گرَم و کیلوگرم برای اولین بار در سالهای اولِ انقلاب فرانسه و برای یکسانسازی واحدهای اندازهگیری در سراسر کشور ابداع شدند. با توجه به اینکه چگالی آب حدود ۱۰۰۰ کیلوگرم بر متر مکعب است، بنابراین، یک کیلوگرم جِرم یک لیترآب خالص در شرایط عادی (هوای ۲۰ درجه) میباشد.
از سال ۱۸۸۹، واحد کیلوگرم بر اساس یک سیلندر ساخته شده از آلیاژ پلاتینیوم – ایریدیوم که با نام مستعار «لو گران کا» به اصطلاح «ک بزرگ» شهرت داشت تعیین می شد. برای حفظ این سیلندر از هر نوع آلودگی و یا تغییر، آن را در چندین محفظه تو در تو در یک خزانه در حومه پاریس نگهداری میکنند.
ولی در روز دوشنبه ۲۰ مِی ۲۰۱۹، «لو گران کا» موقعیت ویژه خود را از دست داد و دیگر معیار اندازه گیری واحد جرم کیلوگرم در جهان نخواهد بود. چرا که سیلندری که مرجع اندازه گیری کیلوگرم است یک جسم فیزیکی است و به مرور زمان جرم آن تغییر می کند.
از این به بعد، کیلوگرم - همراه با واحدهای دیگر کلوین، آمپر، مول، و شمع - به جای استفاده از مرجع ساخته دست انسان، با مبانی فیزیک و خواص اتمی توصیف خواهد شد.
ثابت پلانک - که یکی از مفاهیم اساسی در فیزیک کوانتوم و یک ثابت پایدار دقیق و قابل دسترس جهانی است - جایگزین «لو گران کا» خواهد شد.
The kilogram (also kilogramme) is the base unit of mass in the metric system, formally the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering, and commerce worldwide, and is often simply called a kilo in everyday speech.
The kilogram was originally defined in 1795 as the mass of one litre of water. This was a simple definition, but difficult to use in practice. By the latest definitions of the unit, however, this relationship still has an accuracy of 30 ppm. In 1799, the platinum Kilogramme des Archives replaced it as the standard of mass. In 1879, a cylinder of platinum-iridium, the International Prototype of the Kilogram (IPK) became the standard of the unit of mass for the metric system, and remained so until 20 May 2019. The kilogram is now defined in terms of the second and the metre, based on fixed fundamental constants of nature, as approved by the General Conference on Weights and Measures (CGPM) on 16 November 2018, making the kilogram the last of the metric base units to be defined by a physical artefact.
The replacement of the International Prototype Kilogram as 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 to several competing efforts to develop measurement technology precise enough to warrant replacing the kilogram artefact with a definition based directly on physical fundamental constants. Physical standard masses such as the IPK and its replicas still serve as secondary standards.
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.
This definition makes the kilogram consistent with the older definitions: the mass remains within 30 ppm of the mass of one litre of water.
The development of the first metric system began about 1790. The initial mass unit was the grave, defined in 1793. Within three years it was replaced by the kilogram.
The gram, 1/1000 of a kilogram, was provisionally defined in 1795 as the mass of one cubic centimetre of water at the melting point of ice. The Kilogramme des Archives was manufactured as a prototype in 1799 and served as a basis for the International Prototype Kilogram (IPK) in 1875. It had a mass equal to the mass of 1 dm3 of water under atmospheric pressure and at the temperature of its maximum density, which is approximately 4 °C.
The kilogram was the last SI unit that was directly defined by an artefact rather than fundamental physical properties that could be independently reproduced in different laboratories. Three other base units (cd, A, mol) and 17 derived units (N, Pa, J, W, C, V, F, Ω, S, Wb, T, H, kat, Gy, Sv, lm, lx) in the SI system were defined in relation to the kilogram, and thus its stability was important. The definitions of only eight other named SI units did not depend on the kilogram: those of temperature (K, °C), time and frequency (s, Hz, Bq), length (m), and angle (rad, sr).
Replacement of the International Prototype Kilogram
After the International Prototype Kilogram had been found to vary in mass over time relative to its reproductions, the International Committee for Weights and Measures (CIPM) recommended in 2005 that the kilogram be redefined in terms of a fundamental constant of nature. At its 2011 meeting, the CGPM agreed in principle that the kilogram should be redefined in terms of the Planck constant, h. The decision was originally deferred until 2014; in 2014 it was deferred again until the next meeting. CIPM proposed revised definitions of the SI base units for consideration at the 26th CGPM. The formal vote on 16 November 2018 approved the change.
Name and terminology
The kilogram is the only base SI unit with an SI prefix (kilo) as part of its name. The word kilogramme or kilogram is derived from the Frenchkilogramme, which itself was a learned coinage, prefixing the Greek stem of χίλιοιkhilioi "a thousand" to gramma, a Late Latin term for "a small weight", itself from Greek γράμμα.
The word kilogramme was written into French law in 1795, in the Decree of 18 Germinal,
which revised the provisional system of units introduced by the French National Convention two years earlier, where the gravet had been defined as weight (poids) of a cubic centimetre of water, equal to 1/1000 of a grave. In the decree of 1795, the term gramme thus replaced gravet, and kilogramme replaced grave.
The French spelling was adopted in Great Britain when the word was used for the first time in English in 1795, with the spelling kilogram being adopted in the United States. In the United Kingdom both spellings are used, with "kilogram" having become by far the more common.[Note 2] UK law regulating the units to be used when trading by weight or measure does not prevent the use of either spelling.
