The International System of Units (Système International d'Unités) or SI is comprised of seven base units; the metre, second, kilogram, kelvin, mole, ampere and candela. Together these units allow for the harmonisation of any measurements made around the world, forming a foundation around which cooperation and collaboration on a global scale can take place.

The International System of Units (SI)

Since the 18th century, when a committee of the French Academy of Sciences introduced the first unified measurement system based on the artefacts for the metre and kilogram, these units have been under constant improvement and revision.

On the 20th May 1875, 17 nations signed the ‘Metre Convention’. Signatories to this new treaty agreed to promote the ‘metric system’, to create standards for the metre and the kilogram and to set up an organisation to maintain the standards and coordinate comparisons

In 2018 at the 26th General Conference on Weights and Measures (CGPM) the Metre Convention’s Member States voted to revise the SI, linking all seven units to fundamental properties of nature.

On the 20th May 2019 these changes came into effect ensuring their ongoing refinement and improvement for years to come.

The second (s) is defined as the hyperfine transition frequency of a caesium 133 atom in its unperturbed ground state (ΔνCs): Revised 1967.

The metre is defined by the speed of light in a vacuum (c): Revised (1975) 1983

The candela is defined by the luminous efficacy of a defined visible radiation (Kcd): Revised 1979.

The kilogram is defined in terms of the Planck constant (h): Revised 2019

The ampere is defined in terms of the elementary charge (e): Revised 2019

The Kelvin is defined in terms of the Boltzmann constant (k): Revised 2019

The mole is defined in terms of the Avogadro constant (NA): Revised 2019

What was the redefinition?

In 2019,  after decades of ground-breaking laboratory work, the world’s scientific and technical community redefined four of the seven base units for the International System of Units (SI) linking them to fundamental constants of the universe.