International System of Units

International System of Units
BIPM · Public domain · source
Name	International System of Units
Caption	Posters defining the seven base units.
Established	1960
Governing body	General Conference on Weights and Measures

International System of Units. The International System of Units is the world's most widely used system of measurement, forming the foundation for global science, industry, and commerce. Established and maintained by the General Conference on Weights and Measures, it provides a coherent framework of base and derived units defined by fundamental constants of nature. Its adoption promotes uniformity and precision in data exchange across nations and disciplines.

History and development

The system's origins trace to the French Revolution, which inspired the creation of the metric system to standardize measurements. In 1875, the Metre Convention established the International Bureau of Weights and Measures and the General Conference on Weights and Measures to oversee its development. Throughout the 20th century, evolving from the MKS system and later the Giorgi system, the modern system was formally named and redefined in 1960. A major revision in 2019 redefined several base units in terms of fundamental constants like the Planck constant and the elementary charge, moving away from physical artifacts like the International Prototype of the Kilogram.

Base units

The system is built upon seven base units, each representing a distinct dimension of physical measurement. These are the metre for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. Since the 2019 redefinition, all base units are now defined by fixing the numerical values of fundamental constants, such as the speed of light for the metre and the Boltzmann constant for the kelvin, ensuring their stability and universality.

Derived units

Many other essential units are derived coherently from the base units through mathematical relationships, without numerical factors. Examples include the hertz for frequency, the newton for force, the pascal for pressure, the joule for energy, and the watt for power. Important derived units with special names also include the volt for electric potential, the ohm for electrical resistance, and the tesla for magnetic flux density. The radian and steradian, units for plane and solid angle, are considered derived units of dimension one.

Prefixes and decimal multiples

A set of standard metric prefixes are used to form decimal multiples and submultiples of units, simplifying the expression of very large or very small quantities. Ranging from quetta (10³⁰) to quecto (10^-30), these prefixes include familiar terms like kilo, centi, and milli. Adopted by the General Conference on Weights and Measures, these prefixes, such as mega for 10⁶ or nano for 10^-9, can be applied to any unit, as seen in kilogram, megawatt, or nanometre.

Realisation of units

While definitions are based on constants, the practical creation, or realisation, of units requires specific experimental techniques. For instance, the metre can be realised using laser interferometry, while the second is realised using atomic clocks based on the caesium-133 transition. The kilogram can be realised through instruments like the Kibble balance or the Avogadro project, which involves counting atoms in a sphere of pure silicon. These realisations are coordinated globally by national metrology institutes like the National Institute of Standards and Technology and the Physikalisch-Technische Bundesanstalt.

Usage and conventions

The system is governed by stylistic conventions published by the International Bureau of Weights and Measures in its official brochure, often called the SI Brochure. Unit symbols, like "m" for metre or "A" for ampere, are unaltered in the plural and are written in upright type. A space separates the numerical value from the unit symbol, as in "100 m". The system is the preferred language of measurement in fields from particle physics at CERN to clinical trials overseen by the World Health Organization, and is mandated for use within the European Union through directives.

Category:Systems of units Category:Metrology