The International System of Units (abbreviated SI from the French French is a Romance language spoken as a first language by about 136 million people worldwide. Around 190 million people speak French as a second language, and an additional 200 million speak it as an acquired foreign language. French speaking communities are present in 57 countries and territories. Most native speakers of the language live in Système international d'unités^[1]) is the modern form of the metric system The metric system is an international decimalised system of measurement, first adopted by France in 1791, that is the common system of measuring units used by most of the world. It exists in several variations, with different choices of fundamental units, though the choice of base units does not affect its day-to-day use. Over the last two and is generally a system of units of measurement A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of measurement devised around seven base units The International System of Units defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are: and the convenience of the number ten. It is the world's most widely used system of measurement A system of measurement is a set of units which can be used to specify anything which can be measured and were historically important, regulated and defined because of trade and internal commerce. Scientifically, when later analyzed, some quantities are designated as fundamental units meaning all other needed units can be derived from them,, both in everyday commerce Commerce is a division of trade or production which deals with the exchange of goods and services from producer to final consumer OR commerce is the exchange of goods and services from the point of production to the point of consumption to satisfy human wants. It comprises the trading of something of economic value such as goods, services, and in science Science is a systematic enterprise of gathering knowledge about nature and organizing and condensing that knowledge into testable laws and theories. As knowledge has increased, some methods have proved more reliable than others, and today the scientific method is the standard for science. It includes the use of careful observation, experimentation,.^[2][3]

The older metric system included several groups of units. The SI was developed in 1960 from the old metre-kilogram-second A physical system of units that expresses any given measurement using fundamental units of the metre, kilogram, and/or second system, rather than the centimetre-gram-second The centimetre-gram-second system is a metric system of physical units based on centimetre as the unit of length, gram as a unit of mass, and second as a unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways of extending the CGS system to cover electromagnetism system, which, in turn, had a few variants. Because the SI is not static, units are created and definitions are modified through international agreement among many nations as the technology of measurement progresses, and as the precision of measurements improves.

The system has been nearly globally adopted Metrication refers to the introduction and use of the SI metric system, the international standard for physical measurements. This has involved a long process of independent and systematic conversions of countries from various local systems of weights and measures. Metrication began in France in the 1790s and spread widely during the following two. Three principal exceptions are Burma (Myanmar) Burmese units or the Burmese system is a collection of units used in Burma. Burma is one of only three nations to primarily use a non-metric system of units, the others being Liberia and the United States. While most Burmese units are used solely in the nation, Imperial system units such as the furlong and acre are also used, Liberia Liberia /laɪˈbɪəriə/ , officially the Republic of Liberia, is a country on the west coast of Africa, bordered by Sierra Leone, Guinea, Côte d'Ivoire, and the Atlantic Ocean. As of the 2008 Census, the nation is home to 3,476,608 people and covers 111,369 square kilometres (43,000 sq mi), and the United States Metrication in the United States is the process of introducing the International System of units to replace the customary units of measurement that are the primary units of measurement in the United States. The U.S. is one of three countries that do not officially use the metric system, along with Burma (Myanmar) and Liberia. In the U.S. the. The United Kingdom has officially adopted the International System of Units Metrication is the process of introducing metric units for measurement. Although the first recorded proposal for a decimal system of measure was made by a John Wilkins, a Briton in 1668 and the adoption of metric units has been discussed regularly by Parliament since 1818, it was only in 1965 that the formal policy of metrication started but not with the intention of replacing customary measures entirely.

Realisation of units

It is very important to distinguish between the definition of a unit and its realisation. The definition of each base unit of the SI is carefully drawn up so that it is unique and provides a sound theoretical basis upon which the most accurate and reproducible measurements can be made. The realisation of the definition of a unit is the procedure by which the definition may be used to establish the value and associated uncertainty of a quantity of the same kind as the unit. A description of how the definitions of some important units are realised in practice is given on the BIPM website.^[4]

A coherent SI derived unit can be expressed in SI base units with no numerical factor other than the number 1.^[5] The coherent SI derived unit of resistance, the ohm, symbol Ω, for example, is uniquely defined by the relation Ω = m²·kg·s⁻³·A⁻², which follows from the definition of the quantity electrical resistance The electrical resistance of an object is a measure of its attraction to the passage of a steady electric current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material. However, "any method consistent with the laws of physics could be used to realise any SI unit."^[6] (p. 111).

