The updated International System of Units (SI)

Richard Davis (Bureau International des Poids et Mesures)

The SI has been with us since 1960, but its roots go much deeper. For the SI to remain relevant to its most demanding users, the definitions of its seven “base units” (from which all others can be derived) have been updated from time to time. Notable examples are the second (defined in 1960 by astronomical observations ; redefined in 1967 by the frequency of an atomic transition) and the metre (defined in 1960 by a wavelength of light emitted by a krypton lamp ; redefined in 1983 by the speed of light in vacuum, combined with the definition of the second). Another major revision of the SI is expected to be approved later this year and to come into force in May, 2019. Four base units will be redefined : the kilogram, the ampere, the kelvin and the mole. In particular, the kilogram is defined at present as the mass of an object manufactured in 1889, but will be redefined by the Planck constant in combination with the definitions of the metre and second, which are unchanged. My presentation will take a global look at the revised SI and its impact on the scientific community.

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