Editing Magnetic field (section)

==History==
{{Main|History of electromagnetic theory}}
{{see also|Timeline of electromagnetism and classical optics}}
[[Image:Descartes magnetic field.jpg|thumb|upright=1.2|One of the first drawings of a magnetic field, by [[René Descartes]], 1644, showing the Earth attracting [[lodestone]]s. It illustrated his theory that magnetism was caused by the circulation of tiny helical particles, "threaded parts", through threaded pores in magnets.]]

===Early developments===
While magnets and some properties of magnetism were known to ancient societies, 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 the resulting field lines crossed at two points he named those points "poles" in analogy to Earth's poles. He also articulated the principle that magnets always have both a north and south pole, no matter how finely one slices them.<ref>{{cite book | doi=10.1007/978-1-4020-4423-6_261 | chapter=Peregrinus, Petrus (Flourished 1269) | title=Encyclopedia of Geomagnetism and Paleomagnetism | date=2007 | last1=Chapman | first1=Allan | pages=808–809 | publisher=Springer | location=Dordrecht | isbn=978-1-4020-3992-8 }}</ref>{{refn|group="note"|His ''Epistola Petri Peregrini de Maricourt ad Sygerum de Foucaucourt Militem de Magnete'', which is often shortened to ''Epistola de magnete'', is dated 1269 C.E.}}

Almost three centuries later, [[William Gilbert (astronomer)|William Gilbert]] of [[Colchester]] replicated Petrus Peregrinus' work and was the first to state explicitly that Earth is a magnet.<ref name=Whittaker1910>{{cite book|last=Whittaker|first=E. T.| author-link=E. T. Whittaker|title=[[A History of the Theories of Aether and Electricity]]|year=1910|publisher=[[Dover Publications]] | isbn=978-0-486-26126-3}}</ref>{{rp|34}} Published in 1600, Gilbert's work, ''[[De Magnete]]'', helped to establish magnetism as a science.

===Mathematical development===
[[File:Hans Christian Ørsted, Der Geist in der Natur, 1854.tiff|thumb|[[Hans Christian Ørsted]], ''Der Geist in der Natur'', 1854]]

In 1750, [[John Michell]] stated that magnetic poles attract and repel in accordance with an [[inverse square law]]<ref name=Whittaker1910 />{{rp|p=56}} [[Charles-Augustin de Coulomb]] experimentally verified this in 1785 and stated explicitly that north and south poles cannot be separated.<ref name=Whittaker1910 />{{rp|p=59}} 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.<ref name=Whittaker1910 />{{rp|p=64}} In this model, a magnetic {{math|'''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. [[Hans Christian Ørsted]] demonstrated that a current-carrying wire is surrounded by a circular magnetic field.{{refn|group="note"|During a lecture demonstration on the effects of a current on a campus needle, Ørsted showed that when a current-carrying wire is placed at a right angle with the compass, nothing happens. When he tried to orient the wire parallel to the compass needle, however, it produced a pronounced deflection of the compass needle. By placing the compass on different sides of the wire, he was able to determine the field forms perfect circles around the wire.<ref name=Whittaker1910 />{{rp|p=85}}}}<ref>{{cite encyclopedia|last1=Williams|first1=L. Pearce|date=1974|page=185| location=New York|title=Oersted, Hans Christian|url=https://archive.org/details/dictionaryofscie10gill/page/184|encyclopedia=Dictionary of Scientific Biography|editor-last1=Gillespie|editor-first1=C. C.|publisher=Charles Scribner's Sons}}</ref> 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.<ref name=Whittaker1910 />{{rp|p=87}}<ref>{{cite book|last1=Blundell|first1=Stephen J.|title=Magnetism: A Very Short Introduction|date=2012|publisher=OUP Oxford|isbn=9780191633720|page=31}}</ref> 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 the forces were inversely proportional to the perpendicular distance from the wire to the magnet.<ref name=Tricker23>{{cite book|last1=Tricker|first1=R. A. R.|title=Early electrodynamics|url=https://archive.org/details/earlyelectrodyna0000tric|url-access=registration|date=1965|publisher=Pergamon|location=Oxford|page=[https://archive.org/details/earlyelectrodyna0000tric/page/23 23]}}</ref><ref name=Whittaker1910 />{{rp|p=86}} [[Laplace]] later deduced a law of force based on the differential action of a differential section of the wire,<ref name=Tricker23/><ref>{{cite journal| last1=Erlichson| first1=Herman|title=The experiments of Biot and Savart concerning the force exerted by a current on a magnetic needle| journal=American Journal of Physics|date=1998|volume=66|issue=5|page=389| doi=10.1119/1.18878| bibcode=1998AmJPh..66..385E |doi-access=free}}</ref> which became known as the [[Biot–Savart law]], as Laplace did not publish his findings.<ref>{{cite book|last1=Frankel| first1=Eugene|title=Jean-Baptiste Biot: The career of a physicist in nineteenth-century France|date=1972|publisher=Doctoral dissertation| location=Princeton University|page=334}}</ref>

Extending these experiments, Ampère published his own successful model of magnetism in 1825. In it, he showed the equivalence of electrical currents to magnets<ref name=Whittaker1910 />{{rp|p=88}} and proposed that magnetism is due to perpetually flowing loops of current instead of the dipoles of magnetic charge in Poisson's model.{{refn|group="note"|From the outside, the field of a dipole of magnetic charge has exactly the same form as a current loop when both are sufficiently small. Therefore, the two models differ only for magnetism inside magnetic material.}} 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.<ref name=Whittaker1910 />{{rp|pp=88–92}}

