Editing Electromagnetism (section)

===19th century===
[[File:A Treatise on Electricity and Magnetism Volume 2 003.jpg|thumb|Cover of ''A Treatise on Electricity and Magnetism'']]
Electricity and magnetism were originally considered to be two separate forces. This view changed with the publication of [[James Clerk Maxwell]]'s 1873 ''[[A Treatise on Electricity and Magnetism]]''<ref>{{Cite journal|date=24 April 1873|title=A Treatise on Electricity and Magnetism|url=https://www.nature.com/articles/007478a0|journal=Nature|language=en|volume=7|issue=182|pages=478–480|doi=10.1038/007478a0|bibcode=1873Natur...7..478.|s2cid=10178476|issn=0028-0836|access-date=8 February 2022|archive-date=23 October 2021|archive-url=https://web.archive.org/web/20211023150511/https://www.nature.com/articles/007478a0|url-status=live}}</ref> in which the interactions of positive and negative charges were shown to be mediated by one force. There are four main effects resulting from these interactions, all of which have been clearly demonstrated by experiments:
# Electric charges ''{{vanchor|attract}}'' or ''{{vanchor|repel}}'' one another with a force [[inversely proportional]] to the square of the distance between them: opposite charges attract, like charges repel.<ref>{{Cite web |date=2019-02-06 |title=Why Do Like Charges Repel And Opposite Charges Attract? |url=https://www.scienceabc.com/eyeopeners/like-charges-repel-opposite-charges-attract.html |access-date=2022-08-22 |website=Science ABC |language=en-US |archive-date=2022-08-22 |archive-url=https://web.archive.org/web/20220822120352/https://www.scienceabc.com/eyeopeners/like-charges-repel-opposite-charges-attract.html |url-status=live }}</ref>
# Magnetic poles (or states of polarization at individual points) attract or repel one another in a manner similar to positive and negative charges and always exist as pairs: every north pole is yoked to a south pole.<ref>{{Cite web |title=What Makes Magnets Repel? |url=https://sciencing.com/magnets-repel-7754550.html |access-date=2022-08-22 |website=Sciencing |date=27 December 2020 |language=en |archive-date=2022-09-26 |archive-url=https://web.archive.org/web/20220926214826/https://sciencing.com/magnets-repel-7754550.html |url-status=live }}</ref>
# An electric current inside a wire creates a corresponding circumferential magnetic field outside the wire. Its direction (clockwise or counter-clockwise) depends on the direction of the current in the wire.<ref name=lscience >{{Cite web |author1=Jim Lucas Contributions from Ashley Hamer |date=2022-02-18 |title=What Is Faraday's Law of Induction? |url=https://www.livescience.com/53509-faradays-law-induction.html |access-date=2022-08-22 |website=livescience.com |language=en |archive-date=2022-08-22 |archive-url=https://web.archive.org/web/20220822120351/https://www.livescience.com/53509-faradays-law-induction.html |url-status=live }}</ref>
# A current is induced in a loop of wire when it is moved toward or away from a magnetic field, or a magnet is moved towards or away from it; the direction of current depends on that of the movement.<ref name=lscience />
In April 1820, [[Hans Christian Ørsted]] observed that an electrical current in a wire caused a nearby compass needle to move. At the time of discovery, Ørsted did not suggest any satisfactory explanation of the phenomenon, nor did he try to represent the phenomenon in a mathematical framework. However, three months later he began more intensive investigations.<ref>{{Cite journal|date=1884-02-23|title=History of the Electric Telegraph|url=http://dx.doi.org/10.1038/scientificamerican02231884-6784supp|journal=Scientific American|volume=17|issue=425supp|pages=6784–6786|doi=10.1038/scientificamerican02231884-6784supp|issn=0036-8733|access-date=2022-02-08|archive-date=2024-10-03|archive-url=https://web.archive.org/web/20241003193822/https://www.scientificamerican.com/article/history-of-the-electric-telegraph1/|url-status=live}}</ref><ref>{{Cite book|url=https://www.worldcat.org/oclc/1261807533|title=Volta and the history of electricity|date=2003|publisher=U. Hoepli|editor-first1=Fabio|editor-last1=Bevilacqua|editor-first2=Enrico A.|editor-last2=Giannetto|isbn=88-203-3284-1|location=Milano|oclc=1261807533|access-date=2022-02-08|archive-date=2024-10-03|archive-url=https://web.archive.org/web/20241003193744/https://search.worldcat.org/title/1261807533|url-status=live}}</ref> Soon thereafter he published his findings, proving that an electric current produces a magnetic field as it flows through a wire. The [[CGS]] unit of [[Electromagnetic induction|magnetic induction]] ([[oersted]]) is named in honor of his contributions to the field of electromagnetism.<ref>{{Cite book|last=Roche|first=John J.|url=https://www.worldcat.org/oclc/40499222|title=The mathematics of measurement : a critical history|date=1998|publisher=Athlone Press|isbn=0-485-11473-9|location=London|oclc=40499222|access-date=2022-02-08|archive-date=2024-10-03|archive-url=https://web.archive.org/web/20241003193840/https://search.worldcat.org/title/40499222|url-status=live}}</ref>

