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SI

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{{alternateuses}} Image:SI Brochure Cover.jpg frame|right|Cover of brochure ''[http://www.bipm.org/en/publications/brochure/ The International System of Units]''. The '''International System of Units''' (abbreviated '''SI''' from the French language name ''Système International d'Unités'') is the modern form of the metric system. It is the world's most widely used system of units, both in everyday commerce and in science. The older metric system included several groupings of units. The SI was developed in 1960 from the metre-kilogram-second (MKS) system, rather than the centimetre gram second system of units centimetre-gram-second (CGS) system, which, in turn, had many variants. The SI introduced several newly named units. The SI is not static; it is a living set of standards where units are created and definitions are modified with international agreement as measurement technology progresses. With few exceptions (such as draught beer sales in the United Kingdom), the system is legally being used in every country in the world, and many countries do not maintain official definitions of other units. Metric system in the United States In the United States, industrial use of SI is increasing, but popular use is still limited. In the United Kingdom, metrication conversion to metric units is official policy but not yet complete. Those countries that still recognize non-SI units (e.g. the United States US and United Kingdom UK) have redefined most of their traditional non-SI units conversion of units in terms of SI units.

History
:''See main articles: metre, kilogram, second, ampere, Kelvin, candela, and mole (unit) mole. The metric system was created during the French Revolution. On August 1, 1793 the National Convention adopted the new decimal "metre" with a provisional length as well as the other decimal units with preliminary definitions and terms. On April 7, 1795 (Loi du 18 germinal, an III) the terms ''gramme'' and ''kilogramme'' repaced the former terms "gravet" and "grave". After the 18 Brumaire Year VIII, the metric system was definitively adopted in France by the First Consul Bonaparte, (the later Napoleon I of France Napoleon I) on Decembre 10, 1799 (Loi du 19 frimaire an VIII). During the Metric system#History history of the metric system a number of variations have evolved and their use spread around the world replacing many traditional Systems of measurement measurement systems. By the end of World War II a number of different systems of measurement were still in use throughout the world. Some of these systems were metric system variations whilst others were based on the Imperial unit Imperial and US customary units American systems. It was recognised that additional steps were needed to metrication promote a worldwide measurement system. As a result the 9th General Conference on Weights and Measures (CGPM), in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities. Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units recommended were the metre, kilogram, second, ampere, Kelvin degree (later renamed the kelvin), and the candela. In 1960, the 11th CGPM named the system the ''International System of Units'', abbreviated SI from the French name: ''Le Système International d'Unités''. The seventh base unit, the mole (unit) mole, was added in 1970 by the 14th CGPM. The International System is now either obligatory or permissible throughout the world. It is administered by the standards organisation: the Bureau International des Poids et Mesures (International Bureau of Weights and Measures).

Units
:''Main articles: SI base unit, SI derived unit, SI prefix'' The international system of units consists of a set of units together with a set of SI prefix prefixes. The units of SI can be divided into two subsets. There are the seven SI base unit base units. Each of these base units are dimensionally independent. From these seven base units several SI derived unit other units are derived. In addition to the SI units there are also a set of non-SI units accepted for use with SI. {| class="wikitable" ! colspan="3" |SI base units |- |'''Name''' |'''Symbol''' |'''Quantity''' |- |kilogram |'''kg''' |Mass |- |second |'''s''' |Time |- |metre |'''m''' |Displacement |- |ampere |'''A''' |Current (electricity) Electrical current |- |kelvin |'''K''' |Temperature |- |Mole (unit) mole |'''mol''' |Amount of substance |- |candela |'''cd''' |Luminous intensity |} A SI prefix prefix may be added to units to produce a multiple of the original unit. All multiples are integer powers of ten. For example, ''kilo-'' denotes a multiple of a thousand and ''milli-'' denotes a multiple of a thousandth hence there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined: a millionth of a kilogram is a ''milligram'' not a ''microkilogram''.

