International System of Units
The SI base units form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology.
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 Lord
Other units, such as the litre (US English: liter), are formally not part of the SI, but are accepted for use with SI .
* 1 Seven SI base units * 2 Proposed redefinitions * 3 See also * 4 References * 5 External links
SEVEN SI BASE UNITS
SI base units NAME SYMBOL MEASURE CURRENT (2005) FORMAL DEFINITION HISTORICAL ORIGIN / JUSTIFICATION Dimension symbol
"The metre is the length of the path travelled by light in vacuum
during a time interval of 1 ⁄ 7008299792458000000♠299792458 of
17th CGPM (1983, Resolution 1, CR, 97) 1 ⁄ 10,000,000 of the
distance from the
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 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
"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
Celsius scale : 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) Atomic weight or molecular weight divided by the molar mass constant , 1 g/mol. N
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 HISTORICAL ORIGIN / JUSTIFICATION Dimension symbol
Main article: Proposed redefinition of SI base units Proposed SI System: Dependence of base unit definitions on physical constants with fixed 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
It has long been an objective in metrology to define the kilogram in
terms of a fundamental constant , in the same way that the metre is
now defined in terms of the speed of light . The 21st General
Conference on Weights and Measures (CGPM, 1999) placed these efforts
on an official footing, and recommended "that national laboratories
continue their efforts to refine experiments that link the unit of
mass to fundamental or atomic constants with a view to a future
redefinition of the kilogram." Two possibilities have attracted
particular attention: the
In 2005, the International Committee for Weights and Measures (CIPM) approved preparation of new definitions for the kilogram, the ampere, and the kelvin and it noted the possibility of a new definition of the mole based on the Avogadro constant. The 23rd CGPM (2007) decided to postpone any formal change until the next General Conference in 2011.
In a note to the CIPM in October 2009, Ian Mills, the President of the CIPM Consultative Committee - Units (CCU) catalogued the uncertainties of the fundamental constants of physics according to the current definitions and their values under the proposed new definition . He urged the CIPM to accept the proposed changes in the definition of the kilogram, ampere, kelvin, and mole so that they are referenced to the values of the fundamental constants, namely the Planck constant (h), the electron charge (e), the Boltzmann constant (k), and the Avogadro constant (NA).
* ^ A B
International Bureau of Weights and Measures