ISO 31-5

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 It is superseded by ISO 80000-6.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	${\begin{matrix}{\frac {1}{\mu _{0}{c_{0}}^{2}}}\end{matrix}}$	8.854 187 817... x 10⁻¹² F m⁻¹
Characteristic impedance of vacuum	Z₀	μ₀ c₀	376.730 313 461...Ω