SI
- For other uses, see SI (disambiguation).
There are seven base units and several derived units, together with a set of prefixeses. Non-SI units can be converted to SI units (or vice versa) according to the conversion of units.
Table of contents |
2 Basis 3 SI writing style 4 Units 5 Spelling variations 6 Related topics 7 External links 8 Further reading |
Origin
The units of the SI system are decided by 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.
Basis
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.
SI writing style
- Symbols are written in lower case except for in symbols where the unit is the same as the name of a person, or derived from the name of a person. This means that the symbol for the SI unit for pressure, named after Blaise Pascal, is Pa, whereas the unit itself is written pascal. The official SI brochure lists the symbol for the litre as an allowed exception to the capitalization rules: either capital or lowercase L is acceptable.
- Symbols are written in singular e.g 25 kg (not "25 kgs")
- It is preferable to keep the symbol in upright roman type (for example, kg for kilograms, m for metres), so as to differentiate from mathematical and physical variables (for example, m for mass, l for length).
- A space is left between the numbers and the symbols: 2.21 kg, 7.3·10^{2} m^{2}
- SI uses spaces to separate decimal digits in sets of three. e.g. 1 000 000 or 342 142 (in contrast to the commas or dots used in other systems e.g. 1,000,000 or 1.000.000).
- SI used only a comma as the separator for decimal fractions until 1997. The number "twenty four and fifty one hundredths" would be written as "24,51". In 1997 the CIPM decided that the British full stop (the "dot on the line", or period) would be the decimal separator in text whose main language is English ("24.51"); the comma remains the decimal separator in all other languages.
- The symbol (or prefix) can be used in place of the decimal separator. 2.3k would be 2k3, 4.7A (Amps) would be 4A7, and 0.0047F would be 4m7F.
Units
Base Units
Name | Unit Symbol | Measure Of | 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 |
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. |
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]. |
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 540×10^{12} hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. |
Dimensionless derived units
The following SI units are derived from the base units and are dimensionless.
Name | Unit Symbol | Measure Of | 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 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 r^{2}. There are steradians in a sphere. |
Derived units with special names
Base units can be put together to derive units of measurement for other quantities. Some have been given names.
Name | Unit Symbol | Measure of | Expressed in base units |
hertz | Hz | Frequency | s^{-1} |
newton | N | Force | kg m/s ^{2} |
joule | J | Energy | N m = kg m^{2}/s^{2} |
watt | W | Power | J/s = kg m^{2}/s^{3} |
pascal | Pa | Pressure | N/m^{2} = kg/m s^{2} |
lumen | lm | Luminous flux | cd sr |
lux | lx | Illuminance | cd sr/m^{2} |
coulomb | C | Electric Charge | A s |
volt | V | Electric Potential Difference | J/C = kg m^{2} A^{-1} s^{-3} |
ohm | Ω | Electric resistance | V/A = kg m^{2} A^{-2} s^{-3} |
farad | F | Electric capacitance | A^{2} s^{4} kg^{-1} m^{-2} = ^{-1} s |
weber | Wb | Magnetic flux | kg m^{2}/s^{2} A |
tesla | T | Magnetic flux density | Wb/m^{2} = kg/s^{2} A |
henry | H | Inductance | kg m^{2} A^{-2} s^{-2} = s |
siemens | S | Electric conductance | ^{-1} = kg^{-1} m^{-2} A^{2} s^{3} |
becquerel | Bq | Radioactivity (decays per unit time) | s^{-1} |
gray | Gy | Absorbed dose (of ionising radiation) | J/kg |
sievert | Sv | Dose equivalent (of ionising radiation) | J/kg |
The unit of volume litre, abbreviated L or l and being equal to 0.001 m^{3}, is not an SI unit but is "accepted for use with the International System."
Spelling variations
Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre', 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 abbreviations 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, though that is an older spelling and is falling out of use.
Related topics
- Other measurement systems:
- Metrication
- Metric system in the United States
- Metrology
- UTC (Coordinated Universal Time)
- Binary Prefixes - used to quantify large amounts of computer data
- Orders of magnitude
External links
Official
- SI maintenance agency (home page)
- BIPM reference (SI reference)
- US NIST reference on SI
- SI -- Its History and Use in Science and Industry
- Nelson, Robert A., "The International System of Units Its History and Use in Science and Industry". Via Satellite, February 2000.
- A Dictionary of Units of Measurement by Russ Rowlett
Further reading
- I. Mills, Tomislav Cvitas, Klaus Homann, Nikola Kallay, IUPAC: Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell Science Inc 1993, ISBN 0632035838.