kilogram

kilogram. The kilogram is the base unit of mass in the International System of Units, defined by fixing the numerical value of the Planck constant to exactly 6.62607015×10⁻³⁴ when expressed in the unit J⋅s, which is equal to kg⋅m²⋅s⁻¹. This definition, adopted in 2019, replaced the previous standard based on the International Prototype of the Kilogram, a physical artifact housed at the International Bureau of Weights and Measures near Paris. The unit is essential for science, commerce, and industry worldwide, providing the foundational standard for measurements of mass and derived quantities like force and pressure.

Definition and history

The concept of the kilogram originated during the French Revolution as part of the new metric system, with the original definition in 1795 being the mass of one cubic decimetre of water at the temperature of melting ice. This was soon replaced by the Kilogramme des Archives, a platinum artifact created in 1799. In 1875, the signing of the Metre Convention established the International Bureau of Weights and Measures and led to the creation of the International Prototype of the Kilogram in 1889, a cylinder of platinum-iridium alloy. For over a century, this prototype, kept under controlled conditions at the Pavillon de Breteuil in Sèvres, served as the definitive standard, with all national copies, such as those held by the National Institute of Standards and Technology in the United States and the National Physical Laboratory (United Kingdom), calibrated against it. Historical figures like James Clerk Maxwell and William Thomson, 1st Baron Kelvin had advocated for a definition based on fundamental constants, presaging the eventual shift away from an artifact standard.

Physical realization

Following the 2019 redefinition, the kilogram is no longer tied to a physical object but is realized through experiments that measure the Planck constant with extreme precision. The primary method is the Kibble balance (originally known as the watt balance), an apparatus that compares mechanical power to electrical power, effectively weighing mass against electromagnetic forces. Key developments of this balance were achieved at the National Institute of Standards and Technology and the National Research Council (Canada). An alternative realization method uses the Avogadro project, which involves counting the number of atoms in a nearly perfect sphere of pure silicon-28 to determine the Avogadro constant. This approach was advanced through international collaborations like the International Avogadro Coordination involving institutions such as the Physikalisch-Technische Bundesanstalt in Germany. These realizations allow national metrology institutes to establish their own mass standards traceable to the fundamental constants, ensuring global uniformity without reliance on a single artifact.

Redefinition of 2019

The redefinition was formally voted on and adopted at the 26th General Conference on Weights and Measures held at the International Bureau of Weights and Measures in Versailles in November 2018, with the change taking effect on World Metrology Day, May 20, 2019. This change was part of a broader revision of the International System of Units that also redefined the ampere, kelvin, and mole. The decision was driven by the long-term instability of the International Prototype of the Kilogram, which had been found to drift in mass by microscopic amounts compared to its official copies over decades. The new definition ties the kilogram to the immutable Planck constant, a fundamental constant from quantum mechanics, thereby creating a stable and universal standard. This shift was the culmination of decades of research by metrologists worldwide, including teams at the International Bureau of Weights and Measures, the National Physical Laboratory (United Kingdom), and the National Institute of Standards and Technology.

SI multiples and submultiples

Mass is commonly expressed using standard SI prefixes with the gram (one-thousandth of a kilogram) as a base. For large masses, multiples include the megagram (Mg), equivalent to one tonne as defined in the International System of Units, and the gigagram (Gg). In scientific contexts, the microgram (µg) and milligram (mg) are frequently used in fields like pharmacology and chemistry, while the nanogram (ng) and picogram (pg) are common in molecular biology and toxicology. The metric ton, or tonne, equal to 1000 kilograms, is a standard unit in industries such as shipping, mining, and agriculture, and is recognized by organizations like the International Organization for Standardization. These prefixes, established by the General Conference on Weights and Measures, ensure coherent scaling across all scientific and engineering disciplines.

Related units

Many non-SI units of mass are legally defined in terms of the kilogram. The avoirdupois system's pound (mass), used in the United States and the United Kingdom, is defined as exactly 0.45359237 kilograms by international agreement, such as the Yard and Pound Agreement of 1959. The troy ounce, used for precious metals like gold and silver, is defined as approximately 31.1035 grams. In scientific fields, the atomic mass unit (unified), symbolized 'u' or 'Da' for the dalton, is a tiny unit of mass used in chemistry and physics, defined as one-twelfth the mass of a carbon-12 atom and approximately equal to 1.66053906660×10⁻²⁷ kg. Other historical units, like the German pfund and the French livre, were also aligned to the kilogram during the metrication processes of the 19th and 20th centuries.