Features Metric system

1 features

1.1 universality
1.2 decimal multiples
1.3 realisability , replicable prototypes

1.3.1 metre , kilogram
1.3.2 other base units


1.4 coherence

features

although metric system has changed , developed since inception, basic concepts have hardly changed. designed transnational use, consisted of basic set of units of measurement, known base units. derived units built base units using logical rather empirical relationships while multiples , submultiples of both base , derived units decimal-based , identified standard set of prefixes.

universality

chinese road sign listing distances on expressway in eastern beijing. although primary text in chinese, distances use internationally recognised characters.

at outbreak of french revolution in 1789, countries , cities had own system of measurement. although different countries might have used units of measure same name, such foot, or local language equivalents such pied, fuß, , voet, there no consistency in magnitude of units, nor in relationships multiples , submultiples, modern-day differences between , uk pints , gallons.

the metric system designed universal—in words of french philosopher marquis de condorcet people time . designed ordinary people, engineers worked in human-related measurements , astronomers , physicists worked numbers both small , large, hence huge range of prefixes have been defined in si.

when french government first investigated idea of overhauling system of measurement, concept of universality put practice in 1789: maurice de talleyrand, acting on condorcet s advice, invited john riggs miller, british parliamentarian , thomas jefferson, american secretary of state george washington, work french in producing international standard promoting legislation in respective legislative bodies. however, these overtures failed , custody of metric system remained in hands of french government until 1875.

in languages distinction made, unit names common nouns (i.e. not proper nouns). use character set , follow grammatical rules of language concerned, example kilomètre , kilómetro , each unit has symbol independent of language, example km kilometre , v volts etc.

decimal multiples

in metric system, multiples , submultiples of units follow decimal pattern, concept identified possibility in 1586 simon stevin, flemish mathematician had introduced decimal fractions europe. done @ cost of losing simplicity associated many traditional systems of units division 3 not result in awkward fractions; example 1 third of foot 4 inches, simplicity in 1790 debated, rejected originators of metric system. in 1854, in introduction proceedings of [british] decimal association, mathematician augustus de morgan summarised advantages of decimal-based system on non-decimal system thus: in simple rules of arithmetic, practice pure decimal system, interrupted entrance of other system: column column never carry tens .

a common set of decimal-based prefixes have effect of multiplication or division integer power of ten can applied units large or small practical use. concept of using consistent classical (latin or greek) names prefixes first proposed in report [french revolutionary] commission on weights , measures in may 1793. prefix kilo, example, used multiply unit 1000, , prefix milli indicate one-thousandth part of unit. kilogram , kilometre thousand grams , metres respectively, , milligram , millimetre 1 thousandth of gram , metre respectively. these relations can written symbolically as:

1 mg = 0.001 g
1 km = 1000 m

in days, multipliers positive powers of ten given greek-derived prefixes such kilo- , mega-, , negative powers of ten given latin-derived prefixes such centi- , milli-. however, 1935 extensions prefix system did not follow convention: prefixes nano- , micro-, example have greek roots. during 19th century prefix myria-, derived greek word μύριοι (mýrioi), used multiplier 7004100000000000000♠10000.

when applying prefixes derived units of area , volume expressed in terms of units of length squared or cubed, square , cube operators applied unit of length including prefix, illustrated below.

prefixes not used indicate multiples of second greater 1; non-si units of minute, hour , day used instead. on other hand, prefixes used multiples of non-si unit of volume, litre (l, l) such millilitres (ml).

realisability , replicable prototypes

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.

coherence


james clerk maxwell played major role in developing concept of coherent cgs system , in extending metric system include electrical units.

each variant of metric system has degree of coherence—the various derived units directly related base units without need intermediate conversion factors. example, in coherent system units of force, energy , power chosen equations

hold without introduction of unit conversion factors. once set of coherent units have been defined, other relationships in physics use units automatically true. therefore, einstein s mass-energy equation, e = mc, not require extraneous constants when expressed in coherent units.

the cgs system had 2 units of energy, erg related mechanics , calorie related thermal energy; 1 of them (the erg) bear coherent relationship base units. coherence design aim of si resulting in 1 unit of energy being defined – joule.

in si, coherent system, unit of power watt , defined 1 joule per second . in customary system of measurement, non-coherent, unit of power horsepower , defined 550 foot-pounds per second (the pound in context being pound-force). similarly, neither gallon nor imperial gallon 1 cubic foot or 1 cubic yard— gallon 231 cubic inches , imperial gallon 277.42 cubic inches.

the concept of coherence introduced metric system in third quarter of 19th century; in original form metric system non-coherent—in particular litre 0.001 m , (from hectare derives) 100 m. units of mass , length related each other through physical properties of water, gram having been designed being mass of 1 cubic centimetre of water @ freezing point.

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