His findings resulted in intensive research throughout the scientific community in electrodynamics. They influenced French physicist [[André-Marie Ampère]]'s developments of a single mathematical form to represent the magnetic forces between current-carrying conductors. Ørsted's discovery also represented a major step toward a unified concept of energy.

This unification, which was observed by [[Michael Faraday]], extended by [[James Clerk Maxwell]], and partially reformulated by [[Oliver Heaviside]] and [[Heinrich Hertz]], is one of the key accomplishments of 19th-century [[mathematical physics]].<ref>{{cite book |last1=Darrigol |first1=Olivier |title=Electrodynamics from Ampère to Einstein |date=2000 |publisher=Oxford University Press |location=New York |isbn=0198505949 |url-access=registration |url=https://archive.org/details/electrodynamicsf0000darr }}</ref> It has had far-reaching consequences, one of which was the understanding of the nature of [[light]]. Unlike what was proposed by the electromagnetic theory of that time, light and other [[electromagnetic waves]] are at present seen as taking the form of [[quantum|quantized]], self-propagating [[oscillatory]] electromagnetic field disturbances called [[photon]]s. Different [[frequencies]] of oscillation give rise to the different forms of [[electromagnetic radiation]], from [[radio wave]]s at the lowest frequencies, to visible light at intermediate frequencies, to [[gamma ray]]s at the highest frequencies.

Ørsted was not the only person to examine the relationship between electricity and magnetism. In 1802, [[Gian Domenico Romagnosi]], an Italian legal scholar, deflected a magnetic needle using a Voltaic pile. The factual setup of the experiment is not completely clear, nor if current flowed across the needle or not. An account of the discovery was published in 1802 in an Italian newspaper, but it was largely overlooked by the contemporary scientific community, because Romagnosi seemingly did not belong to this community.<ref>{{cite book|last1=Martins |first1=Roberto de Andrade |title=Nuova Voltiana: Studies on Volta and his Times |chapter=Romagnosi and Volta's Pile: Early Difficulties in the Interpretation of Voltaic Electricity |volume=3 |editor=Fabio Bevilacqua |editor2=Lucio Fregonese |publisher=Università degli Studi di Pavia |pages=81–102 |chapter-url=http://ppp.unipv.it/collana/pages/libri/saggi/nuova%20voltiana3_pdf/cap4/4.pdf |access-date=2010-12-02 |url-status=dead |archive-url=https://web.archive.org/web/20130530200951/http://ppp.unipv.it/Collana/Pages/Libri/Saggi/Nuova%20Voltiana3_PDF/cap4/4.pdf |archive-date=2013-05-30 }}</ref>

An earlier (1735), and often neglected, connection between electricity and magnetism was reported by a Dr. Cookson.<ref>VIII. An account of an extraordinary effect of lightning in communicating magnetism. Communicated by Pierce Dod, M.D. F.R.S. from Dr. Cookson of Wakefield in Yorkshire.
Phil. Trans. 1735 39, 74-75, published 1 January 1735</ref> The account stated:<blockquote>A tradesman at Wakefield in Yorkshire, having put up a great number of knives and forks in a large box ...  and having placed the box in the corner of a large room, there happened a sudden storm of thunder, lightning, &c.  ... The owner emptying the box on a counter where some nails lay, the persons who took up the knives, that lay on the nails, observed that the knives took up the nails. On this the whole number was tried, and found to do the same, and that, to such a degree as to take up large nails, packing needles, and other iron things of considerable weight ...</blockquote> [[E. T. Whittaker]] suggested in 1910 that this particular event was responsible for lightning to be "credited with the power of magnetizing steel; and it was doubtless this which led Franklin in 1751 to attempt to magnetize a sewing-needle by means of the discharge of Leyden jars."<ref>[[E. T. Whittaker|Whittaker, E.T.]] (1910). [[A History of the Theories of Aether and Electricity|A History of the Theories of Aether and Electricity from the Age of Descartes to the Close of the Nineteenth Century]]. Longmans, Green and Company.</ref>