Cover of brochure The International System of Units.

The International System of Units (abbreviated SI from the French Le Système International d'Unités^[1]) is the modern form of the metric system and is generally a system devised around the convenience of the number 10. French ( français,) is a Romance language spoken around the world by 118 million people as a native language and by about 180 to 260 million people The metric system is a decimalised system of measurement. It exists in several variations with different choices of base units, though the choice of base units does It is the world's most widely used system of units, both in everyday commerce and in science. A system of measurment 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 Commerce is a division of trade or production which deals with the exchange of goods and services from producer to final consumer Science (from the Latin scientia, meaning " Knowledge " or "knowing" is the effort to discover, and increase human understanding ^[2] 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. Vacuum permittivity, referred to by international standards organizations as the electric constant, and denoted by the symbol ε0 is a fundamental Physical The vacuum permeability, referred to by international standards organizations as the magnetic constant, and denoted by the symbol μ 0 (also ^[3]

The older metric system included several groups of units. The SI was developed in 1960 from the old metre-kilogram-second (mks) system, rather than the centimetre-gram-second (cgs) system, which, in turn, had a few variants. Year 1960 ( MCMLX) was a Leap year starting on Friday (link will display full calendar of the Gregorian calendar. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units The centimetre-gram-second system ( CGS) is a system of physical units. 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 is nearly universally employed, and most countries do not even maintain official definitions of any other units. A notable exception is the United States, which still uses many old units in addition to SI. The United States of America —commonly referred to as the In the United Kingdom, conversion to metric units is government policy, but the transition is not yet complete. The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located Metrication (or metrification refers to the introduction of the SI metric system as the International standard for physical measurements—a long-term series Those countries that still recognise non-SI units (e. g. the US and UK) have redefined their traditional non-SI units in terms of SI units. Conversion of units refers to conversion factors between different Units of measurement for the same Quantity.

Three nations have not officially adopted the International System of Units as their primary or sole system of measurement: Liberia, Myanmar and the United States. Liberia, officially the Republic of Liberia, is a country on the west coast of Africa, bordered by Sierra Leone, Guinea, Côte d'Ivoire Burma, officially the Union of Myanmar ( pjìdàunzṵ mjàmmà nàinŋàndɔ̀ is the largest country by geographical area in mainland Southeast Asia. The United States of America —commonly referred to as the

Realisation of units

It is important to distinguish between the definition of a unit and its realization. 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 realization 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 realized in practice is given on the BIPM website,

SI Practical Realization brochure

A coherent SI derived unit is defined uniquely only in terms of SI base units. For example, the coherent SI derived unit of resistance, the ohm, symbol Ω, is uniquely defined by the relation Ω = m² kg s⁻³ A⁻², which follows from the definition of the quantity electrical resistance. However any method consistent with the laws of physics could be used to realize any SI unit. ^[4]

History

Main articles: Metre, Grave (mass), Kilogram, Second, Ampere, Kelvin, Mole (unit), Candela, and Coulomb

The metric system was conceived by a group of scientists (among them, Antoine-Laurent Lavoisier, who is known as the "father of modern chemistry") which had been commissioned by King Louis XVI of France to create a unified and rational system of measures. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International 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 The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic The mole (symbol mol) is a unit of Amount of substance: it is an SI base unit, and almost the only unit to be used to measure this The candela (kanˈdɛlə /-ˈdiːlə/ symbol cd) is the SI base unit of Luminous intensity; that is power emitted by a light source in a particular The coulomb (symbol C) is the SI unit of Electric charge. It is named after Charles-Augustin de Coulomb. The metric system is a decimalised system of measurement. It exists in several variations with different choices of base units, though the choice of base units does Louis XVI ( 23 August 1754 – 21 January 1793) Louis-Auguste de France, ruled as King of France and Navarre This article is about the country For a topic outline on this subject see List of basic France topics. After the French Revolution, the system was adopted by the new government. The French Revolution (1789–1799 was a period of political and social upheaval in the History of France, during which the French governmental structure previously an ^[5] 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. Events 30 BC - Octavian (later known as Augustus enters Alexandria, Egypt, bringing it under the control of the Roman Year 1793 ( MDCCXCIII) was a Common year starting on Tuesday (link will display the full calendar of the Gregorian calendar (or a Common On April 7, 1795 (Loi du 18 germinal, an III) the terms gramme and kilogramme replaced the former terms "gravet" (correctly "milligrave") and "grave". Events 529 - First draft of Corpus Juris Civilis (a fundamental work in Jurisprudence) is issued by Eastern Roman Emperor Year 1795 ( MDCCXCV) was a Common year starting on Thursday (link will display the full calendar of the Gregorian calendar (or a 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 December 10, 1799 (a month after Napoleon's coup d'etat), the metric system was definitively adopted in France. Events 1041 - Empress Zoe of Byzantium elevates her adoptive son to the throne of the Eastern Roman Empire as Michael V Year 1799 ( MDCCXCIX) was a Common year starting on Tuesday (link will display the full calendar of the Gregorian calendar (or a 18 Brumaire, the coup of 18 Brumaire or sometimes simply Brumaire refers to the Coup d'état by which General Napoleon Bonaparte overthrew This article is about the country For a topic outline on this subject see List of basic France topics.

