Difference between revisions of "System of units"

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The International System of Units(abbreviated SI from Lang-fr<ref>Resolution of the International Bureau of Weights and Measures establishing the International System of Units</ref>) is the modern form of the Metric system and is generally a system of Units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units. The SI was established in 1960, based on the metre-kilogram-second system, rather than the centimetre-gram-second system, which, in turn, had a few variants. The SI is declared as an evolving system, thus prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses, and as the precision of measurements improves.

SI is the world's most widely used system of measurement, which is used both in everyday Commerce and in Science.<ref>Official BIPM definitions</ref><ref>Essentials of the SI: Introduction</ref><ref>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 metre 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.

History

The Metric system was conceived by a group of scientists (among them, Antoine-Laurent Lavoisier, who is known as the "father of modern chemistry") who had been commissioned by the Assemblée nationale and Louis XVI of France to create a unified and rational system of measures.<ref>Cite web</ref> 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 and on 22 June 1799, after Pierre Méchain and Jean-Baptiste Delambre completed their survey, the definitive standard metre was deposited in the French National Archives. 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 led to the signing in 1875 of the Metre Convention, a treaty that established three international organizations to oversee the keeping of metric standards:

• General Conference on Weights and Measures (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 (Bureau international des poids et mesures or BIPM) – an international metrology centre at Sèvres in France; and
• International Committee for Weights and Measures (Comité international des poids et mesures or CIPM)—an administrative committee that meets annually at the BIPM.

The history of the metric system has seen a number of variations, and has spread around the world, to replace many traditional measurement systems. At the end of World War II, a number of different systems of measurement were still in use throughout the world. Some of these systems were metric-system variations, whereas others were based on customary systems. It was recognised that additional steps were needed to promote a worldwide measurement system. As a result, the 9th General Conference on Weights and Measures (CGPM), in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities.

Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units that were recommended are the Metre, Kilogram, Second, Ampere, degree Kelvin (later renamed Kelvin), and Candela. In 1960, the 11th CGPM named the system the International System of Units, abbreviated SI from the French name, Lang. The seventh base unit, the mole, was added in 1971 by the 14th CGPM.

One of the CIPM committees, the CCU, has proposed a number of changes to the definitions of the base units used in SI.<ref name="draft">Cite web</ref> The CIPM meeting of October 2010 found that the proposal was not complete,<ref>Cite web</ref> and it is expected that the CGPM will consider the full proposal in 2015.

Units and prefixes

The International System of Units consists of a set of units together with a set of SI. The units are divided into two classes—base units and derived units. There are seven SI, each representing, by convention, different kinds of physical quantities.

Derived units are formed from multiplication and division of the seven base units and other derived units<ref name=SI>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.</ref> and are unlimited in number;<ref>SIBrochure8th</ref> for example, the SI derived unit of speed is metre per second, m/s. Some derived units have special names; for example, the 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. The Radian and Steradian, once given special status, are now considered dimensionless derived units.<ref name=SI/>

A prefix may be added to a unit to produce a multiple of the original unit. All multiples are integer powers of ten, and beyond a hundred(th) all are integer powers of a thousand. 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, and multiples of the kilogram are named as if the gram was the base unit. Thus a millionth of a metre is a micrometre, not a millimillimetre, and a millionth of a kilogram is a milligram, not a microkilogram.

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 non-coherent units such as the Litre.