## SI base unit |

The **SI base units** and their physical quantities are the

The SI base units form a set of mutually independent dimensions as required by

The names and symbols of SI base units are written in lowercase, except the symbols of those named after a person, which are written with an initial capital letter. For example, the *metre* (US English: *meter*) has the symbol m, but the *kelvin* has symbol K, because it is named after *ampere* with symbol A is named after

Several other units, such as the *liter*), are formally not part of the SI, but are

- seven si base units
- proposed redefinitions
- see also
- references
- external links

Name | Symbol | Measure | Current (2005) formal definition^{[1]} |
Historical origin / justification | Dimension symbol |
---|---|---|---|---|---|

m | "The metre is the length of the path travelled by light in vacuum during a time interval of 1 / 792458 of a second." 29917th CGPM (1983, Resolution 1, CR, 97) |
1 / 000000 of the distance from the 10 |
L | ||

kg | "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 |
M | ||

s | "The second is the duration of 192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." 913th 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 |
T | ||

A | "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 ×10^{−7} 29th 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 |
I | ||

K | "The kelvin, unit of thermodynamic temperature, is the fraction 1 / of the thermodynamic temperature of the 273.1613th 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 ^{2}H per mole of ^{1}H, 0.000 379 9 mole of ^{17}O per mole of ^{16}O, and 0.002 005 2 mole of ^{18}O per mole of ^{16}O."(Added by CIPM in 2005) |
The |
Θ | ||

mol | "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." |
N | |||

cd | "The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency ×10^{12} hertz and that has a radiant intensity in that direction of 1 / 683 watt per 54016th CGPM (1979, Resolution 3; CR, 100) |
The |
J | ||

Name | Symbol | Measure | Current (2005) formal definition^{[1]} |
Historical origin / justification | Dimension symbol |

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中文: 国际单位制基本单位