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ISO 31-5 is the part ofinternational standardISO 31 that defines names and symbols forquantities andunits related toelectricity and magnetism. It is superseded byISO 80000-6.

Some of its definitions are below, with values taken fromNIST values of the constants:

Name	Symbol	Definition	Value
Speed of light in vacuum	c0	299 792 458 m s−1	299 792 458 m s−1
Magnetic constant	μ0	4π × 10−7 N A−2	12.566 370 614... x 10−7 N A−2
Electric constant	ε0	${\displaystyle \begin{array}{c}\frac{1}{{\mu }_0{c_0}^2}\end{array}}$	8.854 187 817... x 10−12 F m−1
Characteristic impedance of vacuum	Z0	μ0c0	376.730 313 461...Ω

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