History

The metric system The metric system is an international decimalised system of measurement, first adopted by France in 1791, that is the common system of measuring units used by most of the world. It exists in several variations, with different choices of fundamental units, though the choice of base units does not affect its day-to-day use. Over the last two was conceived by a group of scientists (among them, Antoine-Laurent Lavoisier Antoine-Laurent de Lavoisier ; (French pronunciation: [ɑ̃twan lɔʁɑ̃ də lavwazje]), the "father of modern chemistry", was a French nobleman prominent in the histories of chemistry and biology. He stated the first version of the law of conservation of mass, recognized and named oxygen (1778) and hydrogen (1783), abolished the, who is known as the "father of modern chemistry") who had been commissioned by Louis XVI Louis XVI ruled as King of France and Navarre from 1774 until 1791, and then as King of the French from 1791 to 1792. Suspended and arrested during the Insurrection of 10 August 1792, he was tried by the National Convention, found guilty of high treason, and executed by guillotine on 21 January 1793. He was the only king of France ever to be of France to create a unified and rational system of measures. After the French Revolution The French Revolution was a period of radical social and political upheaval in French and European history. The absolute monarchy that had ruled France for centuries collapsed in three years. French society underwent an epic transformation as feudal, aristocratic, and religious privileges evaporated under a sustained assault from liberal political, the system was adopted by the new government.^[7] On 1 August 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 7 April 1795 (Loi du 18 germinal, an III) the terms gramme and kilogramme replaced the former terms gravet (correctly milligrave) and grave A grave is a metallic reference standard of one thousand grams that was used for a few years until it was replaced by the kilogram standard in 1799. On 10 December 1799 (a month after Napoleon's coup d'état), the metric system was definitively adopted in France.

The desire for international cooperation on metrology Metrology (from Ancient Greek metron and logos (study of)) is the science of measurement. Metrology includes all theoretical and practical aspects of measurement led to the signing in 1875 of the Metre Convention, a treaty A treaty is an agreement under international law entered into by actors in international law, namely sovereign states and international organizations. A treaty may also be known as: agreement, protocol, covenant, convention, exchange of letters, etc. Regardless of the terminology, all of these international agreements under international law are which established three international organizations An international organization is an organization with an international membership, scope, or presence. There are two main types: to oversee the keeping of metric standards:

• General Conference on Weights and Measures The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units (SI) under the terms of the Convention du Mètre (Metre Convention) of 1875. It meets in Sèvres (in the southwestern suburbs of Paris) (Conférence générale des poids et mesures or CGPM) - a meeting every four to six years of delegates from all member states;
• International Bureau of Weights and Measures The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units (SI) under the terms of the Metre Convention (Convention du Mètre). The organisation is usually referred to by its French initialism, BIPM (Bureau international des poids et mesures or BIPM) - an international metrology centre at Sèvres Sèvres is a commune in the southwestern suburbs of Paris, France. It is located 9.9 km from the center of Paris in France; and
• International Committee for Weights and Measures The International Committee for Weights and Measures is the English name of the Comité international des poids et mesures . It consists of eighteen persons from Member States of the Metre Convention (Convention du Mètre). Its principal task is to ensure world-wide uniformity in units of measurement and it does this by direct action or by (Comité international des poids et mesures or CIPM) - an administrative committee which meets annually at the BIPM.

