# Measuring Planck's Constant

## Redefining the International System of Units (SI) in 2018

In the "New SI" four of the SI base units – namely the kilogram, the ampere, the kelvin and the mole – will be redefined in terms of constants; the new definitions will be based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (kB), and the Avogadro constant (NA), respectively. Further, the definitions of all seven base units of the SI will also be uniformly expressed using the explicit-constant formulation, and specific mises en pratique will be drawn up to explain the realization of the definitions of each of the base units in a practical way.

Key to SI Definition Constants:
Δν = the unperturbed ground state hyperfine transition frequency of the caesium-133 atom ΔνCs is 9,192,631,770 hertz
c = the speed of light in vacuum c is exactly 299,792,458 metres per second
h = the Planck constant h is exactly 6.626 06X x 10–34 joule seconds
e = the elementary charge e is exactly 1.602 17X x 10–19 coulombs
kB = the Boltzmann constant k is exactly 1.380 6X x 10–23 joules per kelvin
NA = the Avogadro constant NA is exactly 6.022 14X x 1023 reciprocal moles
Kcd = the luminous efficacy Kcd of monochromatic radiation of frequency 540 x 1012 Hz is exactly 683 lumens per watt
Note: X represents the yet unknown final digit(s).