The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of thePlanck constanth to be6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where themetre and thesecond are defined in terms ofc andΔνCs.
1793: Thegrave (the precursor of the kilogram) was defined as the mass of 1litre (dm3) of water, which was determined to be 18841grains.[11]
1795: the gram (1/1000 of a kilogram) was provisionally defined as the mass of one cubiccentimetre of water at the melting point of ice.[12]
1799: TheKilogramme des Archives was manufactured as a prototype. It had a mass equal to the mass of 1 dm3 of water at the temperature of its maximum density, which is approximately 4 °C.[13]
The kilogram is the onlybase SI unit with anSI prefix (kilo) as part of its name. The wordkilogramme orkilogram is derived from theFrenchkilogramme,[15] which itself was a learned coinage, prefixing theGreek stem ofχίλιοι‹See RfD›khilioi "a thousand" togramma, a Late Latin term for "a small weight", itself from Greekγράμμα.[16] The wordkilogramme was written into French law in 1795, in theDecree of18 Germinal,[17]which revised the provisional system of units introduced by the FrenchNational Convention two years earlier, where thegravet had been defined as weight (poids) of a cubic centimetre of water, equal to 1/1000 of agrave.[18] In the decree of 1795, the termgramme thus replacedgravet, andkilogramme replacedgrave.[13]
The French spelling was adopted in Great Britain when the word was used for the first time in English in 1795,[19][15] with the spellingkilogram being adopted in the United States. In the United Kingdom both spellings are used, with "kilogram" having become by far the more common.[1] UK law regulating the units to be used whentrading by weight or measure does not prevent the use of either spelling.[20]
In the 19th century the French wordkilo, ashortening ofkilogramme, was imported into the English language where it has been used to mean both kilogram[21] and kilometre.[22] Whilekilo as an alternative is acceptable, toThe Economist for example,[23] the Canadian government'sTermium Plus system states that "SI (International System of Units) usage, followed in scientific and technical writing" does not allow its usage and it is described as "a common informal name" on Russ Rowlett's Dictionary of Units of Measurement.[24][25] When theUnited States Congress gave the metric system legal status in 1866, it permitted the use of the wordkilo as an alternative to the wordkilogram,[26] but in 1990 revoked the status of the wordkilo.[27]
The SI system was introduced in 1960 and in 1970 theBIPM started publishing theSI Brochure, which contains all relevant decisions and recommendations by theCGPM concerning units. TheSI Brochure states that "It is not permissible to use abbreviations for unit symbols or unit names ...".[28][Note 2]
For use with east Asian character sets, the SI symbol is encoded as a single Unicode character,U+338F㎏SQUARE KG in theCJK Compatibility block.
TheSI system after the 2019 redefinition: the kilogram is now fixed in terms of thesecond, thespeed of light and thePlanck constant; furthermore theampere no longer depends on the kilogramAKibble balance, which was originally used to measure thePlanck constant in terms of the IPK, can now be used to calibrate secondary standard weights for practical use.
The replacement of theInternational Prototype of the Kilogram (IPK) as the primary standard was motivated by evidence accumulated over a long period of time that the mass of the IPK and its replicas had been changing; the IPK had diverged from its replicas by approximately 50 micrograms since their manufacture late in the 19th century. This led toseveral competing efforts to develop measurement technology precise enough to warrant replacing the kilogram artefact with a definition based directly on physical fundamental constants.[6]
Prior to the redefinition, the kilogram and several other SI units based on the kilogram were defined by a man-made metal artifact: theKilogramme des Archives from 1799 to 1889, and the IPK from 1889 to 2019.[6]
In 1960, themetre, previously similarly having been defined with reference to a single platinum-iridium bar with two marks on it, was redefined in terms of an invariant physical constant (the wavelength of a particular emission of light emitted bykrypton,[30] and later thespeed of light) so that the standard can be independently reproduced in different laboratories by following a written specification.
At the 94th Meeting of the CIPM in 2005, it was recommended that the same be done with the kilogram.[31]
In October 2010, the CIPM voted to submit a resolution for consideration at theGeneral Conference on Weights and Measures (CGPM), to "take note of an intention" that the kilogram be defined in terms of thePlanck constant,h (which has dimensions of energy times time, thus mass × length2 / time) together with other physical constants.[32][33] This resolution was accepted by the 24th conference of the CGPM[34] in October 2011 and further discussed at the 25th conference in 2014.[35][36] Although the Committee recognised that significant progress had been made, they concluded that the data did not yet appear sufficiently robust to adopt the revised definition, and that work should continue to enable the adoption at the 26th meeting, scheduled for 2018.[35] Such a definition would theoretically permit any apparatus that was capable of delineating the kilogram in terms of the Planck constant to be used as long as it possessed sufficient precision, accuracy and stability. TheKibble balance is one way to do this.[37]
As part of this project, a variety of verydifferent technologies and approaches were considered and explored over many years. Some of these approaches were based on equipment and procedures that would enable the reproducible production of new, kilogram-mass prototypes on demand (albeit with extraordinary effort) using measurement techniques and material properties that are ultimately based on, or traceable to, physical constants. Others were based on devices that measured either the acceleration or weight of hand-tuned kilogram test masses and that expressed their magnitudes in electrical terms via special components that permit traceability to physical constants. All approaches depend on converting a weight measurement to a mass and therefore require precise measurement of the strength of gravity in laboratories (gravimetry). All approaches would have precisely fixed one or more constants of nature at a defined value.[citation needed]
Because an SI unit may not have multiple prefixes (seeSI prefix), prefixes are added togram, rather than the base unitkilogram, which already has a prefix as part of its name.[38] For instance, one-millionth of a kilogram is 1mg (one milligram), not 1μkg (one microkilogram).
