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| Units of mass | |
|---|---|
| Name | Units of mass |
| Quantity | Mass |
| SI unit | kilogram (kg) |
| Introduced | Ancient times to present |
Units of mass
Units of mass are standardized quantities used to express the amount of matter in physical objects; they underpin measurement in International System of Units, metrology institutions, trade, science, and engineering. Systems for mass evolved through interactions among ancient Egypt, Babylon, Roman Empire, Medieval Europe, and modern nation-states leading to international agreements such as the Treaty of the Meter and work by organizations like the International Bureau of Weights and Measures and the National Institute of Standards and Technology. Standards for mass affect legal frameworks, industrial production, and scientific research at institutions such as CERN, MIT, Caltech, and NASA.
Early mass units emerged in civilizational centers including Ancient Egypt, Mesopotamia, Indus Valley Civilization, and the Han dynasty, where societies used artifacts, commodity measures, and balances tied to local trade and taxation. Classical and medieval units—such as those used in the Roman Empire and by merchants in Medieval Europe—varied regionally until reforms promoted uniformity; reforms include the decimalization movements linked to the French Revolution and the creation of the Metric system. The 19th and 20th centuries saw international cooperation via the Congress of Paris (1878), the Metre Convention (1875), and establishment of the International System of Units led by the International Committee for Weights and Measures. Advances in physics and quantum standards at NIST, BIPM, and research centers such as University of Oxford, Imperial College London, and ETH Zurich culminated in the 2019 redefinition of the kilogram based on the Planck constant and instruments like the Kibble balance.
Major systems include the International System of Units with the kilogram as the base, the former Metric system variants, and customary systems such as the Imperial units and United States customary units. Standardization bodies—BIPM, ISO, IEC, ASTM International, and national metrology institutes like PTB, LNE, and NPL—provide protocols, legal metrology frameworks, and traceability chains. The modern kilogram is realized by fixing the Planck constant and implementing measurements via the Kibble balance and silicon sphere experiments undertaken by teams at NIST, INRIM, and METAS. International accords such as agreements under the Metre Convention and protocols from OIML coordinate global legal and technical harmonization.
Common SI-derived and non-SI units include the kilogram (kg), gram (g), tonne (t), pound (lb), ounce (oz), stone, carat, atomic mass unit (u), Dalton (Da), and electronvolt-equivalent mass used in particle physics. Conversion relationships—such as 1 kg = 2.2046226218488 lb, 1 t = 1000 kg, 1 u ≈ 1.66053906660×10^−27 kg—are applied in contexts ranging from pharmacopoeia dosage standards to aerospace payloads and automotive manufacturing. Units like the slug (unit) appear in historical engineering texts and American practice; specialized units include the long ton and short ton tied to maritime and industrial history involving ports and navies such as Royal Navy and United States Navy logistical records.
Mass measurement uses balances, scales, and inertial methods. Traditional mass comparison employed balance scales exemplified in archeological collections at museums like the British Museum; precision mass metrology now uses the Kibble balance, electrostatic balances, hydrostatic weighing, and mass spectrometry at facilities including CERN and national labs. Instruments require environmental control such as temperature and vibration isolation used in laboratories at NPL and PTB. Emerging techniques in micro- and nano-scales leverage atomic force microscopy groups at IBM Research and ion trap experiments from laboratories like Max Planck Institute for Quantum Optics and Harvard University for inertial mass determinations of single particles.
Achieving high precision relies on traceability chains linking working standards to primary standards maintained by BIPM, calibrated using protocols from ISO/IEC 17025 and guidance by OIML. Uncertainty analysis follows frameworks developed by ISO, BIPM guides, and metrologists at NIST, employing statistical treatment of Type A and Type B uncertainties and propagation methods taught in institutions like University of Cambridge and Stanford University. Calibration laboratories participate in key comparisons organized under the CCM to quantify degrees of equivalence; quality assurance in industries references standards such as ISO 9001 and ISO 17025.
Mass units underpin trade standards in commodities governed by bodies like the World Trade Organization and commodity exchanges in cities such as London and New York City. Industries from pharmaceuticals regulated by agencies like the European Medicines Agency and FDA to aerospace companies like Boeing and Airbus rely on certified mass standards for compliance and safety. Mass metrology informs research at facilities such as CERN, LIGO Laboratory, and European Space Agency, and supports sectors including logistics firms like Maersk and DHL for freight measurement, and standards in manufacturing adopted by Siemens and General Electric.
Category:Measurement