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 It is superseded by ISO 80000-6.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 |
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 |