<P> The International System of Units consists of a set of base units, a set of derived units with special names, and a set of decimal - based multipliers that are used as prefixes . The term SI Units covers all three categories, but the term coherent SI units includes only base units and coherent derived units . </P> <P> The SI base units are the building blocks of the system and all other units are derived from them . When Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass, length and time . Giorgi later identified the need for an electrical base unit . Theoretically any one of electric current, potential difference, electrical resistance, electrical charge or a number of other quantities could have provided the base unit, with the remaining units then being defined by the laws of physics . In the event, the unit of electric current was chosen for SI . Another three base units (for temperature, substance and luminous intensity) were added later . </P> <Table> SI base units <Tr> <Th> Unit name </Th> <Th> Unit symbol </Th> <Th> Quantity name </Th> <Th> Definition (incomplete) </Th> <Th> Dimension symbol </Th> </Tr> <Tr> <Th> metre </Th> <Td> m </Td> <Td> length </Td> <Td> <Ul> <Li> Original (1793): 1 / 7007100000000000000 ♠ 10 000 000 of the meridian through Paris between the North Pole and the Equator . </Li> <Li> Interim (1960): 7006165076373000000 ♠ 1650763. 73 wavelengths in a vacuum of the radiation corresponding to the transition between the 2p and 5d quantum levels of the krypton - 86 atom . </Li> <Li> Current (1983): The distance travelled by light in a vacuum in 1 / 7008299792458000000 ♠ 299 792 458 second . </Li> </Ul> </Td> <Td> </Td> </Tr> <Tr> <Th> kilogram </Th> <Td> kg </Td> <Td> mass </Td> <Td> <Ul> <Li> Original (1793): The grave was defined as being the weight (mass) of one cubic decimetre of pure water at its freezing point . </Li> <Li> Current (1889): The mass of the International Prototype Kilogram (Le Grand K). </Li> </Ul> </Td> <Td> </Td> </Tr> <Tr> <Th> second </Th> <Td> s </Td> <Td> time </Td> <Td> <Ul> <Li> Original (Medieval): 1 / 7004864000000000000 ♠ 86 400 of a day . </Li> <Li> Interim (1956): 1 / 7007315569259747000 ♠ 31 556 925. 9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time . </Li> <Li> Current (1967): The duration of 7009919263177000000 ♠ 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium - 133 atom . </Li> </Ul> </Td> <Td> </Td> </Tr> <Tr> <Th> ampere </Th> <Td> </Td> <Td> electric current </Td> <Td> <Ul> <Li> Original (1881): A tenth of the electromagnetic CGS unit of current . The (CGS) electromagnetic unit of current is that current, flowing in an arc 1 cm long of a circle 1 cm in radius, that creates a field of one oersted at the centre . </Li> <Li> Current (1946): The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 6993200000000000000 ♠ 2 × 10 newtons per metre of length . </Li> </Ul> </Td> <Td> </Td> </Tr> <Tr> <Th> kelvin </Th> <Td> </Td> <Td> thermodynamic temperature </Td> <Td> <Ul> <Li> Original (1743): The centigrade scale is obtained by assigning 0 ° C to the freezing point of water and 100 ° C to the boiling point of water . </Li> <Li> Interim (1954): The triple point of water (0.01 ° C) defined to be exactly 273.16 K . </Li> <Li> Current (1967): 1 / 273.16 of the thermodynamic temperature of the triple point of water </Li> </Ul> </Td> <Td> Θ </Td> </Tr> <Tr> <Th> mole </Th> <Td> mol </Td> <Td> amount of substance </Td> <Td> <Ul> <Li> Original (1900): The molecular weight of a substance in mass grams . </Li> <Li> Current (1967): The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon - 12 . </Li> </Ul> </Td> <Td> </Td> </Tr> <Tr> <Th> candela </Th> <Td> cd </Td> <Td> luminous intensity </Td> <Td> <Ul> <Li> Original (1946): The value of the new candle is such that the brightness of the full radiator at the temperature of solidification of platinum is 60 new candles per square centimetre . </Li> <Li> Current (1979): The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 7014540000000000000 ♠ 5.4 × 10 hertz and that has a radiant intensity in that direction of 1 / 683 watt per steradian . </Li> </Ul> </Td> <Td> J </Td> </Tr> <Tr> <Td_colspan="5"> <Dl> <Dt> Note </Dt> </Dl> <Ol> <Li> Jump up ^ Interim definitions are given here only when there has been a significant difference in the definition . </Li> <Li> Jump up ^ Despite the prefix "kilo -", the kilogram is the base unit of mass . The kilogram, not the gram, is used in the definitions of derived units . Nonetheless, units of mass are named as if the gram were the base unit . </Li> <Li> Jump up ^ In 1954 the unit of thermodynamic temperature was known as the "degree Kelvin" (symbol ° K; "Kelvin" spelt with an upper - case "K"). It was renamed the "kelvin" (symbol "K"; "kelvin" spelt with a lower case "k") in 1967 . </Li> <Li> Jump up ^ When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles . </Li> </Ol> <P> The original definitions of the various base units in the above table were made by the following authorities: </P> <Dl> <Dd> <Ul> <Li> FG = French Government </Li> <Li> IEC = International Electrotechnical Commission </Li> <Li> ICAW = International Committee on Atomic Weights </Li> </Ul> </Dd> </Dl> <P> All other definitions result from resolutions by either CGPM or the CIPM and are catalogued in the SI Brochure . </P> </Td> </Tr> </Table> <Tr> <Th> Unit name </Th> <Th> Unit symbol </Th> <Th> Quantity name </Th> <Th> Definition (incomplete) </Th> <Th> Dimension symbol </Th> </Tr>

What is the standard unit of length in the si system