kilogram

kilogram
Coyau · CC BY-SA 3.0 · source
Name	kilogram
Quantity	mass
Standard	International System of Units
Defined	20 May 2019 (by fixed numerical value of the Planck constant)
Derived from	Planck constant, metre, second

kilogram The kilogram is the SI base unit of mass historically defined by a physical artefact and since 2019 defined by a fixed value of the Planck constant. It serves as the principal reference for mass in scientific metrology, international trade, industrial standards and scientific research. National metrology institutes coordinate through international organizations to realize, compare and disseminate the unit for commerce, medicine, aerospace and fundamental physics.

Definition and SI role

The kilogram is one of seven SI base units alongside metre, second, ampere, kelvin, mole and candela in the system managed by the International Bureau of Weights and Measures and ratified by the General Conference on Weights and Measures. As the SI base unit for mass it underpins derived units such as the newton and the joule and features in international agreements including standards set by the International Organization for Standardization and conventions of the International Telecommunication Union for measurement traceability. Its modern definition ties mass to fundamental constants used in precision experiments at laboratories like the National Institute of Standards and Technology, the National Physical Laboratory (United Kingdom), and the Physikalisch-Technische Bundesanstalt.

History of the kilogram

Origins trace to proposals during the late 18th century French metric reforms under figures like Jean-Baptiste Delambre and Pierre-Simon Laplace and legislation of the French First Republic that established metric prototypes. The International Prototype Kilogram, a platinum–iridium cylinder produced in 1879 at the International Bureau of Weights and Measures, became the global artefact after the Metre Convention and subsequent treaties. Throughout the 19th and 20th centuries national prototypes held by institutions such as the Bureau International des Poids et Mesures and the United States National Prototype Kilogram in the United States formed the calibration chain used by industrial powers including United Kingdom, Germany, France, Japan, and Italy.

Redefinition and realization (2019)

A global move to definitions based on fundamental constants culminated in a resolution at the 26th meeting of the General Conference on Weights and Measures where member states adopted a new SI in 2018, implemented on 20 May 2019. The kilogram was redefined by fixing the numerical value of the Planck constant and relating mass to the metre and second via quantum mechanical principles. Realizations use experimental apparatus such as the Kibble balance and silicon-sphere experiments at laboratories including NIST, NPL, PTB, LNE, and CIPM-affiliated institutes to realize the unit traceable to the fixed Planck constant.

Measurement and calibration

Practical dissemination of the kilogram involves comparison chains between national measurement institutes using mass comparators, vacuum balance systems, and frequency-stabilized lasers linked to atomic clocks like those at BIPM and NIST. Calibration laboratories follow protocols from bodies such as the International Organization for Standardization and the International Committee for Weights and Measures while interlaboratory comparisons coordinate through the Mutual Recognition Arrangement and key comparisons organized by the BIPM. Techniques include electrostatic and electromagnetic mass balances, artifact comparisons, and quantum electrical standards employing the Josephson effect and quantum Hall effect to connect electrical measurements to the Planck constant.

Applications and usage

The kilogram underlies measurement in industries and sectors represented by institutions and treaties such as World Trade Organization agreements on metrology for trade, medical dosing standards in hospitals accredited by bodies like Joint Commission, pharmaceutical manufacturing regulated by agencies including the Food and Drug Administration, and aerospace engineering by organizations such as European Space Agency and NASA. Research in particle physics at facilities such as CERN and materials science at universities like Massachusetts Institute of Technology relies on precise mass standards for experiments and calibrations. International commerce, agricultural commodity trading, and safety-critical manufacturing use kilogram-based mass standards maintained by national metrology institutes.

Symbols, units and derived quantities

The official symbol for the unit is "kg". Derived SI units that include mass are the newton (kg·m/s^2) for force, the joule (kg·m^2/s^2) for energy, and the pascal (kg·m^-1·s^-2) for pressure, all linked to the kilogram through definitions of the metre and second. Non-SI units historically related to the unit include the gram, the tonne, and customary units like the pound (mass), which are reconciled through internationally agreed conversion factors adopted in standards by bodies such as the International Organization for Standardization and national standards agencies.

Category:Units of mass