The history of the metric system has seen a number of variations, whose use has spread around the world, to replace many traditional measurement systems. The metric system is a decimalised system of measurement. It exists in several variations with different choices of base units, though the choice of base units does A system of measurment 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 At the end of World War II a number of different systems of measurement were still in use throughout the world. World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including Some of these systems were metric-system variations, while others were based on the Imperial and American systems. Imperial units or the Imperial system is a collection of units first defined in the British Weights and Measures Act of 1824 US customary units, also known in the United States as English units or Imperial units (in reference to the British Empire) (but see English It was recognized that additional steps were needed to promote a worldwide measurement system. Metrication (or metrification refers to the introduction of the SI metric system as the International standard for physical measurements—a long-term series 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. The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures ( CGPM, never GCWM The International Committee for Weights and Measures is the English name of the Comité international des poids et mesures ( CIPM, sometimes written in English

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, degree Kelvin (later renamed the kelvin), and the candela. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic The candela (kanˈdɛlə /-ˈdiːlə/ symbol cd) is the SI base unit of Luminous intensity; that is power emitted by a light source in a particular 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, was added in 1971 by the 14th CGPM. The mole (symbol mol) is a unit of Amount of substance: it is an SI base unit, and almost the only unit to be used to measure this

Future development

The ISO standard ISO 31 contains recommendations for the use of the International System of Units; for applications in electrical applications additionally the IEC standard IEC 60027 has to be taken into account. International Standard ISO 31 ( Quantities and units International Organization for Standardization, 1992 is the most widely respected style guide for the The International Electrotechnical Commission ( IEC) is a not-for-profit, non-governmental international Standards organization that prepares and publishes IEC 60027 (formerly IEC 27) is the International Electrotechnical Commission 's standard on Letter symbols to be used in electrical technology. As of 2008, work is proceeding to integrate both standards into a joint standard Quantities and Units in which the quantities and equations used with SI are to be referred as the International System of Quantities (ISQ). International standard ISO 80000 or IEC 80000 (depending on which of the two international standards bodies International Organization for Standardization

Units

Main articles: SI base unit, SI derived unit, and SI prefix

The international system of units consists of a set of units together with a set of prefixes. The International System of Units (SI defines seven dimensionally independent SI base units. SI derived units are part of the SI system of measurement units and are derived from the seven SI base units They are derived from SI basic units/defined An SI prefix (also known as a metric prefix) is a name or associated symbol that precedes a unit of measure (or its symbol to form a Decimal multiple or An SI prefix (also known as a metric prefix) is a name or associated symbol that precedes a unit of measure (or its symbol to form a Decimal multiple or The units of SI can be divided into two subsets. There are seven base units. The International System of Units (SI defines seven dimensionally independent SI base units. Each of these base units is nominally dimensionally independent. From these seven base units several other units are derived. SI derived units are part of the SI system of measurement units and are derived from the seven SI base units They are derived from SI basic units/defined In addition to the SI units there is also a set of non-SI units accepted for use with SI. The following units are not SI units but are "accepted for use with the International System

