Editing Speed of light (section)

=== Electromagnetic constants ===
An option for deriving ''c'' that does not directly depend on a measurement of the propagation of electromagnetic waves is to use the relation between ''c'' and the [[vacuum permittivity]] ''ε''<sub>0</sub> and [[vacuum permeability]] ''μ''<sub>0</sub> established by Maxwell's theory: ''c''<sup>2</sup>&nbsp;=&nbsp;1/(''ε''<sub>0</sub>''μ''<sub>0</sub>). The vacuum permittivity may be determined by measuring the [[capacitance]] and dimensions of a [[capacitor]], whereas the value of the vacuum permeability was historically fixed at exactly {{val|4|end=π|e=-7|u=H.m-1}} through the definition of the [[ampere (unit)|ampere]]. [[Edward Bennett Rosa|Rosa]] and [[Noah Ernest Dorsey|Dorsey]] used this method in 1907 to find a value of {{val|299710|22|u=km/s}}. Their method depended upon having a standard unit of electrical resistance, the "international [[ohm]]", and so its accuracy was limited by how this standard was defined.<ref name="Essen1948"/><ref name="RosaDorsey">{{Cite journal |last1=Rosa |first1=E. B. |author-link=Edward Bennett Rosa |last2=Dorsey |first2=N. E. |author-link2=Noah Ernest Dorsey |year=1907 |title=A new determination of the ratio of the electromagnetic to the electrostatic unit of electricity |journal=Bulletin of the Bureau of Standards |volume=3 |issue=6 |page=433 |doi=10.6028/bulletin.070 |doi-access=free}}</ref>