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.

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