A prefix may be added to a unit to produce a multiple of the original unit. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International Length is the long Dimension of any object The length of a thing is the distance between its ends its linear extent as measured from end to end Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic Thermodynamic temperature is the absolute measure of Temperature and is one of the principal parameters of Thermodynamics. The mole (symbol mol) is a unit of Amount of substance: it is an SI base unit, and almost the only unit to be used to measure this The amount of substance, n, of a sample or system is a Physical quantity which is proportional to the number of elementary entities present The candela (kanˈdɛlə /-ˈdiːlə/ symbol cd) is the SI base unit of Luminous intensity; that is power emitted by a light source in a particular In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a Light source in a particular direction per unit Solid An SI prefix (also known as a metric prefix) is a name or associated symbol that precedes a unit of measure (or its symbol to form a Decimal multiple or 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 (. An SI prefix (also known as a metric prefix) is a name or associated symbol that precedes a unit of measure (or its symbol to form a Decimal multiple or yotta- (symbol Y) is an SI prefix in the SI ( System of units) denoting 1024 or 1 000 000 000 000 000 000 000 000 zetta- (symbol Z) is an SI prefix in the SI ( System of units) denoting 1021 or 1 000 000 000 000 000 000 000 exa- (symbol E) is a prefix in the SI system of units denoting 1018 or 1 000 000 000 000 000 000 In Physics and Mathematics, peta- (symbol P) is a prefix in the SI ( System of units) denoting 1015 teras- (symbol T) is a prefix in the SI system of units denoting 1012, or 1000000000000 (1 trillion For other meanings see Giga (disambiguation Giga- (symbol G is a prefix in the SI system of units denoting 109 Mega- (symbol M) is an SI prefix in the SI system of units denoting a factor of 106, 1000000 (one Million Kilo- (symbol k is a prefix in the SI and other systems of units denoting 103 or 1000 hecto- or hecta- (symbol h) is a SI prefix in the SI system of units denoting a factor of 102 (100 deca- or deka- is an SI prefix (symbol da) in the SI system of units also a Numerical prefix (from Greek δέκα deci- (symbol d) is a prefix in the SI system of units denoting a factor of 10−1 (1/10 centi- (symbol c) is a SI prefix in the SI system of units denoting a factor of 10−2 or 1/100 "Milli" redirects here for the village in Azerbaijan see Birinci Milli; for similar-sounding words see Millie. micro- ( µ) is a prefix in the SI and other systems of units denoting a factor of 10&minus6 (one Millionth. nano- is a prefix (symbol n) in the SI system of units denoting a factor of 10−9 pico- (symbol p) is a prefix denoting a factor of 10-12 in the International System of Units (SI femto- is an SI prefix (symbol f) in the SI system of units denoting a factor of 10−15 or 0 atto- (symbol a) is an SI prefix to a unit and means that it is 10−18 times this unit zepto- (symbol z) is a prefix in the SI system of units denoting a factor of 10−21 yocto- (symbol y) is an SI prefix in the SI system of units denoting a factor of 10−24 or 0 ) unless at the end of a sentence.
• Symbols are written in upright (Roman) type (m for metres, l for litres), so as to differentiate from the italic type used for variables (m for mass, l for length). In Typography, "roman" type has two principal meanings both stemming from the stylistic origin of text typefaces from inscriptional capitals used in In Typography, italic type /ɪˈtælɪk/ or /aɪˈtælɪk/ refers to cursive Typefaces based on a stylized form of calligraphic Handwriting. By consensus of international standards bodies, this rule is applied independent of the font used for surrounding text. ^[7]
• Symbols for units are written in lower case, except for symbols derived from the name of a person. Lower case (also lower-case or lowercase) minuscule, or small letters are the smaller form of letters as opposed to upper For example, the unit of pressure is named after Blaise Pascal, so its symbol is written "Pa" whereas the unit itself is written "pascal". Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Blaise Pascal (blɛz paskal (June 19 1623 &ndash August 19 1662 was a French Mathematician, Physicist, and religious Philosopher 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 litre or liter (see spelling differences) is a unit of Volume. The following is a list of Sovereign states and territories where English is an Official language, in order of Population. The American National Institute of Standards and Technology recommends that "L" be used instead, a usage which is common in the US, Canada, Australia (but not elsewhere). This has been accepted as an alternative by the CGPM since 1979. The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures ( CGPM, never GCWM Year 1979 ( MCMLXXIX) was a Common year starting on Monday (link displays the 1979 Gregorian calendar) The cursive is occasionally seen, especially in Japan and Greece, but this is not currently recommended by any standards body. For a topic outline on this subject see List of basic Japan topics. Greece (Ελλάδα transliterated: Elláda, historically, Ellás,) officially the Hellenic Republic (Ελληνική Δημοκρατία A standards organization, standards body, standards development organization or SDO is any entity whose primary activities are developing coordinating For more information, see Litre. The litre or liter (see spelling differences) is a unit of Volume.
