Background Proposed redefinition of SI base units

the basic structure of si developed on period of 170 years (1791 1960). since 1960 technological advances have made possible address various weaknesses in si, such dependence on artefact define kilogram.

development of si

during years of french revolution, leaders of french national constituent assembly decided introduce new system of measurement based on principles of logic , natural phenomena. specifically, metre defined 1 ten-millionth of distance north pole equator, , kilogram mass of 1 thousandth of cubic metre of pure water. although these definitions chosen nobody own units, not measured sufficient convenience or precision practical use. instead copies created in form of mètre des archives , kilogramme des archives best attempt @ fulfilling these principles.

in 1875, time use of metric system had become widespread in europe , in latin america, twenty industrially developed nations met convention of metre. result signing of treaty of metre under 3 bodies set take custody of international prototype kilogram , metre , regulate comparisons national prototypes. were:

cgpm (general conference on weights , measures / conférence générale des poids et mesures)—the conference meets every 4 6 years , consists of delegates of nations had signed convention. discusses , examines arrangements required ensure propagation , improvement of international system of units , endorses results of new fundamental metrological determinations.
cipm (international committee weights , measures / comité international des poids et mesures)—the committee consists of eighteen eminent scientists, each different country, nominated cgpm. cipm meets annually , tasked advise cgpm. cipm has set number of sub-committees, each charged particular area of interest. 1 of these, consultative committee units (ccu), amongst other things, advises cipm on matters concerning units of measurement.
bipm (international bureau weights , measures / bureau international des poids et mesures)—the bureau provides safe keeping of international prototype kilogram , metre, provides laboratory facilities regular comparisons of national prototypes international prototype , secretariat cipm , cgpm.

the first cgpm (1889) formally approved use of 40 prototype metres , 40 prototype kilograms british firm johnson matthey standards mandated convention of metre. 1 of each of these nominated lot international prototypes, other copies retained cgpm working copies , rest distributed member nations use national prototypes. @ regular intervals national prototypes compared , recalibrated against international prototype. in 1921 convention of metre revised , mandate of cgpm extended provide standards units of measure, not mass , length. in ensuing years cgpm took on responsibility providing standards of electric current (1946), luminosity (1946), temperature (1948), time (1956) , molar mass (1971).

mass drift on time of national prototypes k21–k40, plus 2 of international prototype kilogram s (ipk s) sister copies: k32 , k8(41). mass changes relative ipk.

the 9th cgpm (1948) instructed cipm make recommendations single practical system of units of measurement, suitable adoption countries adhering metre convention . recommendations based on mandate presented 11th cgpm (1960) formally accepted , given name système international d unités , abbreviation si .

impetus change

changing underlying principles behind definition of si base units not without precedent. 11th cgpm (1960) defined si metre in terms of wavelength of krypton-86 radiation, replacing pre-si metre bar. 13th cgpm (1967) replaced original definition of second (which based on back-calculation of earth s rotation in year 1900) definition based on frequency of radiation emitted between 2 hyperfine levels of ground state of caesium 133 atom. , 17th cgpm (1983) replaced 1960 definition of metre 1 based on second, giving exact definition of speed of light in units of metres per second.

over years, drifts of 6992200000000000000♠2×10 kilograms per annum in national prototype kilograms relative international prototype kilogram have been detected. there no way of determining whether national prototypes gaining mass or whether ipk losing mass. @ 21st meeting of cgpm (1999), national laboratories urged investigate ways of breaking link between kilogram , specific artefact. newcastle university metrologist peter cumpson has since identified mercury vapour absorption or carbonaceous contamination possible causes of drift.

independently of drift having been identified, avogadro project , development of watt balance promised methods of indirectly measuring mass high precision. these projects provided tools enable alternative means of redefining kilogram.

a report published in 2007 consultative committee thermometry (cct) cipm noted current definition of temperature has proved unsatisfactory temperatures below 20 kelvins , temperatures above 1300 kelvins. committee of view boltzmann constant provided better basis temperature measurement did triple point of water, overcame these difficulties.

at 23rd meeting (2007), cgpm mandated cipm investigate use of natural constants basis units of measure rather artefacts in use. following year endorsed international union of pure , applied physics (iupap). @ meeting of ccu held in reading, united kingdom, in september 2010, resolution , draft changes si brochure presented next meeting of cipm in october 2010 agreed in principle. cipm meeting of october 2010 found conditions set general conference @ 23rd meeting have not yet been met. reason cipm not propose revision of si @ present time ; however, cipm presented resolution consideration @ 24th cgpm (17–21 october 2011) agree new definitions in principle, not implement them until details have been finalised. resolution accepted conference, , in addition cgpm moved date of 25th meeting forward 2015 2014. @ 25th meeting (18–20 november 2014), found despite [the progress in necessary requirements] data not yet appear sufficiently robust cgpm adopt revised si @ 25th meeting , postponing revision next meeting in 2018.

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