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, in its broadest sense, any systematic knowledge that is capable of resulting in a correct prediction or reliable outcome. In this sense, science may refer to a highly skilled technique, technology, or practice.[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.

Three nations have not officially adopted the International System of Units as their primary or sole system of measurement: Burma 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 The United States customary system is the most commonly used system of measurement in the United States. It is similar but not identical to the British Imperial units. The U.S. is the only industrialized nation that does not mainly use the metric system in its commercial and standards activities, although the International System of Units (SI,.

Contents

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.

Countries by date of metrication

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:

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, 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

Main articles: SI base unit 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:, SI derived unit The International System of Units specifies a set of seven base units from which all other units of measurement are formed. These other units are called SI derived units and are also considered part of the standard, and SI prefix 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 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.

SI base units[9][10]
Name Unit symbol Quantity Symbol
metre m length l (a lowercase L)
kilogram kg mass m
second s time t
ampere A electric current I (a capital i)
kelvin K thermodynamic temperature T
candela cd luminous intensity Iv (a capital i with lowercase v subscript)
mole mol amount of substance n

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.

Standard prefixes for the SI units of measure
Multiples Name deca- hecto- kilo- mega- giga- tera- peta- exa- zetta- yotta-
Symbol da h k M G T P E Z Y
Factor 100 101 102 103 106 109 1012 1015 1018 1021 1024
Subdivisions Name deci- centi- milli- micro- nano- pico- femto- atto- zepto- yocto-
Symbol d c m μ n p f a z y
Factor 100 10−1 10−2 10−3 10−6 10−9 10−12 10−15 10−18 10−21 10−24

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