The history of the metric system The metric system is an international decimalised system of measurement, first adopted by France in 1791, that is the common system of measuring units used by most of the world. It exists in several variations, with different choices of fundamental units, though the choice of base units does not affect its day-to-day use. Over the last two has seen a number of variations, whose use has spread around the world, to replace many traditional measurement systems A system of measurement is a set of units which can be used to specify anything which can be measured and were historically important, regulated and defined because of trade and internal commerce. Scientifically, when later analyzed, some quantities are designated as fundamental units meaning all other needed units can be derived from them,. At the end of World War II Albania · Australia · Austria · Azerbaijan · Belarus · Belgium · Brazil · Bulgaria · Burma · Cambodia · Canada · Ceylon (Sri Lanka) · Channel Islands · China · Czechoslovakia · Denmark · Dutch East Indies · Egypt · Estonia · Finland · France · Germany · Gibraltar · Greece · Greenland · Hong Kong · Hungary · Iceland · a number of different systems of measurement were still in use throughout the world. Some of these systems were metric-system variations, whereas others were based on customary systems. It was recognised that additional steps were needed to promote a worldwide measurement system Metrication refers to the introduction and use of the SI metric system, the international standard for physical measurements. This has involved a long process of independent and systematic conversions of countries from various local systems of weights and measures. Metrication began in France in the 1790s and spread widely during the following two. As a result the 9th General Conference on Weights and Measures The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units (SI) under the terms of the Convention du Mètre (Metre Convention) of 1875. It meets in Sèvres (in the southwestern suburbs of Paris) (CGPM), in 1948, asked the International Committee for Weights and Measures The International Committee for Weights and Measures is the English name of the Comité international des poids et mesures . It consists of eighteen persons from Member States of the Metre Convention (Convention du Mètre). Its principal task is to ensure world-wide uniformity in units of measurement and it does this by direct action or by (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 that were recommended are the metre The metre , symbol m, is the base unit of length in the International System of Units (SI). Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole, its definition has been periodically refined to reflect growing knowledge of metrology. Since 1983, it is defined as the distance travelled by light in a, kilogram The kilogram is the base unit of mass in the International System of Units (SI, from the French Le Système International d’Unités),[Note 2] which is the modern standard governing the metric system. The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK),[Note 3] which is almost exactly equal to the mass, second The second , sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time. It may be measured using a clock, ampere The ampere is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics. In practice, its name is often shortened to amp, degree Kelvin The kelvin is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic (absolute) temperature scale referenced to absolute zero, the absence of all thermal energy. So by definition, the temperature of a substance at absolute zero is zero kelvin (0 K). The secondary reference point on the Kelvin (later renamed the kelvin), and the candela The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function (a standardized model of the sensitivity of the human eye to different wavelengths, also known as the luminous efficiency function). A common candle emits light with a luminous intensity of. 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 The mole is the SI base unit of amount of substance; one of a few units used to measure this physical quantity. The name "mole" is an 1897 translation of the German Mol, coined by Wilhelm Ostwald in 1893, although the related concept of equivalent mass had been in use at least a century earlier. The name is assumed to be derived from the, was added in 1971 by the 14th CGPM.

Future development

ISO 31 International Standard ISO 31 was the most widely respected style guide for the use of physical quantities and units of measurement, and formulas involving them, in scientific and educational documents worldwide[citation needed]. In most countries, the notations used in mathematics and science textbooks at schools and universities follow closely contains recommendations for the use of the International System of Units; for electrical applications, in addition, IEC 60027 IEC 60027 is the International Electrotechnical Commission's standard on Letter symbols to be used in electrical technology. It consists of several parts: has to be taken into account. As of 2008^[update], work is proceeding to integrate both standards into a joint standard Quantities and Units International standard ISO 80000 or IEC 80000 , successor of ISO 31 and partially of IEC 60027, is the most widely respected style guide for the use of physical quantities and units of measurement, and formulas involving them, in scientific and educational documents worldwide. In most countries, the notations used in mathematics and science in which the quantities and equations used with SI are to be referred as the International System of Quantities (ISQ).^[8]

A readable discussion of the present units and standards is found at Brian W. Petley International Union of Pure and Applied Physics I.U.P.A.P.- 39 (2004).