• The SI rule for pluralising units is that symbols of units are not pluralised, for example "25 kg" (not "25 kgs"). ^[7]
  • The American National Institute of Standards and Technology has defined guidelines for using the SI units in its own publications and for other users of the SI. ^[8] These guidelines give guidance on pluralizing unit names: the plural is formed by using normal English grammar rules, for example, "henries" is the plural of "henry". English grammar is a body of rules ( Grammar) specifying how phrases and sentences are constructed in the English language. The henry (symbol H is the SI unit of Inductance. It is named after Joseph Henry (1797-1878 the American scientist who discovered electromagnetic The units lux, hertz, and siemens are exceptions from this rule: they remain the same in singular and plural. LUX is the principal centre for the promotion and distribution of experimental Film and Video works in the UK. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. The siemens (symbol S is the SI derived unit of Electric conductance. Note that this rule only applies 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". ^[9][10] Exceptions are the symbols for plane angular degrees, minutes and seconds (°, ′ and ″), which are placed immediately after the number with no intervening space.
• Spaces may be used as a thousands separator (1 000 000) in contrast to commas or periods (1,000,000 or 1. In a positional Numeral system, the decimal separator is a Symbol used to mark the boundary between the integral and the fractional 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). In writing a space () is a blank area that is devoid of content which separates words letters numbers and punctuation
• Any line break inside a number, inside a compound unit or between number and unit should be avoided, but if necessary the latter option should be used.
• 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". The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures ( CGPM, never GCWM In a positional Numeral system, the decimal separator is a Symbol used to mark the boundary between the integral and the fractional A full stop or period (sometimes stop, full point, decimal point, or dot) is the Punctuation mark commonly placed at the In practice, the decimal point is used in English and the comma in most other European languages. Most of the many Languages of Europe belong to the Indo-European Language family.
• Symbols for derived units formed from multiple units by multiplication are joined with a space or centre dot (·), for example "N m" or "N·m". An interpunct ( ·) is a small dot used for Interword separation in ancient Latin script, being perhaps the first consistent visual representation of word boundaries ^[11]
• Symbols formed by division of two units are joined with a solidus (⁄), or given as a negative exponent. The solidus ( ⁄) is a punctuation mark that is not found on standard keyboards For example, the "metre per second" can be written "m/s", "m s⁻¹", "m·s⁻¹" or Only one solidus should be used, i. e. "kg·m⁻¹·s⁻²" is preferable to "kg/m/s²", and "kg/m·s²" is something else. 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 Computing, a keyboard is an Input device partially modelled after the typewriter keyboard which uses an arrangement of buttons or keys
• 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. is a language spoken by over 130 million people in Japan and in Japanese emigrant communities This article is mainly about the spoken Korean language See Hangul for details on the native Korean writing system CJK is a collective term for Chinese, Japanese, and Korean, which constitute the main East Asian languages. Unicode includes these in its CJK Compatibility and Letterlike Symbols subranges for back compatibility, without necessarily recommending future usage. In Computing, Unicode is an Industry standard allowing Computers to consistently represent and manipulate text expressed in most of the world's
• 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. The long and short scales are two different numerical systems used throughout the world Short scale is the English translation of the French The long and short scales are two different numerical systems used throughout the world Short scale is the English translation of the French The long and short scales are two different numerical systems used throughout the world Short scale is the English translation of the French SI therefore recommends avoiding these terms [1]. However, no alternative is suggested by the International Bureau of Weights and Measures (BIPM). The International Bureau of Weights and Measures ( Bureau international des poids et mesures, in French) is an international Standards organization, one