SI writing style
*Symbols are written in lower case, except for symbols derived from the name of a person. For example, the unit of pressure is named after Blaise Pascal, so its symbol is written "Pa" whereas the unit itself is written "pascal". The one exception is the litre, whose original symbol "l" is dangerously similar to the numeral "1". The National Institute of Standards and Technology NIST recommends that "L" be used instead, a usage which is common in the U.S., Canada and Australia, and has been accepted as an alternative by the General Conference on Weights and Measures CGPM. The cursive "â„“" is occasionally seen, especially in Japan, but this is not currently recommended by any standards body. For more information, see Litre#Symbol Litre. *Symbols are written without grammatical markers when used with singular numerals: i.e. "25 kg", not "25 kgs". Pluralization would be language dependent; "s" plurals (as in French and English) are particularly undesirable since "s" is the symbol of the second. Other cases may be marked in a language-dependent manner, e.g. Finnish ''25 kg:lla'' = ''25 kilogrammalla'' "with 25 kg". *Symbols do not have an appended period (.) unless at the end of a sentence. *It is preferable to write symbols in upright Roman type (m for metres, L for litres), so as to differentiate from the italic type used for mathematical variables (''m'' for mass, ''l'' for length). *A space should separate the number and the symbol, e.g. "2.21 kg", "7.3×10² m²", "22 °C" [http://physics.nist.gov/Pubs/SP811/sec07.html]. Exceptions are the symbols for plane angular degrees, minutes and seconds (°, ′ and ″), which are placed immediately after the number with no intervening space. *Spaces should be used to group decimal digits in threes, e.g. 1 000 000 or 342 142 (in contrast to the commas or dots used in other systems, e.g. 1,000,000 or 1.000.000). This is presumably to reduce confusion. *The 10th resolution of General Conference on Weights and Measures CGPM in 2003 declared that "the symbol for the decimal marker shall be either the full stop point on the line or the comma on the line". In practice, the full stop is used in English, and the comma in most other European languages. *Symbols for derived units formed from multiple units by multiplication are joined with a space or centre dot (·), e.g. N m or N·m. *Symbols formed by division of two units are joined with a slash (punctuation) solidus (/), or given as a negative exponent. For example, the "metre per second" can be written "m/s", "m s^-1", "m·s^-1" or $\backslash frac\{\backslash mbox\{m\}\}\{\backslash mbox\{s\}\}$. A solidus should not be used if the result is ambiguous, i.e. "kg·m^-1·s^-2" is preferable to "kg/m/s²".

Spelling variations
*Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre' in keeping with standard American English spelling. In addition, the official US spelling for the SI prefix 'deca' is 'deka'. *The unit 'gram' is also sometimes spelled 'gramme' in English-speaking countries other than the United States, though that is an older spelling and its use is declining.

Cultural issues
The swift worldwide adoption of the metric system as a tool of economy and everyday commerce was based mainly on the lack of customary systems in many countries to adequately describe some concepts, or as a result of an attempt to standardize the many regional variations in the customary system. International factors also affected the adoption of the metric system, as many countries increased their trade. Scientifically, it provides ease when dealing with very large and small quantities because it lines up so well with our decimal numeral system. Cultural differences can be represented in the local everyday uses of metric units. For example, bread is sold in one-half, one or two kilogram sizes in many countries, but you buy them by multiples of one hundred grams in the former USSR. In some countries, the informal ''cup'' measurement has become 250 mL, and prices for items are sometimes given per 100 g rather than per kilogram. A profound cultural difference between physicists and engineers, especially radio engineers, existed prior to the adoption of the metre-kilogram-second (MKS) system and hence its descendent, SI. Engineers work with volts, amperes, ohm (unit) ohms, farads, and coulomb coulombs, which are of great practical utility, while the centimetre gram second system of units centimetre-gram-second (CGS) units, which, though appropriate for theoretical physics, can be inconvenient for electrical engineering usage and are largely unfamiliar to householders using appliances rated in volts and watts. In the U.S., blood glucose measurements are recorded in milligrams per deciliter (mg/dL); in Europe, the standard is millimole/litre (mmol/L). The fine-tuning that has happened to the metric base units over the past 200 years, as experts have tried periodically to refine the metric system to fit the best scientific research, does not affect the everyday use of metric units. Since most non-SI units, such as the U.S. customary units, are nowadays defined in terms of SI units, any change in the definition of the SI units results in a change of the definition of the older units as well.

Trade
The European Union has a directive as a result of which non-SI markings will be banned after 2009 December 31 on any goods imported into the European Union. This applies to all markings on products, enclosed directions and papers, packaging, and advertisements.

See also
*Units of measurement *Weights and measures *Mesures usuelles *Metrified English unit *History of measurement *Other systems of measurement: **Imperial unit Imperial units **U.S. customary units **Metre-tonne-second system of units **Chinese unit Chinese system of units **Planck units **Atomic units **Geometrized unit system Geometrized units *CODATA *Metrication *Metric system in the United States *Metrology *Coordinated Universal Time Coordinated Universal Time (UTC) *Binary prefixes - used to quantify large amounts of computer data *Orders of magnitude *ISO 31