Units

The international system of units consists of a set of units together with a set of prefixes The International System of Units specifies a set of unit prefixes known as SI prefixes or metric prefixes. An SI prefix is a name that precedes a basic unit of measure to indicate a decimal multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol. The SI prefixes are standardized by the International. The units of SI can be divided into two subsets. There are seven base units: Each of these base units represents, at least in principle, different kinds of physical quantities. From these seven base units, several other units are derived. In addition to the SI units, there is also a set of non-SI units accepted for use with SI which includes some commonly used units such as the litre.

A prefix may be added to a unit 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 do not have an appended period/full stop (.).
• Symbols are written in upright (Roman) type (m for metres, s for seconds), so as to differentiate from the italic type used for variables (m for mass, s for displacement). By consensus of international standards bodies, this rule is applied independent of the font used for surrounding text.^[11]
• Symbols for units 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". All symbols of prefixes larger than 10³ (kilo) are also uppercase.
  • The one exception is the litre, whose original symbol "l" is unsuitably similar to the numeral "1" or the uppercase letter "i" (depending on the typeface used), at least in many English-speaking countries. The American National Institute of Standards and Technology recommends that "L" be used instead, a usage which is common in the US, Canada and Australia (but not elsewhere). This has been accepted as an alternative by the CGPM since 1979. The cursive ℓ is occasionally seen, especially in Japan and Greece, but this is not currently recommended by any standards body. For more information, see litre.
• The SI rule is that symbols of units are not pluralised, for example "25 kg" (not "25 kgs").^[11]
  • The American National Institute of Standards and Technology has defined guidelines for American users of the SI.^[12][13] These guidelines give guidance on pluralising unit names: the plural is formed by using normal English grammar rules, for example, "henries" is the plural of "henry".^[12]:31 The units lux, hertz, and siemens are exceptions from this rule: They remain the same in singular and plural. Note that this rule applies only to the full names of units, not to their symbols.
• A space separates the number and the symbol; e.g., "2.21 kg", "7.3×10² m²", "22 K". This rule explicitly includes the percent sign (%). Exceptions are the symbols for plane angular degrees, minutes and seconds (°, ′ and ″), which are placed immediately after the number with no intervening space.^[14][15]
• The 10th resolution of CGPM in 2003 declared that "the symbol for the decimal marker shall be either the point on the line or the comma on the line." In practice, the decimal point is used in English-speaking countries as well as most of Asia and the comma in most continental European languages.
• Spaces may be used as a thousands separator (1000000) in contrast to commas or periods (1,000,000 or 1.000.000) in order to reduce confusion resulting from the variation between these forms in different countries. In print, the space used for this purpose is typically narrower than that between words (commonly a thin space).
• Any line-break inside a number, inside a compound unit, or between number and unit should be avoided, but, if necessary, the last-named option should be used.
• Symbols for derived units (formed from multiple units by multiplication) are joined with a centre dot (·), dot (.)^[16], or a non-break space, for example, "N·m", "N.m", or "N m".^[17]
• Symbols formed by division of two units are joined with a 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 . Only one solidus should be used; e.g., "kg⁄(m·s²)" or "kg·m⁻¹·s⁻²" are acceptable but "kg⁄m⁄s²" is ambiguous and unacceptable. Many computer users will type the / character provided on computer keyboards, which in turn produces the Unicode character U+002F, which is named solidus but is distinct from the Unicode solidus character, U+2044.
• In Chinese, Japanese, and Korean language computing (CJK), some of the commonly used units, prefix-unit combinations, or unit-exponent combinations have been allocated predefined single characters taking up a full square. Unicode includes these in its CJK Compatibility and Letterlike Symbols subranges for back compatibility, without necessarily recommending future usage.
• When writing dimensionless quantities, the terms 'ppb' (parts per billion) and 'ppt' (parts per trillion) are recognised as language-dependent terms, since the value of billion and trillion can vary from language to language. SI, therefore, recommends avoiding these terms.^[18] However, no alternative is suggested by the International Bureau of Weights and Measures (BIPM).

