page_title: Magnetic field
section_title: History
hierarchical_section_title: Magnetic field / History
caption_attribution_description: English: Drawing of a magnetic field by French philosopher René Descartes, from his Principia Philosophiae, 1644. This was one of the first drawings of the concept of a magnetic field. It shows the magnetic field of the Earth (D) attracting several round lodestones (I, K, L, M, N) and illustrates his theory of magnetism. Descartes proposed that magnetic attraction was caused by the circulation of tiny helical particles, "threaded parts" (shown), which circulated through parallel threaded pores in magnets, in through the South pole (A), out through the North pole (B), and then through the space around the magnet (G, H) back to the South pole. Oppositely threaded particles circulated in the opposite direction. When the "threaded parts" came near a lodestone or piece of iron, they passed through its pores, causing magnetic force. Alterations to image: cropped, and cloned out intruding text around edges of drawing
context_page_description: A magnetic field is a vector field that describes the magnetic influence on an electric charge of other moving charges or magnetized materials. A charge that is moving in a magnetic field experiences a force perpendicular to its own velocity and to the magnetic field. The effects of magnetic fields are commonly seen in permanent magnets, which pull on magnetic materials such as iron, and attract or repel other magnets, creating a torque. In addition, a magnetic field that varies with location will exert a force on a range of non-magnetic materials by affecting the motion of their outer atomic electrons. Magnetic fields surround magnetized materials, and are created by electric currents such as those used in electromagnets, and by electric fields varying in time. Since both strength and direction of a magnetic field may vary with location, they are described as a map assigning a vector to each point of space or, more precisely—because of the way the magnetic field transforms under mirror reflection—as a field of pseudovectors.
  In electromagnetics, the term "magnetic field" is used for two distinct but closely related vector fields denoted by the symbols B and H.
context_section_description: Although magnets and magnetism were studied much earlier, the research of magnetic fields began in 1269 when French scholar Petrus Peregrinus de Maricourt mapped out the magnetic field on the surface of a spherical magnet using iron needles. Noting that the resulting field lines crossed at two points he named those points "poles" in analogy to Earth's poles. He also clearly articulated the principle that magnets always have both a north and south pole, no matter how finely one slices them.
  Almost three centuries later, William Gilbert of Colchester replicated Petrus Peregrinus's work and was the first to state explicitly that Earth is a magnet. 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. Charles-Augustin de Coulomb experimentally verified this in 1785 and stated explicitly that the north and south poles cannot be separated. Building on this force between poles, Siméon Denis Poisson (1781–1840) created the first successful model of the magnetic field, which he presented in 1824. 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 in 1820 challenged this foundation of magnetism, though. Hans Christian Ørsted demonstrated that a current-carrying wire is surrounded by a circular magnetic field. Then André-Marie Ampère showed that parallel wires with currents attract one another if the currents are in the same direction and repel if they are in opposite directions. Finally, Jean-Baptiste Biot and Félix Savart announced empirical results about the forces that a current-carrying long, straight wire exerted on a small magnet, determining that the forces were inversely proportional to the perpendicular distance from the wire to the magnet. Laplace later deduced, but did not publish, a law of force based on the differential action of a differential section of the wire, which became known as the Biot–Savart law.
  Extending these experiments, Ampère published his own successful model of magnetism in 1825. In it, he showed the equivalence of electrical currents to magnets and proposed that magnetism is due to perpetually flowing loops of current instead of the dipoles of magnetic charge in Poisson's model. This has the additional benefit of explaining why magnetic charge can not be isolated. Further, Ampère derived both Ampère's force law describing the force between two currents and Ampère's law, which, like the Biot–Savart law, correctly described the magnetic field generated by a steady current. Also in this work, Ampère 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 Ampère's force law. 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 Ampère's model and induction. Further, he derived how H and B relate to each other.
  The reason the letters H and B are used for the two magnetic fields has been a source of some debate among science historians. Most agree that Kelvin avoided M to prevent confusion with the SI fundamental unit of length, the Metre, abbreviated "m". Others believe the choices were purely random.
  Between 1861 and 1865, James Clerk Maxwell developed and published Maxwell's equations, which explained and united all of classical electricity and magnetism. The first set of these equations was published in a paper entitled On
caption: One of the first drawings of a magnetic field, by René Descartes, 1644, showing the Earth attracting lodestones.  It illustrated his theory that magnetism was caused by the circulation of tiny helical particles, "threaded parts", through threaded pores in magnets.
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