In the 19th century the French word kilo, a shortening of kilogramme, was imported into the English language where it has been used to mean both kilogram and kilometre. While kilo as an alternative is acceptable, to The Economist for example, the Canadian government's Termium 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. When the United States Congress gave the metric system legal status in 1866, it permitted the use of the word kilo as an alternative to the word kilogram, but in 1990 revoked the status of the word kilo.
During the 19th century, the standard system of metric units was the centimetre–gram–second system of units, treating the gram as the fundamental unit of mass and the kilogram as a derived unit.
In 1901, however, following the discovery by James Clerk Maxwell that electric measurements could not be explained solely in terms of the three fundamental units of length, mass and time, Giovanni Giorgi proposed a new standard system that would include a fourth fundamental unit to measure quantities in electromagnetism.
In 1935 this was adopted by the IEC as the Giorgi system, now also known as MKS system,
and in 1946 the CIPM approved a proposal to adopt the ampere as the electromagnetic unit of the "MKSA system".:109,110
In 1948 the CGPM commissioned the CIPM "to make recommendations for a single practical system of units of measurement, suitable for adoption by all countries adhering to the Metre Convention". This led to the launch of SI in 1960 and the subsequent publication of the "SI Brochure", which stated that "It is not permissible to use abbreviations for unit symbols or unit names ...".[Note 3]
The CGS and MKS systems co-existed during much of the early-to-mid 20th century, but as a result of the decision to adopt the "Giorgi system" as the international system of units in 1960, the kilogram is now the SI base unit for mass, while the definition of the gram is derived from that of the kilogram.
The International Committee for Weights and Measures (CIPM) approved a redefinition of the SI base units in November 2018 that defines the kilogram by defining the Planck constant to be exactly 6.62607015×10−34 kg⋅m2⋅s−1, effectively defining the kilogram in terms of the second and the metre. The new definition took effect on 20 May 2019.
In 1960, the metre, 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 by krypton, and later the speed of light) so that the standard can be independently reproduced in different laboratories by following a written specification.
In October 2010, the CIPM voted to submit a resolution for consideration at the General Conference on Weights and Measures (CGPM), to "take note of an intention" that the kilogram be defined in terms of the Planck constant, h (which has dimensions of energy times time) together with other physical constants. This resolution was accepted by the 24th conference of the CGPM in October 2011 and further discussed at the 25th conference in 2014. 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. 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. The Kibble balance is one way to do this.
As part of this project, a variety of very different 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 which 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 the precise measurement of the strength of gravity in laboratories. All approaches would have precisely fixed one or more constants of nature at a defined value.
Because SI prefixes may not be concatenated (serially linked) within the name or symbol for a unit of measure, SI prefixes are used with the unit gram, not kilogram, which already has a prefix as part of its name. For instance, one-millionth of a kilogram is 1mg (one milligram), not 1μkg (one microkilogram).
The microgram is typically abbreviated "mcg" in pharmaceutical and nutritional supplement labelling, to avoid confusion, since the "μ" prefix is not always well recognised outside of technical disciplines.[Note 5] (The expression "mcg" is also the symbol for an obsolete CGS unit of measure known as the "millicentigram", which is equal to 10μg.)
In the United Kingdom, because serious medication errors have been made from the confusion between milligrams and micrograms when micrograms has been abbreviated, the recommendation given in the Scottish Palliative Care Guidelines is that doses of less than one milligram must be expressed in micrograms and that the word microgram must be written in full, and that it is never acceptable to use "mcg" or "μg".
The hectogram (100 g) is a very commonly used unit in the retail food trade in Italy, usually called an etto, short for ettogrammo, the Italian for hectogram.
The former standard spelling and abbreviation "deka-" and "dk" produced abbreviations such as "dkm" (dekametre) and "dkg" (dekagram). The abbreviation "dkg" (10 g) is still used in parts of central Europe in retail for some foods such as cheese and meat.
The unit name megagram 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 name tonne 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." The unit name megatonne or megaton (Mt) is often used in general-interest literature on greenhouse gas emissions, whereas the equivalent unit in scientific papers on the subject is often the teragram (Tg).
^Gramme, le poids absolu d'un volume d'eau pure égal au cube de la centième partie du mètre, et à la température de la glace fondante; The term poids absolu was used alongside masse for the concept of "mass" (which latter term had first been introduced in its strict physical sense in English in 1704).
^Wood, B. (November 3–4, 2014). "Report on the Meeting of the CODATA Task Group on Fundamental Constants"(PDF). BIPM. p. 7. [BIPM director Martin] Milton responded to a question about what would happen if ... the CIPM or the CGPM voted not to move forward with the redefinition of the SI. He responded that he felt that by that time the decision to move forward should be seen as a foregone conclusion.
^ ab"Kilogram". Oxford English Dictionary. Oxford University Press. Retrieved November 3, 2011.
^Fowlers, HW; Fowler, FG (1964). The Concise Oxford Dictionary. Oxford: The Clarendon Press.
Greek γράμμα (as it were γράφ-μα, Doric γράθμα) means "something written, a letter", but it came to be used as a unit of weight, apparently equal to 1/24 of an ounce (1/288 of a libra, which would correspond to about 1.14 grams in modern units), at some time during Late Antiquity. French gramme was adopted from Latin gramma, itself quite obscure, but found in the Carmen de ponderibus et mensuris (8.25) attributed by Remmius Palaemon (fl. 1st century), where it is the weight of two oboli (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 work Geoponica 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.
The metre (mètre) on which this definition depends was itself defined as the ten-millionth part of a quarter of Earth's meridian, given in traditional units as 3 pieds, 11.44 lignes (a ligne being the 12th part of a pouce (inch), or the 144th part of a pied.
^Peltier, Jean-Gabriel (1795). "Paris, during the year 1795". Monthly Review. 17: 556. Retrieved August 2, 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