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ISO 31-5 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to electricity and magnetism. It is superseded by ISO 80000-6.

Some of its definitions are below, with values taken from NIST 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 ${\displaystyle {\begin{matrix}{\frac {1}{\mu _{0}{c_{0))^{2))}\end{matrix))}$ 8.854 187 817... x 10−12 F m−1
Characteristic impedance of vacuum Z0 μ0 c0 376.730 313 461...Ω