The SI base units form a set of mutually independent dimensions as required bydimensional 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, themetre has the symbol m, but thekelvin has symbol K, because it is named afterLord Kelvin and theampere with symbol A is named afterAndré-Marie Ampère.
The day is divided into 24 hours, each hour divided into 60 minutes, each minute divided into 60 seconds. A second is1 / (24 × 60 × 60) of theday. Historically, a day was defined as themean solar day; i.e., the average time between two successive occurrences of local apparent solarnoon.
"The metre, symbol m, is the SI unit oflength. It is defined by taking the fixed numerical value of thespeed of light in vacuumc to be299792458 when expressed in the unitm s−1, where the second is defined in terms of∆νCs."[1]
"The kilogram, symbol kg, is the SI unit ofmass. It is defined by taking the fixed numerical value of thePlanck constanth to be6.62607015×10−34 when expressed in the unitJ s, which is equal tokg m2 s−1, where the metre and the second are defined in terms ofc and ∆νCs."[1]
The mass of onelitre ofwater at the temperature of melting ice. A litre is one thousandth of a cubic metre.
"The ampere, symbol A, is the SI unit ofelectric current. It is defined by taking the fixed numerical value of theelementary chargee to be1.602176634×10−19 when expressed in the unit C, which is equal toA 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 ofsilver nitrate.
"The kelvin, symbol K, is the SI unit ofthermodynamic temperature. It is defined by taking the fixed numerical value of theBoltzmann constantk to be1.380649×10−23 when expressed in the unitJ K−1, which is equal tokg m2 s−2 K−1, where the kilogram, metre and second are defined in terms ofh,c and ∆νCs."[1]
TheCelsius scale: the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0 K isabsolute zero).
"The mole, symbol mol, is the SI unit ofamount of substance. One mole contains exactly6.022 140 76 × 1023 elementary entities. This number is the fixed numerical value of theAvogadro constant,NA, when expressed in the unit mol−1 and is called theAvogadro number.
The amount of substance, symboln, 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]
"The candela, symbol cd, is the SI unit ofluminous intensity in a given direction. It is defined by taking the fixed numerical value of theluminous efficacy of monochromatic radiation of frequency540×1012 Hz,Kcd, to be 683 when expressed in the unitlm W−1, which is equal tocdsr W−1, orcd sr kg−1 m−2 s3, where the kilogram, metre and second are defined in terms ofh,c and ∆νCs."[1]
Thecandlepower, which is based on the light emitted from a burning candle of standard properties.
TheSI system after 1983, but before the 2019 revision: Dependence of base unit definitions on other base units (for example, themetre is defined as the distance travelled bylight in a specific fraction of asecond), with the constants of nature and artefacts used to define them (such as the mass of theIPK for the kilogram).New SI: Dependence of base unit definitions onphysical constants with fixed numerical values and on other base units that are derived from the same set of constants. Arrows are shown in the opposite direction compared to typicaldependency graphs, i.e. in this chart means depends on.
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 theMetre 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; themole, theampere, and thecandela were linked through their definitions to the mass of theInternational Prototype of the Kilogram, a roughly golfball-sizedplatinum–iridium cylinder stored in a vault near Paris.
It has long been an objective inmetrology to define the kilogram in terms of afundamental constant, in the same way that the metre is now defined in terms of thespeed of light. The 21stGeneral 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: thePlanck constant and theAvogadro constant.
In 2005, theInternational 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.[2] The 23rd CGPM (2007) decided to postpone any formal change until the next General Conference in 2011.[3]
In a note to the CIPM in October 2009,[4] Ian Mills, the President of the CIPMConsultative Committee – Units (CCU) catalogued the uncertainties of the fundamental constants of physics according to the current definitions and their values under the proposednew definition. He urged the CIPM to accept the proposed changes in the definition of thekilogram,ampere,kelvin, andmole so that they are referenced to the values of the fundamental constants, namely thePlanck constant (h), theelementary charge (e), theBoltzmann constant (k), and theAvogadro constant (NA).[5] This approach was approved in 2018, only after measurements of these constants were achieved with sufficient accuracy.