In 1831, [[Michael Faraday]] discovered [[electromagnetic induction]] when he found that a changing magnetic field generates an encircling electric field, formulating what is now known as [[Faraday's law of induction]].<ref name=Whittaker1910 />{{rp|pp=189–192}} Later, [[Franz Ernst Neumann]] proved that, for a moving conductor in a magnetic field, induction is a consequence of Ampère's force law.<ref name=Whittaker1910 />{{rp|p=222}} In the process, he introduced the magnetic vector potential, which was later shown to be equivalent to the underlying mechanism proposed by Faraday.<ref name=Whittaker1910 />{{rp|p=225}}

In 1850, [[Lord Kelvin]], then known as William Thomson, distinguished between two magnetic fields now denoted {{math|'''H'''}} and {{math|'''B'''}}. The former applied to Poisson's model and the latter to Ampère's model and induction.<ref name=Whittaker1910 />{{rp|p=224}} Further, he derived how {{math|'''H'''}} and {{math|'''B'''}} relate to each other and coined the term ''permeability''.<ref name=Whittaker1910 />{{rp|p=245}}<ref>[http://www.physik.uni-augsburg.de/lehrstuehle/did/Physik/Schulservice/Physikerkalender/06_26_Lord-Kelvin-of-Largs.pdf Lord Kelvin of Largs]. physik.uni-augsburg.de. 26 June 1824</ref>

Between 1861 and 1865, [[James Clerk Maxwell]] developed and published [[Maxwell's equations]], which explained and united all of [[classical theory|classical]] electricity and magnetism. The first set of these equations was published in a paper entitled ''[[:CCommons:File:On Physical Lines of Force.pdf|On Physical Lines of Force]]'' in 1861. These equations were valid but incomplete. Maxwell completed his set of equations in his later 1865 paper ''[[A Dynamical Theory of the Electromagnetic Field]]'' and demonstrated the fact that light is an [[electromagnetic wave]]. [[Heinrich Hertz]] published papers in 1887 and 1888 experimentally confirming this fact.<ref name=h202>Huurdeman, Anton A. (2003) ''The Worldwide History of Telecommunications''. Wiley. {{ISBN|0471205052}}. p. 202</ref><ref>{{cite web|url=http://www.hhi.fraunhofer.de/fraunhofer-hhi-the-institute/about-us/history-of-hhi/the-most-important-experiments.html|publisher=Fraunhofer Heinrich Hertz Institute|title=The most important Experiments – The most important Experiments and their Publication between 1886 and 1889|access-date=19 February 2016}}</ref>

===Modern developments===

In 1887, Tesla developed an [[induction motor]] that ran on [[alternating current]]. The motor used [[Polyphase system|polyphase]] current, which generated a [[rotating magnetic field]] to turn the motor (a principle that Tesla claimed to have conceived in 1882).<ref>{{cite book|title=Networks of Power: Electrification in Western Society, 1880–1930|publisher=JHU Press|page=117|url=https://books.google.com/books?id=g07Q9M4agp4C&pg=PA117|isbn=9780801846144|date=March 1993 }}</ref><ref>Thomas Parke Hughes, ''Networks of Power: Electrification in Western Society, 1880–1930'', pp. 115–118</ref><ref>{{cite book | url=https://books.google.com/books?id=1AsFdUxOwu8C&pg=PA204|title=Robert Bud, Instruments of Science: An Historical Encyclopedia | page=204|access-date=18 March 2013|isbn=9780815315612|last1=Ltd|first1=Nmsi Trading|author2 = Smithsonian Institution | year=1998 |publisher=Taylor & Francis }}</ref> Tesla received a patent for his electric motor in May 1888.<ref>{{US patent|381968}}</ref><ref>{{Cite journal | last1=Porter|first1=H. F. J.|last2=Prout|first2=Henry G.|date=January 1924|title=A Life of George Westinghouse| url=http://dx.doi.org/10.2307/1838546|journal=The American Historical Review| volume=29| issue=2|page=129|doi=10.2307/1838546| jstor=1838546|hdl=2027/coo1.ark:/13960/t15m6rz0r|issn=0002-8762|hdl-access=free}}</ref> In 1885, [[Galileo Ferraris]] independently researched rotating magnetic fields and subsequently published his research in a paper to the ''Royal Academy of Sciences'' in [[Turin]], just two months before Tesla was awarded his patent, in March 1888.<ref>{{Cite web |title=Galileo Ferraris (March 1888) ''Rotazioni elettrodinamiche prodotte per mezzo di correnti alternate'' (Electrodynamic rotations by means of alternating currents), memory read at Accademia delle Scienze, Torino, in ''Opere di Galileo Ferraris'', Hoepli, Milano, 1902 vol I pages 333 to 348 |url=http://ieeemilestones.ethw.org/images/a/a0/1-Galileo_Ferraris_Rotating-field--Rotazioni_Elettrodinamiche.pdf |url-status=dead |archive-url=https://web.archive.org/web/20210709182151/http://ieeemilestones.ethw.org/images/a/a0/1-Galileo_Ferraris_Rotating-field--Rotazioni_Elettrodinamiche.pdf |archive-date=9 July 2021 |access-date=2 July 2021}}</ref>

The twentieth century showed that classical electrodynamics is already consistent with special relativity, and extended classical electrodynamics to work with quantum mechanics. [[Albert Einstein]], in his paper of 1905 that established relativity, showed that both the electric and magnetic fields are part of the same phenomena viewed from different reference frames. Finally, the emergent field of [[quantum mechanics]] was merged with electrodynamics to form [[quantum electrodynamics]], which first formalized the notion that electromagnetic field energy is quantized in the form of photons.