Realisability and replicable prototypes Metric system

the metre defined 1 ten millionth of distance between north pole , equator through paris.

the base units used in metric system must realisable, ideally reference natural phenomena rather unique artefacts. each of base units in si accompanied mise en pratique [practical realisation] published bipm describes in detail @ least 1 way in base unit can measured. possible, definitions of base units developed laboratory equipped proper instruments able realise standard without reliance on artefact held country. in practice, such realisation done under auspices of mutual acceptance arrangement (maa).

metre , kilogram

in original version of metric system base units derived specified length (the metre) , weight [mass] of specified volume ( ⁄7003100000000000000♠1000 of cubic metre) of pure water. de facto french government of day, assemblée nationale constituante, considered defining metre length of pendulum has period of 1 second @ 45°n , altitude equal sea level. altitude , latitude specified accommodate variations in gravity; specified latitude compromise between latitude of london (51° 30 n), paris (48° 50 n) , median parallel of united states (38°n) accommodate variations. mathematician borda persuaded assembly survey having ends @ sea level , based on meridian spanned @ least 10% of earth s quadrant more appropriate such basis.

one litre equivalent volume of cube edges of 10 cm , kilogram designed 1 litre of water @ melting point of ice.

the available technology of 1790s made impracticable use these definitions basis of kilogram , metre, prototypes represented these quantities insofar practicable manufactured. on 22 june 1799 these prototypes adopted definitive reference pieces, deposited in archives nationales , became known mètre des archives , kilogramme des archives. copies made , distributed around france. these artefacts replaced in 1889 new prototypes manufactured under international supervision. insofar possible, new prototypes exact copies of original prototypes, used later technology ensure better stability. 1 of each of kilogram , metre prototypes chosen lot serve definitive international reference piece remainder being distributed signatories of metre convention. in 1889 there no accepted theory regarding nature of light 1960 wavelength of specific light spectra give more accurate , reproducible value prototype metre. in year prototype metre replaced formal definition defines metre in terms of wavelength of specified light spectra. 1983 accepted speed of light in vacuum constant , constant provided more reproducible procedure measuring length. therefore, metre redefined in terms of speed of light. these definitions give better reproducibility , allow anyone, anywhere suitably equipped laboratory, make standard metre.

other base units

none of other base units rely on prototype – based on phenomena directly observable , had been in use many years before formally becoming part of metric system.

the second first became de facto base unit within metric system when, in 1832, carl friedrich gauss used it, centimetre , gram derive units associated values of absolute measurements of earth s magnetic field. second, if based on earth s rotation, not constant earth s rotation slowing down—in 2008 solar day 0.002 s longer in 1820. had been known many years; consequently in 1952 international astronomical union (iau) defined second in terms of earth s rotation in year 1900. measurements of time made using extrapolation readings based on astronomy. launch of si in 1960, 11th cgpm adopted iau definition. in years followed, atomic clocks became more reliable , precise; , in 1968 13th cgpm redefined second in terms of specific frequency emission spectrum of caesium 133 atom, component of atomic clocks. provided means measure time associated astronomical phenomena rather using astronomical phenomena basis time measurements made.

the cgs absolute unit of electric current, abampere, had been defined in terms of force between 2 parallel current-carrying wires in 1881. in 1940s, international electrotechnical commission adopted mks variant of definition ampere, adopted in 1948 cgpm.

temperature has been based on observable phenomena—in 1744 degree centigrade based on freezing , boiling points of water. in 1948 cgpm adopted centigrade scale, renamed celsius temperature scale name , defined in terms of triple point of water.

when mole , candela accepted cgpm in 1971 , 1975 respectively, both had been defined third parties reference phenomena rather artefacts.

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