Other pages in this series (7)
M6
Exact-value transitions
Constants that became exact, and exact ones that became measured
Takeaway
- With the 2019 revision of the SI, the Planck constant, elementary charge, Boltzmann constant and Avogadro constant are exact from the 2018 edition (fixed by definition rather than measured; uncertainty zero).
- Conversely, the vacuum magnetic permeability μ₀ and the molar mass constant went from exact to measured. Uncertainty does not vanish; it moves to other quantities.
- Definition
- Constants whose status changed between editions, from measured (with an uncertainty) to exact (fixed by definition rather than measured, uncertainty zero), or the reverse, and when.
- Formula
- A change in the exact flag between two consecutive editions.
- Denominator
- The 31 quantities on this site.
- How to read it
- The speed of light became exact when the meter was redefined in terms of light in 1983, and has zero uncertainty from the 1986 edition. With the 2019 revision of the SI, the Planck constant, elementary charge, Boltzmann constant and Avogadro constant, and the Faraday constant, molar gas constant and Stefan–Boltzmann constant computed from them, are exact from the 2018 edition. Conversely, the vacuum magnetic permeability μ₀ and the molar mass constant M_u were exact up to the 2014 edition and are measured from 2018.
- Caveats
- A constant leaves the M1–M4 calculations from the edition in which it becomes exact. Becoming exact does not mean knowledge of the quantity is complete: the uncertainty moves to the quantities derived from it.
| Name | Edition | Transition | Relative standard uncertainty (when measured) |
|---|---|---|---|
| speed of light in vacuum | 1973→1986 | measured → exact | 4.003e-09 |
| Faraday constant | 2014→2018 | measured → exact | 6.115e-09 |
| molar mass constant | 2014→2018 | exact → measured | 3.000e-10 |
| Avogadro constant | 2014→2018 | measured → exact | 1.229e-08 |
| molar gas constant | 2014→2018 | measured → exact | 5.773e-07 |
| elementary charge | 2014→2018 | measured → exact | 6.117e-09 |
| Planck constant | 2014→2018 | measured → exact | 1.222e-08 |
| Boltzmann constant | 2014→2018 | measured → exact | 5.722e-07 |
| vacuum magnetic permeability | 2014→2018 | exact → measured | 1.512e-10 |
| Stefan–Boltzmann constant | 2014→2018 | measured → exact | 2.293e-06 |