SI base unit
Symbol | Name | Base quantity |
---|---|---|
s | second | time |
m | metre | length |
kg | kilogram | mass |
A | ampere | electric current |
K | kelvin | thermodynamic temperature |
mol | mole | amount of substance |
cd | candela | luminous intensity |
The SI base units are the standard
The SI base units form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology. [citation needed]
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 has the symbol m, but the
Definitions
On 20 May 2019, as the final act of the
Name | Symbol | Measure | Post-2019 formal definition[1] | Historical origin / justification | Dimension symbol |
---|---|---|---|---|---|
second | s | time | "The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, ∆νCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1."[1] | The day is divided into 24 hours, each hour divided into 60 minutes, each minute divided into 60 seconds. A second is 1 / (24 × 60 × 60) of the mean solar day; i.e., the average time between two successive occurrences of local apparent solar noon .
|
T |
metre | m | length | "The metre, symbol m, is the SI unit of | 1 / 10000000 of the distance from the Earth's equator to the North Pole measured on the meridian arc through Paris. | L |
kilogram | kg | mass | "The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of c and ∆νCs."[1] | The mass of one litre of water at the temperature of melting ice. A litre is one thousandth of a cubic metre. | M |
ampere | A | electric current | "The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634×10−19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆νCs."[1] | 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. | I |
kelvin | K | thermodynamic temperature | "The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380649×10−23 when expressed in the unit J K−1, which is equal to kg m2 s−2 K−1, where the kilogram, metre and second are defined in terms of h, c and ∆νCs."[1] | The Celsius scale: the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0 K is absolute zero ).
|
Θ |
mole | mol | amount of substance | "The mole, symbol mol, is the SI unit of Avogadro number .
The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles."[1] |
molecular weight divided by the molar mass constant , 1 g/mol.
|
N |
candela | cd | luminous intensity | "The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×1012 Hz, Kcd, to be 683 when expressed in the unit lm W−1, which is equal to cd sr W−1, or cd sr kg−1 m−2 s3, where the kilogram, metre and second are defined in terms of h, c and ∆νCs."[1] | The candlepower, which is based on the light emitted from a burning candle of standard properties. | J |
2019 redefinition of the SI base units
New base unit definitions were adopted on 16 November 2018, and they became effective on 20 May 2019. The definitions of the base units have been modified several times since the Metre Convention in 1875, and new additions of base units have occurred. Since the redefinition of the metre in 1960, the kilogram had been the only base unit still defined directly in terms of a physical artefact, rather than a property of nature. This led to a number of the other SI base units being defined indirectly in terms of the mass of the same artefact; the mole, the ampere, and the candela were linked through their definitions to the mass of the International Prototype of the Kilogram, a roughly golfball-sized platinum–iridium cylinder stored in a vault near Paris.
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 attracted particular attention: the Planck constant and the Avogadro constant.
In 2005, the
In a note to the CIPM in October 2009,
See also
- International vocabulary of metrology
- International System of Quantities
- Non-SI units mentioned in the SI
- Metric prefix
- Physical constant
References
- ^ a b c d e f g h "The International System of Units (SI), 9th Edition" (PDF). Bureau International des Poids et Mesures. 2019.
- International Committee for Weights and Measures (2005). "Recommendation 1: Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants" (PDF). Archived from the original (PDF) on 7 August 2011.)
{{cite web}}
: CS1 maint: numeric names: authors list (link - ^ 23rd General Conference on Weights and Measures (2007). Resolution 12: On the possible redefinition of certain base units of the International System of Units (SI).
- ^ Ian Mills, President of the CCU (October 2009). "Thoughts about the timing of the change from the Current SI to the New SI" (PDF). CIPM. Retrieved 23 February 2010.
- ^ Ian Mills (29 September 2010). "Draft Chapter 2 for SI Brochure, following redefinitions of the base units" (PDF). CCU. Retrieved 1 January 2011.