Spelling variations

• The official US spellings for "deca" and "metre" is "deka" and "meter" respectively. ^[12]
• In some English-speaking countries, the unit "ampere" is often shortened to amp (singular) or amps (plural) in informal writing.

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. Conversion of units refers to conversion factors between different Units of measurement for the same Quantity. There are several compilations of conversion factors; see, for example Appendix B of NIST SP 811. ^[8]

Length, mass and temperature convergence

Specific gravity is commonly expressed in SI units or in reference to water. Since a cube with sides of 1 dm has volume of 1 dm³, which is 1 L and, when filled with water, has a mass of 1 kg, water has an approximate specific gravity of 1 kg/L, which is equal to 1 g/cm³ and 1 t/m³, and will freeze at 0 °C at 1 atmosphere of pressure. The litre or liter (see spelling differences) is a unit of Volume.

Note that this is only an approximate definition of the kg, as the volume 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 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. Scientifically, it simplifies dealing with very large and small quantities since it lines up so well with the decimal numeral system. The decimal ( base ten or occasionally denary) Numeral system has ten as its base. A numeral system (or system of numeration) is a Mathematical notation for representing numbers of a given set by symbols in a consistent manner

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. ^[13] 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. ^[14]
• The year is specifically not included but has a recommended conversion factor. ^[15]
• The Celsius temperature scale; kelvins are rarely employed in everyday use. The Celsius Temperature scale was previously known as the centigrade scale.
• Electric energy is often billed in kilowatt-hours instead of megajoules. Electric energy is the potential energy associated with the conservative Coulomb forces between Charged particles contained within a system, where
• 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 at the equator). A nautical mile or sea mile is a unit of Length. It corresponds approximately to one minute of Latitude along any meridian. In addition to these, Annex 5 of the Convention on International Civil Aviation permits the "temporary use" of the foot for altitude. The Convention on International Civil Aviation, also known as the Chicago Convention, established the International Civil Aviation Organization (ICAO a specialized A foot (plural feet or foot; symbol or abbreviation ft or sometimes &prime – the prime symbol) is a non-SI unit Altitude is the Elevation of a point or object from a known level or datum (plural data
• Astronomical distances measured in astronomical units, parsecs, and light-years instead of, say, petametres (a light-year is about 9. Astronomical distances are distances in Outer space, occurring on a much larger scale than those on Earth. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the History The first direct measurements of an object at interstellar distances were undertaken by German Astronomer Friedrich Wilhelm Bessel in 1838 A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by 461 Pm or about 9 461 000 000 000 000 m).
• Atomic scale units used in physics and chemistry, such as the ångström, electronvolt, atomic mass unit and barn. An ångström or angstrom (symbol Å) (ˈɔːŋstrəm Swedish: ˈɔ̀ŋstrœm is an internationally recognized non- SI unit of length equal The unified atomic mass unit ( u) or Dalton ( Da) or sometimes universal mass unit, is an unit of Mass used to express A barn (symbol b) is a unit of Area. While the barn is not an SI unit it is accepted (although discouraged for use with the SI
• Some physicists still use the centimetre-gram-second (CGS) units, with their associated non-SI electric units. A physicist is a Scientist who studies or practices Physics. Physicists study a wide range of physical phenomena in many branches of physics spanning The centimetre-gram-second system ( CGS) is a system of physical units.
• In some countries the informal cup measurement has become 250 ml. The cup is a unit of measurement for volume used in cooking to measure bulk foods such as Granulated sugar (dry measurement and liquids ( Fluid measurement Likewise, a 500 g "metric pound" is used in many countries. The pound or pound-mass (abbreviation lb, lbm, or sometimes in the United States #) is a unit of Mass Liquids, especially alcoholic ones, are often sold in units whose origins are historical (for example, beer in pints in the UK, champagne in Jeroboams in France). The pint is an English unit of Volume or capacity in the imperial system and United States customary units. A wine bottle is a Bottle used for holding wine generally made of Glass.
• In the US blood glucose measurements are recorded in milligrams per decilitre (mg/dL); in Canada, Australia, New Zealand, Oceania and Europe, the standard is millimole per litre (mmol/L) or mM (millimolar). Blood sugar, used in a physiological context is a misnomer and misleading
• Blood pressure is measured in mmHg instead of Pa. Blood pressure is also the title of a short story by Damon Runyan in Guys and Dolls and Other Stories The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere.

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 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. US customary units, also known in the United States as English units or Imperial units (in reference to the British Empire) (but see English

Trade

The European Union has a directive^[16] banning non-SI markings after 31 December 2009 on any goods imported into the European Union. The European Union ( EU) is a political and economic union of twenty-seven member states, located primarily in The Units of Measure Directive, sometimes referred to as the Metric Directive and more formally known as "Council Directive 80/181/EEC of 20 December 1979 on the approximation Events 406 – Vandals, Alans and Suebians cross the Rhine, beginning an invasion of Gallia. This article is about the year For the film see 2009 Lost Memories. This applies to all markings on products, enclosed directions and papers, packaging and advertisements. However, on September 11, 2007, the EU announced that the United Kingdom would be excepted from this directive and Imperial measurements would still be permitted indefinitely alongside with the metric system as supplementary indications. Events 9 - The Battle of the Teutoburg Forest ends 506 - The Bishops of Visigothic Gaul Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located Imperial units or the Imperial system is a collection of units first defined in the British Weights and Measures Act of 1824 ^[17]

See also

References

Further reading

• International Union of Pure and Applied Chemistry (1993). The International Union of Pure and Applied Chemistry ( IUPAC) (aɪjuːpæk or ay-yoo-pec) is an international Non-governmental organization Quantities, Units and Symbols in Physical Chemistry (2nd Edn). Quantities Units and Symbols in Physical Chemistry Third Edition (ISBN 978-0-85404-433-7 also known as the Green Book, Prepared for publication by E Oxford: Blackwell Science. ISBN 0-632-03583-8. Electronic version.

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 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 Categorized 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. The Open Directory Project ( ODP) also known as dmoz (from directory

Research

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

Pro-metric pressure 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