SI (International System of Units)

SI (International System of Units)
Name	SI (International System of Units)
Introduced	1960
Based on	Metre Convention (1875)
Overseen by	International Bureau of Weights and Measures

SI (International System of Units) The SI is the modern form of the metric system and the internationally agreed system for measurement. It underpins scientific research, industrial manufacturing, and international trade across institutions such as the United Nations, World Trade Organization, and European Union, and it is foundational to work at laboratories like the National Institute of Standards and Technology and the International Bureau of Weights and Measures.

History

The roots of the system trace to the French Revolution, the adoption of the decimal metric proposals by the National Convention and the work of scientists such as Jean-Baptiste Joseph Delambre and Pierre Méchain who determined the meridian measurement that led to the metre; subsequent internationalization was fostered by the Metre Convention (1875) and the convening of the International Committee for Weights and Measures. Key developments occurred at conferences where delegates from nations including United Kingdom, Germany, United States, France, and Japan negotiated standard definitions; notable figures involved include James Clerk Maxwell and William Thomson, 1st Baron Kelvin. The formal adoption of the name and structure occurred at the 11th General Conference on Weights and Measures in 1960, influenced by institutions like the International Organization for Standardization and later refined by advances at facilities such as the CERN and the Max Planck Institute.

Structure and base units

The SI is organized around seven base units that define the system's dimensional foundation, a framework used in laboratories like the National Physical Laboratory and in standards set by bodies including the International Electrotechnical Commission. The seven base units are the metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (thermodynamic temperature), mole (amount of substance), and candela (luminous intensity). These units are realized through artefacts, experiments, and constants tied to work performed at institutions such as the Bureau International des Poids et Mesures, Physikalisch-Technische Bundesanstalt, and Laboratoire national de métrologie et d'essais.

Derived units and prefixes

Derived units in SI, such as the newton, joule, and pascal, are algebraic combinations of the base units and are widely used across disciplines from research at the Royal Society to applied engineering at the Siemens AG and General Electric. A set of standardized prefixes, from yocto- to yotta-, scales quantities by powers of ten and facilitates communication among organizations like the International Monetary Fund and companies such as IBM and Intel. Derived units sometimes carry special names honoring scientists—newton for Isaac Newton, joule for James Prescott Joule, and tesla for Nikola Tesla—reflecting the historical linkage between measurement and scientific achievement observed by academies such as the French Academy of Sciences and the Royal Society of London.

Definitions and realization

Definitions of SI units have evolved from physical artefacts to operational definitions based on fundamental constants, a transition culminating in the 2019 redefinition where constants like the speed of light, Planck constant, and elementary charge were fixed. This redefinition relied on precision experiments at facilities including the National Institute of Standards and Technology, International Bureau of Weights and Measures, and research groups at universities such as Massachusetts Institute of Technology, University of Oxford, and ETH Zurich. Realization methods involve technologies like the Kibble balance, X-ray crystal density determination used by teams at the National Research Council (Canada), and quantum electrical standards advanced in collaborations among Physikalisch-Technische Bundesanstalt, National Physical Laboratory, and NIST.

Governance and standardization

Governance of the SI rests with the General Conference on Weights and Measures and advisory bodies including the International Committee for Weights and Measures, with technical support from the International Bureau of Weights and Measures. National metrology institutes such as NIST, PTB, LNE, and VNIIM implement and disseminate standards, coordinating through organizations like the International Organization for Standardization and the International Electrotechnical Commission. Historical treaty frameworks such as the Metre Convention provide legal underpinning, while international scientific collaborations—e.g., projects at CERN, ITER, and multinational standardization efforts by the European Committee for Standardization—drive practical harmonization.

Usage and adoption worldwide

SI is the statutory system in most countries and is the preferred system for science and international commerce involving entities like the World Health Organization, World Meteorological Organization, and multinational corporations such as Siemens, Boeing, and Toyota. Some nations retain limited exceptions for customary units historically used in sectors linked to institutions like the United States customary units systems; nevertheless, industries from pharmaceuticals regulated by agencies like the European Medicines Agency to aerospace firms working with NASA and Roscosmos increasingly rely on SI. International education and publishing across universities including Harvard University, Peking University, University of Tokyo, and University of Cambridge teach and require SI for reproducibility, while standards compliance is audited by bodies such as ISO and national accreditation agencies.

Category:Units of measurement