## Site announcements

### On the revision of the International System of Units (SI)

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The General Conference on Weights and Measures (CGPM), at its 26th meeting, decides that, effective from 20 May 2019, the International System of Units, the SI, is the system of units in which:
1. the unperturbed ground state hyperfine transition frequency of the caesium 133 atom $$\Delta \nu_{Cs}$$ is 9 192 631 770 Hz,
2. the speed of light in vacuum c is 299 792 458 m/s,
3. the Planck constant h is 6.626 070 15 × 10-34 J s,
4. the elementary charge e is 1.602 176 634 × 10-19 C,
5. the Boltzmann constant k is 1.380 649 × 10-23 J/K,
6. the Avogadro constant NA is 6.022 140 76 × 1023 mol-1,
7. the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, is 683 lm/W,
where the hertz, joule, coulomb, lumen, and watt, with unit symbols Hz, J, C, lm, and W, respectively, are related to the units second, metre, kilogram, ampere, kelvin, mole, and candela, with unit symbols s, m, kg, A, K, mol, and cd, respectively, according to Hz = s–1, J = kg m2 s–2, C = As, lm = cd m2 m–2 = cd sr, and W = kg m2 s–3
Starting from the new definition of the SI described above in terms of fixed numerical values of the defining constants, definitions of each of the seven base units are deduced by taking, as appropriate, one or more of these defining constants to give the following set of definitions, effective from 20 May 2019:
• The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the cesium frequency $$\Delta\nu_{Cs}$$, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s–1.
• The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m/s, where the second is defined in terms of $$\Delta\nu_{Cs}$$.
• The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the metre and the second are defined in terms of c and $$\Delta\nu_{Cs}$$.
• The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10–19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of $$\Delta\nu_{Cs}$$.
• The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10–23 when expressed in the unit J K–1, which is equal to kg m2 s–2 K–1, where the kilogram, metre and second are defined in terms of h, c and $$\Delta\nu_{Cs}$$.
• The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 × 1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol–1 and is called the Avogadro number. The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.
• The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W–1, which is equal to cd sr W–1, or cd sr kg–1 m–2 s3, where the kilogram, metre and second are defined in terms of h, c and $$\Delta\nu_{Cs}$$.