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| SI system | |
|---|---|
| Name | International System of Units |
| Abbreviation | SI |
| Established | 1960 |
| Base units | metre, kilogram, second, ampere, kelvin, mole, candela |
| Maintained by | International Bureau of Weights and Measures |
| Website | International Bureau of Weights and Measures |
SI system
The SI system is the modern, internationally adopted framework for measurement used across science, industry, and commerce. It provides a coherent set of base units and derived units, supported by definitions linked to physical constants and maintained by international institutions. The system underpins reproducible measurement in laboratories, Organisation Internationale de Métrologie Légale, European Union, United States, Japan and other jurisdictions, enabling interoperability among National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, National Physical Laboratory (United Kingdom), Bureau International des Poids et Mesures and research infrastructures such as CERN.
The system developed from the late 18th-century reforms of the French Revolution when the National Convention and figures like Talleyrand promoted decimalization and the metre and kilogram prototypes. Subsequent milestones include the 1875 signing of the Metre Convention, establishing international coordination via the International Bureau of Weights and Measures and the 1889 international prototype kilogram. The 1960 General Conference on Weights and Measures formalized the "International System" naming at the General Conference on Weights and Measures (1960), while later sessions—most notably the 1983 and 2019 meetings—revised unit definitions, linking them to constants through work by scientists at National Institute of Standards and Technology, International Committee for Weights and Measures, and metrologists such as researchers associated with Max Planck Society and CNRS. The 2019 redefinition replaced artefact-based standards with definitions tied to constants like the Planck constant and the speed of light as adopted by delegates at the General Conference on Weights and Measures (2018).
SI defines seven base units that serve as the foundation for all other quantities. The metre, redefined in 1983, is realized via the speed of light in vacuum and optical techniques developed in laboratories including National Research Council (Canada) facilities. The kilogram was redefined in 2019 by fixing the Planck constant following advances in the Kibble balance experiments at institutes such as National Physical Laboratory (United Kingdom) and NIST. The second is defined by the hyperfine transition frequency of caesium-133 atoms, a standard used in International Atomic Time and atomic clocks at institutions like Physikalisch-Technische Bundesanstalt. The ampere now links to the elementary charge and electrical quantum standards developed in Quantum Hall effect and single-electron devices investigated by teams at PTB and NPL. The kelvin is tied to the Boltzmann constant following determinations using acoustic and dielectric-constant gas thermometry by groups at National Institute of Standards and Technology and Laboratoire national de métrologie et d'essais. The mole is defined by a fixed Avogadro constant, important for chemical metrology conducted at universities like Massachusetts Institute of Technology and University of Tokyo. The candela is based on luminous efficacy and photometric standards maintained in standards labs such as National Physical Laboratory (India).
Derived units arise from algebraic combinations of base units and include familiar measures like the newton, joule, pascal, and watt, used in engineering contexts at organizations including Siemens AG, Airbus, and Siemens Energy. Coherent derived units simplify dimensional analysis in research at MIT, Imperial College London, and ETH Zurich. SI also specifies decimal prefixes from yocto- to yotta- for multiples and submultiples, conventions important for industries represented by International Electrotechnical Commission and IEEE standards committees. Specialized units such as becquerel and gray are widely used in facilities like International Atomic Energy Agency laboratories and CERN radiation monitoring programs.
The SI rests on principles of coherence, reproducibility, and universal accessibility promulgated by the General Conference on Weights and Measures and the International Committee for Weights and Measures. Coherence ensures derived units are consistent with base units without additional factors, a concept central to treatments in texts by scholars at University of Cambridge and Harvard University. Definitions have evolved from material artefacts to constants of nature, e.g., fixing values for the speed of light, Planck constant, elementary charge, Boltzmann constant, and Avogadro constant, reflecting theoretical developments stemming from work by Max Planck, Albert Einstein, and experimentalists in quantum metrology. The SI adopts conventions for quantity symbols and unit names used in publications from institutions such as Nature Publishing Group and American Physical Society.
Realization refers to procedures by which units are physically implemented in laboratories, often via primary standards like atomic clocks, Kibble balances, and optical frequency combs pioneered at NIST, PTB, SYRTE, and LNE. International comparisons, proficiency testing, and calibration hierarchies are coordinated through key comparisons organized by the Bureau International des Poids et Mesures and regional metrology organizations such as EURAMET and APMP. Traceability chains link national measurement institutes, industrial calibration services, and instrumentation manufacturers including Fluke Corporation and Keysight Technologies to ensure consistency in measurements underpinning trade regulated by bodies like the World Trade Organization.
Governance is exercised by the General Conference on Weights and Measures, advised by the International Committee for Weights and Measures and technical committees drawn from national metrology institutes such as NIST, PTB, and NMI Australia. Revisions arise from scientific evidence evaluated through interlaboratory comparisons and published results in journals including Physical Review Letters and Metrologia. The 2019 revision exemplifies consensus decision-making with input from international research consortia, committees, and expert panels convened under the Metre Convention framework.
SI enables interoperability across sectors: precision manufacturing at companies like Toyota and Siemens, pharmaceutical metrology at Pfizer and GlaxoSmithKline, climate science models produced by groups at NASA and European Space Agency, and telecommunications standards developed by 3GPP and ITU. Education systems at universities including Stanford University and University of Oxford teach SI-based curricula that support international research collaborations like Large Hadron Collider experiments. The system’s universality facilitates international trade overseen by World Trade Organization rules, regulatory compliance in agencies such as European Medicines Agency, and reproducibility in science published across global journals.