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The International System of Units, (symbol: SI) (for the French phrase Système International d'Unités), is the most widely used system of units. It is used for everyday commerce in virtually every country of the world except the United States. SI was selected from the existing Metre-Kilogram-Second system of units (MKS), with the addition of extra units, rather than the older Centimetre-Gram-Second system of units (CGS). SI is sometimes referred to as the metric system (especially in the United States, which has not widely adopted it, and the UK, where conversion is incomplete). However, not all metric units of measurement are accepted as SI units.
There are seven base units and several derived units, together with a set of prefixes. Non-SI units can be converted to SI units (or vice versa) according to the conversion of units.
The units of the SI system are decided by a series of international conferences organised by the Bureau International des Poids et Mesures (International Office of Weights and Measures). The SI system was first given its name in 1960, and last added to in 1971.
The true origins of the SI or metric system date back to approximately 1640. It was invented by French scientists, and was given a huge boost in popularity by the French Revolution of 1789. The metric system tried to choose units which were non-arbitrary, merging well with the revolution's official ideology of "Pure Reason". The layout of the metric system may have been based on the idealistic world-view of ancient Greeks, who theorized that there were four basic elements: earth, water, fire and air.
The most important unit is that of length: one metre was supposed to be equal to 1/40,000,000thyof the equatorial circumference of the Earth. This is approximately 10% longer than one yard so it is a practical unit of length. Later on, a platinum rod with a rigid, X-shaped cross section was produced to serve as the easy-to-check standard for one metre's length. Later on, a multiple of a specific radiation wavelength was introduced to abstractly define the (unchanged) length of the metre unit, and finally the metre was defined as the distance travelled by light in a vacuum in a specific period of time.
The unit of mass is the kilogram, wich was defined by a cube filled with densest (which is +4° Celsius) distilled pure water and having sides equal to 1/10th of a metre. This volume contains one kilogram of water. One kilogram is about 10% heavier than two pounds (2 lbs), thus it is practical for everyday uses. The cubic space is also known as one litre so volume of different liquids can be compared. Later on, a platinum-iridium metal cylinder was manufactured to serve as the one kilogram weight standard and remained so ever since. Everybody need to know about these.
The unit of temperature became the centigrade or inverted Celsius grade, which means the mercury scale is divided into 100 equal length parts between the water-ice mixture and the boiling point of pure, distilled water. Boiling water thus becomes one hundred centigrades hot and freezing is zero degrees. This is the metric unit of temperature in everyday use. A hundred year later, scientists discovered there is an absolute minimum temperature and nothing can be colder than that. This prompted experts to relocate the zero place to mark this temperature, thus creating the Kelvin scale.
The metric unit of time remained the second. One definition of day is 86,400 seconds. The formal definition of the second has been changed several times for enhanced scientific requirements (astronomic observations, tuning fork clock, quartz clock and then caesium atomic clock) but wristwatch users remain relatively unaffected.
The swift worldwide adoption of the metric system as a tool of economy and everday commerce illustrates the easiness of calculations performed within its framework. Meanwhile, the awkwardness of working with customary or imperial units may have contributed to the fast development and rapid commercialization of machine computing technologies in Britain and the USA. Today, computers are highly advanced and no longer need an old measurement system to justify their existance.
Cultural differences can be represented in the local everyday uses of metric units. For example, bread is sold in one-half, one or two kilogram sizes in most countries, but you buy them by multiples of one hundred grams in the former USSR.
Non-scientific people should not be put off by the fine-tuning that has happened to the metric base units over the past two hundred years, as experts regularly tried to refine the metric system to fit the best scientific researcher (e.g. MKG to CGS to SI system changes or the invention of Kelvin scale). These changes almost never affect the everyday use of metric units. Misrepresentation of these adjustments has been one common tactic of diehard fans of the U.S. customary units against metrication.
SI is built on seven SI base units, the kilogram, metre, second, ampere, kelvin, mole, and candela. These are used to define various SI derived units.
SI also defines a number of SI prefixes to be used with the units: these combine with any unit name to give subdivisions and multiples. For example, the prefix kilo denotes a multiple of a thousand, so the kilometre is 1 000 metres, the kilogram 1 000 grams, and so on. Note that a millionth of a kilogram is a milligram, not a microkilogram.
The system can legally be used in every country in the world, and in many countries its use is obligatory. Those countries that still give official recognition to non-SI units (e.g. the US and UK) define them in terms of SI units; for example, the inch is defined to be exactly 0.0254 metres. It was adopted by the 11th General Conference on Weights and Measures (CGPM) in 1960. (See weights and measures for a history of the development of units of measurement.)
