kilogram (SI base unit)

kilogram (SI base unit)
Name	kilogram
Quantity	mass
Defined	2019-05-20 (since)
Definition	defined by fixing the numerical value of the Planck constant h to 6.62607015×10^−34 J·s
Unit1	gram
Conv1	1 kg = 1000 g

kilogram (SI base unit).

The kilogram is the International System of Units (SI) base unit for mass. It serves as the primary coherent unit in measurements underpinning physics, chemistry, engineering, and international trade through metrology institutions such as the International Bureau of Weights and Measures and standards bodies like the International Organization for Standardization and Bureau International des Poids et Mesures laboratories. Its definition, practical realizations, and dissemination involve concepts and organizations across modern science including the Planck constant, the International Prototype of the Kilogram, and national metrology institutes such as the National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, and National Physical Laboratory.

Definition and properties

The kilogram is defined by fixing the numerical value of the Planck constant h to 6.62607015×10^−34 when expressed in the unit J·s, which is kg·m^2·s^−1, thus linking the kilogram to the metre and the second. This definition ties mass to quantum physics and fundamental constants used in theories developed by figures and institutions such as Albert Einstein, Max Planck, and the Coulomb law context used in precision electromagnetism at facilities like National Metrology Institute of Japan and Laboratoire national de métrologie et d'essais. The kilogram is coherent with SI base units including the metre, second, and ampere, enabling derived units like the newton and joule to be expressed consistently. Properties relevant to practice include invariance under changes in material artifacts, traceability via electromagnetic and quantum standards, and compatibility with precision measurement methods employed by the International Committee for Weights and Measures and national standards laboratories.

History and redefinitions

Early metric mass units arose during the French Revolution with the metric system and the original kilogram was defined as the mass of one litre of water at the temperature of maximum density. The International Prototype of the Kilogram (IPK), created and maintained at the Bureau International des Poids et Mesures in Sèvres, replaced that practical water definition and served as the mass standard from the late 19th century until 2019. Debates and experiments involving institutions such as the National Research Council (Canada), Observatoire de Paris, International Avogadro Coordination, and scientists influenced proposals to redefine the kilogram in terms of fixed constants including the Avogadro constant and the Planck constant. The 2019 redefinition, adopted by the General Conference on Weights and Measures, re-established the kilogram by fixing h, thereby completing a shift analogous to prior redefinitions of the metre (linked to the speed of light) and the second (linked to atomic transitions such as the cesium standard).

Realization and measurement

Realization of the kilogram occurs through experiments that relate mass to the fixed Planck constant, primarily via the Kibble balance (formerly called the watt balance) and the X-ray crystal density (XRCD) or Avogadro project approach using silicon spheres. Kibble balance experiments at metrology institutes including the National Institute of Standards and Technology, National Research Council (Canada), Physikalisch-Technische Bundesanstalt, and National Physical Laboratory equate mechanical power and electromagnetic power using quantum electrical standards tied to the Josephson effect and quantum Hall effect. XRCD experiments use ultra-pure silicon single crystals produced and measured with interferometry methods developed at institutions like Institut National de Métrologie, du Mesures et d'Essais and NMIJ. Precision measurement relies on collaborations with particle physics and condensed-matter research centers such as CERN and Max Planck Institute for Quantum Optics for crystalline and quantum metrology expertise.

Units and multiples

The kilogram is the SI base unit for mass; coherent multiples and submultiples use SI prefixes standardized by International Organization for Standardization recommendations. Common multiples include the megagram (tonne) used in international commerce and industry, while common submultiples include the gram (g), milligram (mg), and microgram (µg). Derived units combining mass with other base quantities produce units such as the newton (kg·m·s^−2) for force and the joule (kg·m^2·s^−2) for energy, which are essential in standards applied by organizations like the International Electrotechnical Commission and research institutions such as Imperial College London and Massachusetts Institute of Technology.

Applications and significance

Accurate mass measurement affects science, industry, and commerce—from fundamental physics experiments at facilities like Fermilab and CERN to pharmaceutical dosing regulated by agencies such as the Food and Drug Administration and European Medicines Agency. Precision mass standards enable trade infrastructures overseen by bodies like the World Trade Organization and quality systems used by corporations such as Siemens and General Electric. In research, linking the kilogram to h supports high-precision determinations of constants like the Boltzmann constant and Avogadro constant, which underpin thermodynamics, chemistry, and materials science at universities and laboratories including Harvard University and ETH Zurich.

Standards and international custody

Custody and dissemination of the kilogram standard involve the International Bureau of Weights and Measures and national metrology institutes including National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, National Physical Laboratory, International Organization for Standardization, and regional bodies like the European Association of National Metrology Institutes. Following the 2019 redefinition, international traceability is maintained by intercomparisons, calibration services, and published measurement protocols coordinated through the General Conference on Weights and Measures and working groups of the International Committee for Weights and Measures to ensure uniformity for science, industry, and commerce.

Category:SI base units