Spelling variations

• The official US spellings for deca, metre, and litre are deka, meter, and liter, respectively.^[19]
• In some English-speaking countries, the unit ampere is often shortened to amp (singular) or amps (plural) in informal writing as well as on many electrical appliances. Secs may sometimes be seen instead of s or seconds.

Conversion factors

The relationship between the units used in different systems is determined by convention or from the basic definition of the units. Conversion of units from one system to another is accomplished by use of a conversion factor. There are several compilations of conversion factors; see, for example, Appendix B of NIST SP 811.^[12]

Length, mass, and temperature convergence

Density (specific mass) is commonly expressed in SI units or in reference to water. Since a cube with sides of 1 decimetre has volume of 1 cubic decimetre, which is 1 litre and, when filled with water, has an approximate mass of 1 kilogram, water has an approximate density of 1 kilogram per litre, which is equal to 1 gram per cubic centimetre and 1 tonne per cubic metre, and will freeze at approximately 0 degrees Celsius at 1 atmosphere of pressure.

Note that this is only an approximate definition of the kilogram, as the density of water can change with temperature; the actual definition is based on a specific platinum-iridium cylinder held in a vault at the BIPM in Sèvres, France.

Cultural issues

The near-worldwide adoption of the metric system as a tool of economy and everyday commerce was based to some extent on the lack of customary systems in many countries to adequately describe some concepts, or as a result of an attempt to standardise the many regional variations in the customary system. International factors also affected the adoption of the metric system, as many countries increased their trade. For use in science, it simplifies dealing with very large and small quantities, since it lines up so well with the decimal numeral system.

Many units in everyday and scientific use are not derived from the seven SI base units (metre, kilogram, second, ampere, kelvin, mole, and candela) combined with the SI prefixes. In some cases these deviations have been approved by the BIPM.^[20] Some examples include:

• The many units of time (minute, min; hour, h; day, d) in use besides the SI second, and are specifically accepted for use according to table 6.^[21]
• The year is specifically not included but has a recommended conversion factor.^[22]
• The Celsius temperature scale; kelvins are rarely employed in everyday use.
• Electric energy is often billed in kilowatt-hours instead of megajoules. Similarly, battery charge is often measured as miliamperes-hour (mAh) instead of coulombs.
• The nautical mile and knot (nautical mile per hour) used to measure travel distance and speed of ships and aircraft (1 International nautical mile = 1852 m or approximately 1 minute of latitude). In addition to these, Annex 5 of the Convention on International Civil Aviation permits the "temporary use" of the foot for altitude.
• Astronomical distances measured in astronomical units, parsecs, and light-years instead of, for example, petametres (a light-year is about 9.461 Pm or about 9461000000000000 m).
• Atomic scale units used in physics and chemistry, such as the ångström, electron volt, atomic mass unit and barn.
• Some physicists prefer the centimetre-gram-second (CGS) units, or systems based on physical constants, such as Planck units, atomic units, or geometric units.
• In some countries, the informal cup measurement has become 250 mL. Likewise, a 500 g metric pound is used in many countries. Liquids, especially alcoholic ones, are often sold in units whose origins are historical (for example, pints for beer and cider in glasses in the UK —although pint means 568 mL; champagne in Jeroboams in France).
• A metric mile of 10 km is used in Norway and Sweden. The term metric mile is also used in some English speaking countries for the 1500 m foot race.
• In the US, blood glucose measurements are recorded in milligrams per decilitre (mg/dL), which would normalise to cg/L; in Canada, Australia, New Zealand, Oceania, and Europe, the standard is millimole per litre (mmol/L) or mM (millimolar).
• Blood pressure and atmospheric pressure are usually measured in mmHg and bars, respectively, instead of Pa.

The fine-tuning that has happened to the metric base-unit definitions over the past 200 years, as experts have tried periodically to find more precise and reproducible methods, does not affect the everyday use of metric units. Since most non-SI units in common use, such as the US customary units, are nowadays defined in SI units,^[23] any change in the definition of the SI units results in a change of the definition of the older units, as well.

