Editing Speed of light (section)

=== Connections with electromagnetism ===
{{See also|History of electromagnetic theory|History of special relativity}}
In the 19th century [[Hippolyte Fizeau]] developed a method to determine the speed of light based on time-of-flight measurements on Earth and reported a value of {{val|315000|u=km/s}}.<ref name="guarnieri 7-1">{{Cite journal|last=Guarnieri|first=M.|year=2015|title=Two Millennia of Light: The Long Path to Maxwell's Waves|journal=IEEE Industrial Electronics Magazine|volume=9|issue=2|pages=54–56, 60|doi=10.1109/MIE.2015.2421754|s2cid=20759821}}</ref> His method was improved upon by [[Léon Foucault]] who obtained a value of {{val|298000|u=km/s}} in 1862.<ref name="How"/> In the year 1856, [[Wilhelm Eduard Weber]] and [[Rudolf Kohlrausch]] measured the ratio of the electromagnetic and electrostatic units of charge, 1/{{radic|''ε''<sub>0</sub>''μ''<sub>0</sub>}}, by discharging a [[Leyden jar]], and found that its numerical value was very close to the speed of light as measured directly by Fizeau. The following year [[Gustav Kirchhoff]] calculated that an electric signal in a [[electrical resistance|resistanceless]] wire travels along the wire at this speed.<ref>
{{Cite journal
 |last1=Kirchhoff |first1=G.
 |title=Über die Bewegung der Elektricität
 |url=http://gallica.bnf.fr/ark:/12148/bpt6k15187j/f549.item.r=
 |journal=[[Annalen der Physik]]
 |volume=178
 |issue=12
 |year=1857
 |pages=529–244
 |doi=10.1002/andp.18571781203
 |bibcode=1857AnP...178..529K
}}</ref>

In the early 1860s, Maxwell showed that, according to the theory of electromagnetism he was working on, electromagnetic waves propagate in empty space<ref>See, for example:
* {{Cite book
 |title=College physics: reasoning and relationships
 |first1=Nicholas J.
 |last1=Giordano
 |publisher=Cengage Learning
 |year=2009
 |isbn=978-0-534-42471-8
 |page=787
 |url=https://books.google.com/books?id=BwistUlpZ7cC
}} [https://books.google.com/books?id=BwistUlpZ7cC&pg=PA787 Extract of page 787]
* {{Cite book
 |title=The riddle of gravitation
 |first1=Peter Gabriel
 |last1=Bergmann
 |publisher=Courier Dover Publications
 |year=1992
 |isbn=978-0-486-27378-5
 |page=17
 |url=https://books.google.com/books?id=WYxkrwMidp0C
}} [https://books.google.com/books?id=WYxkrwMidp0C&pg=PA17 Extract of page 17]
* {{Cite book
 |title=The equations: icons of knowledge
 |first1=Sander
 |last1=Bais
 |publisher=Harvard University Press
 |year=2005
 |isbn=978-0-674-01967-6
 |page=[https://archive.org/details/equationsiconsof0000bais/page/40 40]
 |url=https://archive.org/details/equationsiconsof0000bais|url-access=registration
}} [https://archive.org/details/equationsiconsof0000bais/page/40 Extract of page 40]
</ref> at a speed equal to the above Weber/Kohlrausch ratio, and drawing attention to the numerical proximity of this value to the speed of light as measured by Fizeau, he proposed that light is in fact an electromagnetic wave.<ref name=maxwellbio>
{{Cite web
 |last1=O'Connor
 |first1=J. J.
 |last2=Robertson
 |first2=E. F.
 |date=November 1997
 |title=James Clerk Maxwell
 |url=http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Maxwell.html
 |publisher=School of Mathematics and Statistics, [[University of St Andrews]]
 |access-date=13 October 2010
 |url-status=dead
 |archive-url=https://web.archive.org/web/20110128034939/http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Maxwell.html
 |archive-date=28 January 2011
}}</ref> Maxwell backed up his claim with his own experiment published in the 1868 Philosophical Transactions which determined the ratio of the electrostatic and electromagnetic units of electricity.<ref>Campbell, Lewis; Garnett, William; Rautio, James C. "The Life of James Clerk Maxwell", p.&nbsp;544, {{ISBN|978-1-77375-139-9}}.</ref>