External links
''Official''
- BIPM (SI maintenance agency) (home page)
- BIPM brochure (SI reference)
- ISO 1000:1992 ''SI units and recommendations for the use of their multiples and of certain other units'', with its price tag of 99 Swiss francs for a 22 page, coverless pamphlet showing why the public is sometimes a little slow to pick up on their recommendations. ''Information''
- US NIST reference on SI *
- chart
- SI - Its history and use in science and industry
- A Dictionary of Units of Measurement
- Cyrillic transcription of SI symbols * Judson, Lewis B., ''Weights and Measures Standards of the United States: A brief history'', NIST NBS Special Publication 447, orig. iss. October 1963, updated March 1976 ([http://ts.nist.gov/ts/htdocs/200/202/SP%20447.pdf 46 page PDF file])
- Metric system and conversion tables (courtesy French property advice)
- metre-info - an encyclopaedia of all metric units '' Pro-metric pressure groups
- The UK Metric Association
- The US Metric Association '' Pro-customary measures pressure groups
- The British Weights and Measures Association

Further reading
*I. Mills, Tomislav Cvitas, Klaus Homann, Nikola Kallay, IUPAC: ''Quantities, Units and Symbols in Physical Chemistry'', 2nd ed., Blackwell Science Inc 1993, ISBN 0632035838. Category:SI units Category:Systems of units Category:International standards Category:Dimensional analysis {{Link FA|bg}} af:SI ar:نظام دولي للوحدات bg:Международна Ñ?иÑ?тема единици bs:SI sistem ca:Sistema internacional cs:Soustava SI da:SI{{Link FA|da}} de:Internationales Einheitensystem el:Διεθνές σÏ?στημα μονάδων et:SI-süsteem es:Sistema Internacional de Unidades eo:Sistemo Internacia de Unuoj eu:Nazioarteko Unitate Sistema fa:سیستم استاندارد بین‌المللی واحدها fr:Système international d'unités he:SI hu:SI mértékegységrendszer id:SI (satuan ukur) ia:Systema International de Unitates io:Unaj-internaciona sistemo is:Alþjóðlega einingakerfið it:Sistema Internazionale di unità di misura ko:SI 단위계 lv:SI lt:SI lb:SI-Eenheetesystem jbo:treci'e mr:आंतरराषà¥?टà¥?रीय गणना पदà¥?धती nl:SI ja:国際å?˜ä½?ç³» no:SI-systemet nn:SI-systemet pl:UkÅ‚ad SI pt:Sistema Internacional de Unidades ro:Sistemul internaÅ£ional de unităţi ru:СИ sco:SI simple:SI sk:SI sl:Mednarodni sistem enot sr:СИ su:SI fi:Kansainvälinen yksikköjärjestelmä sv:SI th:หน่วยเอสไอ vi:SI {{Link FA|vi}} uk:Міжнародна Ñ?иÑ?тема СІ zh:国际å?•ä½?制 '''Si''', '''si''', or '''SI''' may stand for: {{wiktionarypar3|si|Si|SI}} *''''SI'''', the Système International d'Unités (the 'metric' International System of Units) *one of two Spanish language Spanish words pronounced like "see": **''sí'' (accented) for "yes" and **''si'' (unaccented) for "conditional if" *a French language French word pronounced like "see" meaning "yes, on the contrary", "as," "however," "so," and "if." *in music, '''Si''', the seventh note of ''solfege'' *in chemistry, '''Si''', the chemical symbol for ''silicon'' *in Mohican mythology, '''Si''', the lunar deity *Station Inspector the highest non-commissioned officer Police rank rank in the Singapore Police Force. *Sinhalese language, (ISO 639 alpha-2) *Slovenia (ISO 3166-1 alpha-2 country code, '''SI''') *in United Kingdom British law, a Statutory Instrument *Smithsonian Institution *Socialist International *Spokane International Railroad {{reporting mark|SI}} *''Sports Illustrated'' *Sports Interactive *Signal Integrity *Situationist International *Staten Island, New York Staten Island, a borough of New York City and an island in the Hudson River *Swedish Institute (''Svenska institutet'') *System integrator *Supplemental Instruction *Sexual Intercourse (medical abbreviation, not common usage) *Self-injury or self-harm *'''SI''' is the acronym for Swarm intelligence Swarm Intelligence *In mathematics, '''si''' or Si is the Sine integral *the IATA code for Skynet Airlines *The '''Si''' is a self-replicating artifact in the roguelike game Ancient Domains of Mystery *The '''Si''' designation is used by Honda to denote high-performance variants of models they manufacture *The '''Shift in''' control code in the C0 and C1 control codes C0 control code set. {{2LCdisambig}} de:SI eo:Si fr:SI id:SI is:Si it:Si ko:Si nl:Si ja:SI pl:Si pt:SI sk:Si sl:Si fi:SI vi:SI (định hÆ°á»›ng) see Template:User metric

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