The international system of units (SI)

The SI (Système international dʹunités), adopted by the Conférence générale des poids et mesures in 1960, was the outcome of almost one century of progress in the establishment of a consistent system of measurement units. We present its main features with its base and derived units, which constitutes a coherent set of units related by rule of multiplication and division with no numeric factor other than one. The historical evolution and the definitions of the seven base units are presented together with a short description of how they are realised in practice. The specific case of the units in ionising radiation is discussed, and some hints of what could be the future of the SI are proposed. To cite this article: J. Kovalevsky, T.J. Quinn, C. R. Physique 5 (2004).