International Trade

One of the European Union's (EU) objectives is the creation of a single market for trade. In order to achieve this objective, the EU standardised on using SI as the legal units of measure. At the time of writing (2009) it had issued two units of measurement directives which catalogued the units of measure that might be used for, amongst other things, trade: the first was Directive 71/354/EEC^[24] issued in 1971 which required member states to standardise on SI rather than use the variety of cgs and mks units then in use. The second was Directive 80/181/EEC^{[25][26][27][28][29]} issued in 1979 which replaced the first and which gave the United Kingdom and the Republic of Ireland a number of derogations from the original directive.

The directives gave a derogation from using SI units in areas where other units of measure had either been agreed by international treaty or which were in universal use in worldwide trade. They also permitted the use of supplementary indicators alongside, but not in place of the units catalogued in the directive. In its original form, Directive 80/181/EEC had a cut-off date for the use of such indicators, but with each amendment this date was moved until, in 2009, supplementary indicators have been allowed indefinitely.

See also

References

1. ^ Bureau International des Poids et Mesures
2. ^ Official BIPM definitions
3. ^ An extensive presentation of the SI units is maintained on line by NIST, including a diagram of the interrelations between the derived units based upon the SI units. Definitions of the basic units can be found on this site, as well as the CODATA report listing values for special constants such as the electric constant, the magnetic constant and the speed of light, all of which have defined values as a result of the definition of the metre and ampere.

   In the International System of Units (SI) (BIPM, 2006), the definition of the meter fixes the speed of light in vacuum c₀, the definition of the ampere fixes the magnetic constant (also called the permeability of vacuum) μ₀, and the definition of the mole fixes the molar mass of the carbon 12 atom M(¹²C) to have the exact values given in the table [Table 1, p.7]. Since the electric constant (also called the permittivity of vacuum) is related to μ₀ by ε₀ = 1/μ₀c₀², it too is known exactly.

