International System of Units

International System of Units
BIPM · Public domain · source
Name	International System of Units
Caption	SI base units symbol set
Established	1960
Governing body	International Bureau of Weights and Measures
Based on	Metric system
Units	seven base units

International System of Units is the modern form of the metric system adopted as the global standard for measurement. Developed through multilateral work by national metrology institutes, international organizations, and scientific bodies, it provides coherent units for science, engineering, commerce, and industry. The system underpins standards maintained by institutions such as the International Bureau of Weights and Measures, the General Conference on Weights and Measures, the International Organization for Standardization, and national bodies like the National Institute of Standards and Technology and the Physikalisch-Technische Bundesanstalt.

History

The system evolved from proposals originating in the age of revolution, notably the French Revolution initiatives that produced the Metric system and the original definitions endorsed by the French Academy of Sciences. Subsequent international cooperation involved conferences such as the Metre Convention of 1875 and organizations including the International Committee for Weights and Measures and the International Bureau of Weights and Measures. Key developments include the 1889 prototype metre and kilogram artifacts, the 1948 resumption of scientific metrology after World War II, and the formal adoption of the modern system at the 11th General Conference on Weights and Measures in 1960. Later milestones include the 1983 redefinition of the metre by the 13th General Conference on Weights and Measures and the 2019 redefinition of four base units following recommendations from the Committee on Data for Science and Technology and the Consultative Committee for Units.

Base units and derived units

The system defines seven base units that serve as foundations for derived units. The base units include the second (time), metre (length), kilogram (mass), ampere (electric current), kelvin (thermodynamic temperature), mole (amount of substance), and candela (luminous intensity). Derived units such as the newton, joule, pascal, watt, tesla, and hertz are coherent combinations of base units and are used across disciplines represented by institutions like the International Union of Pure and Applied Chemistry, the International Union of Pure and Applied Physics, and the World Health Organization. Specialized derived units and supplementary units have been endorsed in technical standards by bodies such as the International Electrotechnical Commission and the European Committee for Standardization.

Definitions and realization

Historically some units were realized by physical artifacts such as the International Prototype of the Kilogram, held under the authority of the International Bureau of Weights and Measures. Modern practice anchors units to invariant constants of nature: the metre to the speed of light, the second to the caesium-133 hyperfine transition, the kelvin to the Boltzmann constant, the mole to the Avogadro constant, and the kilogram to the Planck constant. These redefinitions followed precision measurement campaigns by laboratories including the National Physical Laboratory (United Kingdom), the NPL of India, the Bureau International des Poids et Mesures, and the Laboratoire national de métrologie et d'essais. Realization techniques include interferometry used at facilities like the National Institute of Standards and Technology, and Kibble balance experiments performed by teams at the International Avogadro Project and the Swiss Federal Institute of Metrology.

Prefixes and multiples

A standardized set of prefixes provides decimal multiples and submultiples spanning large ranges. Prefixes such as kilo-, mega-, giga-, tera-, pico-, nano-, micro-, and milli- are widely applied in fields represented by organizations like the Institute of Electrical and Electronics Engineers, the European Space Agency, the CERN accelerator community, and the Royal Society. Recent additions to the prefix list have been adopted following proposals from metrology communities tied to institutes such as the Centre National de la Recherche Scientifique and the Max Planck Society. Application of prefixes is governed by usage conventions promoted by standards bodies including the International Organization for Standardization and the International Electrotechnical Commission.

SI unit conventions and usage

Conventions govern unit symbols, capitalization, spacing, and numeric presentation to ensure clarity in publications by entities such as the American Physical Society, the Institute of Physics, and the American Chemical Society. Guidelines cover the use of unit symbols in technical documents, typographical rules in journals like Nature and Science, and national implementations by agencies such as the Bureau International des Poids et Mesures member states and the United States National Institute of Standards and Technology. Legal adoption and statutory usage vary among jurisdictions exemplified by the European Union directives, the United States Metric Conversion Act, and regulatory frameworks influenced by the World Trade Organization.

Governance and revisions

Governance rests with the General Conference on Weights and Measures, supported by the International Committee for Weights and Measures and technical consultative committees such as the Consultative Committee for Units and the Consultative Committee for Thermometry. Revisions arise from advances in physics and technology, debated in forums including the International Union of Pure and Applied Physics, the International Union of Pure and Applied Chemistry, and meetings hosted by the International Bureau of Weights and Measures. Major redefinitions, such as those enacted in 2019, result from coordinated proposals and votes by member states of the Metre Convention, with input from national metrology institutes including the National Research Council (Canada), the Council of the European Union, and regional coalitions like the Asia Pacific Metrology Programme.

Category:Systems of measurement