{wiki}

https://web.archive.org/web/20181119214326/https://www.bipm.org/utils/common/pdf/CGPM-2018/26th-CGPM-Resolutions.pdf gives it in raw:
> * the unperturbed ground state hyperfine transition frequency of the <caesium-133> atom $\Delta v_{Cs}$ is 9 192 631 770 Hz
  * the speed of light in vacuum c is 299 792 458 m/s
  * the <Planck constant> h is 6.626 070 15 × $10^{-34}$ J s
  * the elementary charge e is 1.602 176 634 × $10^{-19}$ C
  * the <Boltzmann constant> k is 1.380 649 × $10^{-23}$ J/K
  * the Avogadro constant NA is 6.022 140 76 × $10^{23}$ mol
  * the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, is 683 lm/W,
The breakdown is:
* actually use some physical constant:
  * > the unperturbed ground state hyperfine transition frequency of the <caesium-133> atom $\Delta v_{Cs}$ is 9 192 631 770 Hz

    Defines the <second> in terms of <caesium-133> experiments. The beauty of this definition is that we only have to count an integer number of discrete events, which is what allows us to make things precise.
  * > the speed of light in vacuum c is 299 792 458 m/s

    Defines the <meter> in terms of <speed of light> experiments. We already had the <second> from the previous definition.
  * > the <Planck constant> h is 6.626 070 15 × $10^{-34}$ J s

    Defines the <kilogram> in terms of the <Planck constant>.
  * > the elementary charge e is 1.602 176 634 × $10^{-19}$ C

    Defines the <Coulomb> in terms of the <electron charge>.
* arbitrary definitions based on the above just to match historical values as well as possible:
  * > the <Boltzmann constant> k is 1.380 649 × $10^{-23}$ J/K

    Arbitrarily defines temperature from previously defined energy (J) to match historical values.
  * > the Avogadro constant NA is 6.022 140 76 × $10^{23}$ mol

    Arbitrarily defines the mol to match historical values. In particular, the <kilogram> is not an exact multiple of the weight of an atom of <hydrogen>.
  * > the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, is 683 lm/W

    Arbitrarily defines the Candela in terms of previous values to match historical records. The most useless unit comes last as you'd expect.


2019 redefinition of the SI base units

{disambiguate=system of units}

A dimension in a <system of units> is something like <length>, <weight> or <time>, without considering how to assign numerical values ot them, which requires <units of measurement> such as the <meter>, <kilogram> or <second>.

Talking about dimensions can be useful when explaining new derived units without worrying about the exact units involved. See e.g. this table: https://en.wikipedia.org/w/index.php?title=Lumen_(unit)&oldid=1233810964#SI_photometric_units


Ciro Santilli @cirosantilli 37

 Incoming links: Meter