   – CODATA report
4. ^ SI Practical Realization brochure
5. ^ Ambler Thompson and Barry N. Taylor, (2008), Guide for the Use of the International System of Units (SI), (Special publication 811), Gaithersburg, MD: National Institute of Standards and Technology, p. 3, footnote 2.
6. ^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 111, ISBN 92-822-2213-6, http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf
7. ^ "The name "kilogram"". http://www1.bipm.org/en/si/history-si/name_kg.html. Retrieved 25 July 2006.
8. ^ SI Brochure
9. ^ Barry N. Taylor & Ambler Thompson Ed. (2008). The International System of Units (SI). Gaithersburg, MD: National Institute of Standards and Technology. pp. 23. http://physics.nist.gov/Pubs/SP330/sp330.pdf. Retrieved 18 June 2008.
10. ^ Quantities Units and Symbols in Physical Chemistry, IUPAC
11. ^ ^{a b} Bureau International des Poids et Mesures (2006). The International System of Units (SI). 8th ed.. http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf. Retrieved 13 February 2008. Chapter 5.
12. ^ ^{a b c} Ambler Thompson & Barry N. Taylor (2008). NIST Special Publication 811: Guide for the Use of the International System of Units (SI). National Institute of Standards and Technology. http://physics.nist.gov/cuu/pdf/sp811.pdf. Retrieved 18 June 2008.
13. ^ Turner, James M. (9 May 2008). May 2008/pdf/E8-11058.pdf "Interpretation of the International System of Units (the Metric System of Measurement) for the United States". Federal Register (National Archives and Records Administration) 73 (96): 28432–3. FR Doc number E8-11058. http://www.gpo.gov/fdsys/pkg/FR-16 May 2008/pdf/E8-11058.pdf. Retrieved 28 October 2009.
14. ^ The International System of Units (SI) (8 ed.). International Bureau of Weights and Measures (BIPM). 2006. p. 133. http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf.
15. ^ Thompson, A.; Taylor, B. N. (July 2008). "NIST Guide to SI Units — Rules and Style Conventions". National Institute of Standards and Technology. http://physics.nist.gov/Pubs/SP811/sec07.html. Retrieved 29 December 2009.
16. ^ National Standard of Canada. Canadian Metric Practice Guide, CAN/CSA-Z234.1-89, 1989, item 3.8.1
17. ^ Barry N. Taylor, Ed. (2001). The International System of Units (SI). Washington, DC: National Institute of Standards and Technology. pp. 30. http://physics.nist.gov/Pubs/SP330/sp330.pdf. Retrieved 15 October 2007.
18. ^ http://www.bipm.org/en/si/si_brochure/chapter5/5-3-7.html
19. ^ "The International System of Units". pp. iii. http://physics.nist.gov/Pubs/SP330/sp330.pdf. Retrieved 27 May 2008.
20. ^ BIPM - Table 8
21. ^ BIPM - Table 6
22. ^ NIST Guide to SI Units - Appendix B9. Conversion Factors
23. ^ Mendenhall, T. C. (1893). "Fundamental Standards of Length and Mass". Reprinted in Barbrow, Louis E. and Judson, Lewis V. (1976). Weights and measures standards of the United States: A brief history (NBS Special Publication 447). Washington D.C.: Superintendent of Documents. Viewed 23 August 2006 at http://physics.nist.gov/Pubs/SP447/ pp. 28–29.
24. ^ "Council Directive of 18 October 1971 on the approximation of laws of the member states relating to units of measurement, (71/354/EEC)". http://eur-lex.europa.eu/Notice.do?mode=dbl&lang=en&lng1=en,nl&lng2=da,de,el,en,es,fr,it,nl,pt,&val=22924:cs&page=1&hwords=. Retrieved 7 February 2009.
25. ^ The Council of the European Communities (21 December 1979). "Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC". http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0181:19791221:EN:PDF. Retrieved 7 February 2009.
26. ^ The Council of the European Communities (20 December 1984). "Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC". http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0181:19841220:EN:PDF. Retrieved 7 February 2009.
27. ^ The Council of the European Communities (30 November 1989). "Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC". http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0181:19891130:EN:PDF. Retrieved 7 February 2009.
28. ^ The Council of the European Communities (9 February 2000). "Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC". http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0181:20000209:EN:PDF. Retrieved 7 February 2009.
29. ^ The Council of the European Communities (27 May 2009). "Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC". http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0181:20090527:EN:PDF. Retrieved 14 September 2009.

Further reading

• International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry, 2nd edition, Oxford: Blackwell Science. ISBN 0-632-03583-8. Electronic version.
• Unit Systems in Electromagnetism
• MW Keller et al. Metrology Triangle Using a Watt Balance, a Calculable Capacitor, and a Single-Electron Tunneling Device

External links

Official

• BIPM Bureau International des Poids et Mesures (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
  • ISO 31/1000/80000
• NIST Official Publications
  • NIST Special Publication 330, 2008 Edition: The International System of Units (SI)
  • NIST Special Pub 814: Interpretation of the SI for the United States and Federal Government Metric Conversion Policy
• Weights and Measures Act, Canada
• IEEE/ASTM SI 10-2002 Standard for Use of the International System of Units (SI): The Modern Metric System (ANSI approved, joint IEEE/ASTM standard)
• Rules for SAE Use of SI (Metric) Units
• National Physical Laboratory, UK

Information

• International System of Units at the Open Directory Project
• EngNet Metric Conversion Chart Online Categorised Metric Conversion Calculator
• A Practical Guide to the International System of Units

History

• LaTeX SIunits package manual gives a historical background to the SI system.

Research

• The metrological triangle
• Recommendation of ICWM 1 (CI-2005)

Pro-metric advocacy groups

• The UK Metric Association
• The US Metric Association
• Canadian Metric Association
• Metrication US

Pro-customary measures pressure groups

• Pro-customary measures groups at the Open Directory Project

Categories: SI units | Systems of units | International standards

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