Amp?re's law which, like the Biot?Savart law, correctly described the magnetic field generated by a steady current. Also in this work, Amp?re introduced the term electrodynamics to describe the relationship between electricity and magnetism.
In 1831, Michael Faraday discovered electromagnetic induction when he found that a changing magnetic field generates an encircling electric field. He described this phenomenon in what is known as Faraday's law of induction. Later, Franz Ernst Neumann proved that, for a moving conductor in a magnetic field, induction is a consequence of Amp?re's force law .[6] In the process he introduced the magnetic vector potential which was later shown to be equivalent to the underlying mechanism proposed by Faraday.
In 1850, Lord Kelvin, then known as William Thomson, distinguished between two magnetic fields now denoted H and B. The former applied to Poisson's model and the latter to Amp?re's model and induction.[7] Further, he derived how H and B relate to each other.