Serious medication errors have been made by confusing milligrams and micrograms when micrograms has been abbreviated.[39] The abbreviation "mcg" rather than the SI symbol "μg" is formally mandated for medical practitioners in the US by theJoint Commission on Accreditation of Healthcare Organizations (JCAHO).[40] In the United Kingdom, theNational Institute for Health and Care Excellence and Scottish Palliative Care Guidelines state that "micrograms" and "nanograms" must both be written in full, and never abbreviated as "mcg" or "μg".[39][41]
The hectogram (100 g) (Italian:ettogrammo oretto) is a very commonly used unit in the retail food trade in Italy.[42][43][44]
The former standard spelling and abbreviation "deka-" and "dk" produced abbreviations such as "dkm" (dekametre) and "dkg" (dekagram).[45] As of 2020,[update] the abbreviation "dkg" (10 g) is still used in parts of central Europe in retail for some foods such as cheese and meat.[46][47][48][49][50]
The unit namemegagram is rarely used, and even then typically only in technical fields in contexts where especially rigorous consistency with the SI standard is desired. For most purposes, the nametonne is instead used. The tonne and its symbol, "t", were adopted by the CIPM in 1879. It is a non-SI unit accepted by the BIPM for use with the SI. According to the BIPM, "This unit is sometimes referred to as 'metric ton' in some English-speaking countries."[51]
^ab"Kilogram".Oxford English Dictionary. Oxford University Press. Archived fromthe original on 30 January 2013. Retrieved3 November 2011.
^Fowlers, HW; Fowler, FG (1964).The Concise Oxford Dictionary. Oxford: The Clarendon Press. Greekγράμμα (as it wereγράφ-μα, Doricγράθμα) means "something written, a letter", but it came to be used as a unit of weight, apparently equal to1/24 of anounce (1/288 of alibra, which would correspond to about 1.14 grams in modern units), at some time during Late Antiquity. Frenchgramme was adopted from Latingramma, itself quite obscure, but found in theCarmen de ponderibus et mensuris (8.25) attributed byRemmius Palaemon (fl. 1st century), where it is the weight of twooboli (Charlton T. Lewis, Charles Short,A Latin Dictionarys.v. "gramma", 1879).Henry George Liddell. Robert Scott.A Greek-English Lexicon (revised and augmented edition, Oxford, 1940)s.v. γράμμα, citing the 10th-century workGeoponica and a 4th-century papyrus edited in L. Mitteis,Griechische Urkunden der Papyrussammlung zu Leipzig, vol. i (1906), 62 ii 27.
^Convention nationale, décret du 1er août 1793, ed. Duvergier,Collection complète des lois, décrets, ordonnances, règlemens avis du Conseil d'état, publiée sur les éditions officielles du Louvre, vol. 6 (2nd ed. 1834),p. 70.Themetre (mètre) on which this definition depends was itself defined as the ten-millionth part of a quarter of Earth'smeridian, given intraditional units as 3pieds, 11.44lignes (aligne being the 12th part of apouce (inch), or the 144th part of apied.
^Peltier, Jean-Gabriel (1795)."Paris, during the year 1795".Monthly Review.17: 556. Retrieved2 August 2018. Contemporaneous English translation of the French decree of 1795
^Tom Stobart,The Cook's Encyclopedia, 1981, p. 525
^J.J. Kinder, V.M. Savini,Using Italian: A Guide to Contemporary Usage, 2004,ISBN0521485568, p. 231
^Giacomo Devoto, Gian Carlo Oli,Nuovo vocabolario illustrato della lingua italiana, 1987,s.v. 'ètto': "frequentissima nell'uso comune:un e. di caffè, un e. di mortadella; formaggio a 2000 lire l'etto"
^U.S. National Bureau of Standards,The International Metric System of Weights and Measures, "Official Abbreviations of International Metric Units", 1932,p. 13
NIST: This particularRueprecht Balance, an Austrian-made precision balance, was used by the NIST from 1945 until 1960
BIPM:The FB‑2 flexure-strip balance, the BIPM's modern precision balance featuring a standard deviation of one ten-billionth of a kilogram (0.1μg)
BIPM:Mettler HK1000 balance, featuring 1μg resolution and a 4kg maximum mass. Also used by NIST and Sandia National Laboratories' Primary Standards Laboratory