The following are the fundamental units from which all others are derived, they are dimensionally independent. The definitions stated below are widely accepted.
| Name | Symbol | Quantity | Definition |
| metre | m | Length | The unit of length is equal to the length of the path travelled by light in a vacuum during the time interval of 1/299 792 458 of a second. This number is exact; the metre is defined this way. |
| kilogram | kg | Mass | The unit of mass is equal to the mass of the international prototype kilogram (a platinum-iridium cylinder) kept at the Bureau International des Poids et Mesures (BIPM), Sèvres, Paris. Note that the kilogram is the only base unit with a prefix; the gram is defined as a derived unit, equal to 1/1 000 of a kilogram. |
| second | s | Time | The unit of time is the duration of exactly 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of caesium-133 atom. |
| ampere | A | Electrical Current | The unit of electrical current is the constant current which, if maintained in two straight parallel conductors, of infinite length and negligible cross-section, placed 1 metre apart in a vacuum, would produce a force between these conductors equal to 2×10 −7 newton per metre of length. |
| kelvin | K | Absolute Temperature | The unit of thermodynamic temperature (or absolute temperature) is the fraction 1/273.16 (exactly) of the thermodynamic temperature at the triple point of water. |
| mole | mol | Amount of substance | The unit of amount of substance is the amount of substance which contains as many elementary entities as there are atoms in 0.012 kilogram of pure carbon-12. (Elementary entities may be atoms, molecules, ions, electrons, or particles.) It is approximately equal to 6.02214199×1023 units. |
| candela | cd | Luminous intensity | The unit of luminous intensity is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. |
The following SI units are derived from the base units and are dimensionless.
| Name | Symbol | Quantity | Definition |
| radian | rad | Angle | The unit of angle is the angle subtended at the centre of a circle by an arc of the circumference equal in length to the radius of the circle. There are <math>2\pi<math> radians in a circle. |
| steradian | sr | Solid angle | The unit of solid angle is the solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere having an area r2. There are <math>4\pi<math> steradians in a sphere. |
Base units can be put together to derive units of measurement for other quantities. Some have been given names.
| Name | Symbol | Quantity | Expressed in base units |
| hertz | Hz | Frequency | s-1 |
| newton | N | Force | kg m s -2 |
| joule | J | Energy | N m = kg m2 s-2 |
| watt | W | Power | J/s = kg m2 s-3 |
| pascal | Pa | Pressure | N/m2 = kg m -1 s-2 |
| lumen | lm | Luminous flux | cd sr |
| lux | lx | Illuminance | cd sr m-2 |
| coulomb | C | Electric Charge | A s |
| volt | V | Electrical potential difference | J/C = kg m2 A-1 s-3 |
| ohm | Ω | Electric resistance | V/A = kg m2 A-2 s-3 |
| farad | F | Electric capacitance | Ω-1 s = A2 s4 kg-1 m-2 |
| weber | Wb | Magnetic flux | kg m2 s-2 A |
| tesla | T | Magnetic flux density | Wb/m2 = kg s-2 A |
| henry | H | Inductance | Ω s = kg m2 A-2 s-2 |
| siemens | S | Electric conductance | Ω-1 = kg-1 m-2 A2 s3 |
| degree Celsius | °C | Celsius temperature | K |
| becquerel | Bq | Radioactivity (decays per unit time) | s-1 |
| gray | Gy | Absorbed dose (of ionising radiation) | J/kg = m2 s-2 |
| sievert | Sv | Equivalent dose (of ionising radiation) | J/kg = m2 s-2 |
| katal | kat | Catalytic activity | mol/s = mol s-1 |
The unit of volume litre, symbolL or l and being equal to 0.001 m3, is not an SI unit but is "accepted for use with the International System."
Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre'. This is in keeping with standard American English spelling (for example, Americans also use 'center' rather than 'centre'; see also American and British English differences). In addition, the official US spelling for the SI prefix 'deca' is 'deka' (again, a variation not recognized by the BIPM).
The US government has approved these spellings for official use, but the BIPM only recognizes the British English spellings as official names for the units. In scientific contexts only the symbols are used; since these are universally the same, the differences do not arise in practice in scientific use.
The unit 'gram' is also sometimes spelled 'gramme' in English-speaking countries other than the United States, though that is an older spelling and use is declining.
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