The
Contents 1 Seven SI base units 2 Proposed redefinitions 3 See also 4 References 5 External links Seven SI base units[edit] SI base units Name Symbol Measure Current (2005) formal definition[1] Historical origin / justification Dimension symbol metre m length "The metre is the length of the path travelled by light in vacuum during a time interval of 1 / 7008299792458000000♠299792458 of a second." 17th CGPM (1983, Resolution 1, CR, 97) 1 / 7007100000000000000♠10000000 of the distance from the Earth's equator to the North Pole measured on the circumference through Paris. L kilogram kg mass "The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram." 3rd CGPM (1901, CR, 70) The mass of one litre of water at the temperature of melting ice. A litre is one thousandth of a cubic metre. M second s time "The second is 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." 13th CGPM (1967/68, Resolution 1; CR, 103) "This definition refers to a caesium atom at rest at a temperature of 0 K." (Added by CIPM in 1997) The day is divided in 24 hours, each hour divided in 60 minutes, each minute divided in 60 seconds. A second is 1 / (24 × 60 × 60) of the day. T ampere A electric current "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 6993200000000000000♠2×10−7 newton per metre of length." 9th CGPM (1948) The original "International Ampere" was defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of silver nitrate. Compared to the SI ampere, the difference is 0.015%. I kelvin
K
thermodynamic temperature
"The kelvin, unit of thermodynamic temperature, is the fraction
1 / 7002273160000000000♠273.16 of the thermodynamic
temperature of the triple point of water."
13th CGPM (1967/68, Resolution 4; CR, 104)
"This definition refers to water having the isotopic composition
defined exactly by the following amount of substance ratios: 0.000 155
76 mole of 2H per mole of 1H, 0.000 379 9 mole of 17O per mole of 16O,
and 0.002 005 2 mole of 18O per mole of 16O."
(Added by CIPM in 2005)
The
mole mol amount of substance "1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is 'mol'. 2. 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." 14th CGPM (1971, Resolution 3; CR, 78) "In this definition, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to." (Added by CIPM in 1980)
candela cd luminous intensity "The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 7014540000000000000♠540×1012 hertz and that has a radiant intensity in that direction of 1 / 683 watt per steradian." 16th CGPM (1979, Resolution 3; CR, 100) The candlepower, which is based on the light emitted from a burning candle of standard properties. J Name Symbol Measure Current (2005) formal definition[1] Historical origin / justification Dimension symbol Proposed redefinitions[edit] Main article: Proposed redefinition of SI base units Proposed SI System: Dependence of base unit definitions on physical constants with fixed numerical values and on other base units that are derived from the same set of constants. The definitions of the base units have been modified several times
since the
International vocabulary of metrology International System of Quantities Non-SI units mentioned in the SI Metric prefix Physical constant Electric constant Magnetic constant References[edit] ^ a b
External links[edit] International Bureau of Weights and Measures National Physical Laboratory NIST -SI v t e SI units Authority:
Base units ampere candela kelvin kilogram metre mole second Derived units with special names becquerel coulomb degree Celsius farad gray henry hertz joule katal lumen lux newton ohm pascal radian siemens sievert steradian tesla volt watt weber Other accepted units astronomical unit bar dalton day decibel degree of arc electronvolt hectare hour litre minute minute of arc neper second of arc tonne atomic units natural units See also Conversion of units Metric prefixes Proposed redefinitions Systems of measurement Book Category v t e SI base quantities Base quantity Quantity SI unit Name Symbol Dimension symbol Unit name (symbol) Example length l, x, r, (etc.) L metre (m) r = 10 m mass m M kilogram (kg) m = 10 kg time, duration t T second (s) t = 10 s electric current I , i I ampere (A) I = 10 A thermodynamic temperature T Θ kelvin (K) T = 10 K amount of substance n N mole (mol) n = 10 mol luminous intensity Iv J candela (cd) Iv = 10 cd Specification The quantity (not the unit) can have a specification: Tmax = 300 K Derived quantity Definition A quantity Q is expressed in the base quantities: Q = f ( l , m , t , I , T , n , I v ) displaystyle Q=fleft( mathit l,m,t,I,T,n,I mathrm _ v right) Derived dimension dim Q = La · Mb · Tc · Id · Θe · Nf · Jg (Superscripts a–g are algebraic exponents, usually a positive, negative or zero integer.) Example Quantity acceleration = l1 · t−2, dim acceleration = L1 · T−2 possible units: m1 · s−2, km1 · Ms−2, etc. See also History of the metric system International System of Quantities Proposed redefinitions Systems of measurement Book |