The international system of units (SI) - gpo.gov

1 A111017fl0flS7 NATLINSTOFSTANDARDS& (SI) (SI)NATIONALBUREAUOFSTANDARDSTheNational BureauofStandards1wasestablishedbyanacto fCongressMarch3, 'soverallgoalistostrengthenandadvancethe Nation' ,theBureauconductsresearchandprovides:(1 )abasisfortheNation'sphysicalmeasurement system ,(2)scientificandtechnologicalserv icesforindustryandgovernment,(3)atechnic albasisforequityintrade,and(4) ,theInstituteforMaterialsResearch,theIns tituteforAppliedTechnology,theInstitutef orComputerSciencesandTechnology, ;coordinatesthatsystemwithmeasurementsys temsofothernations;andfurnishesessential servicesleadingtoaccurateanduniformphysi calmeasurementsthroughouttheNation'sscie ntificcommunity,industry, ,anOfficeofMeas-urementServicesandthefol lowingdivisions:AppliedMathematics Electricity Mechanics Heat OpticalPhysics NuclearSciences2 AppliedRadiation2 QuantumElectronics8 Electromagnetics3 TimeandFrequency3 LaboratoryAstrophysics3 Cryogenics\THEINSTITUTEFORMATERIALSRESEA RCH conductsmaterialsresearchleadingtoimprov edmethodsofmeasurement,standards,anddata onthepropertiesofwell-characterizedmater ialsneededbyindustry,commerce,educationa linstitutions,andGovernment;providesadvi soryandresearchservicestootherGovernment agencies;anddevelops,produces, :AnalyticalChemistry Polymers Metallurgy InorganicMaterials ReactorRadiation.

2 Cooperateswithpublicandprivateorganizati onsleadingtothedevelopmentoftechnologica lstandards(includingmandatorysafetystand ards),codesandmethodsoftest; :EngineeringandProductStandards WeightsandMeasures InventionandInnova-tion ProductEvaluationTechnology ElectronicTechnology TechnicalAnalysis MeasurementEngineering Structures,Materials,andLifeSafety* BuildingEnvironment4 TechnicalEvaluationandApplication* ,acquisition,andeffectiveutilizationofau tomaticdataprocessingequipment;andserves astheprincipalfocuswithintheexecutivebra nchforthedevelopmentofFederalstandardsfo rautomaticdataprocessingequipment,techni ques, :ComputerServices SystemsandSoftware ComputerSystemsEngineering ;promotesthedevelopmentoftheNationalStan dardReferenceDataSystemandasystemofinfor mationanalysiscentersdealingwiththebroad eraspectsoftheNationalMeasurementSystem; :OfficeofStandardReferenceData OfficeofInformationActivities OfficeofTechnicalPublications Library ,Maryland,unlessotherwisenoted;mailingad dressWashington, ,Colorado80302.

3 * (SI) (TranslationapprovedbytheInternationalBu reauofWeightsandMeasuresofitspublication "LeSystemeInternationald'Unites")(Supers edesNBSS pecialPublication3301971and1972 Editions) , , ( ), ,49pages(July1974)CODEN:XNBSAVF orsalebytheSuperintendentofDocuments, , ( :330/3).Price65centsTheInternationalSyst emofUnitsForewordThisdocument,nowpublish edindependentlybytheNationalBureauofStan dards,USA,andHerMajesty'sStationeryOffic e,UK,isatranslationoftheFrenchuLeSysteme Internationald?Unites"publishedbytheInte rnationalBureauofWeightsandMeasures.*Itw aspreparedjointlybytheNationalPhysicalLa boratory,UK,andtheNationalBureauofStanda rds, "caesium","blackbody",and"deca"intheUKpu blicationand"cesium","blackbody",and"dek a" ,encourageitsuseinallrealmsofscience,ind ustry,andcommerce,andsecureuniformityofn omenclaturethroughouttheEnglish-speaking world.

4 *"LeSystemeInternationald'TJniUs,"1970,O FFILIB,48rueGay-Lussac, (Revisededition1973).Translationshaveals obeenpublishedinseveralotherlanguages,na mely,toourknowledge,inGerman,inSpanish,i nPortuguese, (BIPM),inresponsetofrequentrequests,publ ishesthisdocumentcontainingResolutionsan dRecommendationsoftheGeneralConferenceof WeightsandMeasures(CGPM)ontheInternation alSystemofUnits,Explanationshavebeenadde daswellasrelevantextractsfromtheRecommen dationsoftheInternationalOrganizationfor Standardization(ISO) (CCU)oftheInternationalCommitteeofWeight sandMeasures(CIPM) (Reso-lutions,Recommendations,Declaratio ns,etc.) ,consistentwiththetheo-reticaldefinition sgivenhere,whichmetrologiea] (1971) , , ,byitsResolution6,instructedtheCIPM1:"to studytheestablishmentofacompletesetofrul esforunitsofmeasurement";"tofindoutforth ispurpose,byofficialinquiry,theopinionpr evailinginscientific,technical,andeducat ionalcirclesinallcountries"and"tomakerec ommendationsontheestablishmentofapractic alsystemofunitsofmeasurementsuitablefora doptionbyallsignatoriestotheMetreConvent ion.

5 "ThesameGeneralConferencealsolaiddown,by itsResolution7,generalprinciplesforunits ymbols( ,page6) (1954),byitsResolution6,andthe14thCGPM(1 971)byitsResolution3,adoptedasbaseunitso fthis"practicalsystemofunits",theunitsof thefollowingsevenquantities:length,mass, time,electriccurrent,thermodynamictemper ature,amountofsubstance,andluminousinten sity( ,page3).The11thCGPM(1960),byitsResolutio n12,adoptedthenameInternationalSystemofU nits,withtheinternationalabbreviationSI, forthispracticalsystemofunitsofmeasureme ntandlaiddownrulesfortheprefixes( ,page12),thederivedandsupple-mentaryunit s( , ,page11)andothermat-ters, "SIunits","SIprefixes","supplementaryuni ts"areusedinaccordancewithRecommendation 1(1969) , :baseunits,derivedunits, , ,consideringtheadvantagesofasingle,pract ical,worldwidesystemforinternationalrela tions,forteachingandforscientificwork,de cidedtobasetheInternationalSystemonachoi ceofsevenwell-definedunitswhichbyconvent ionareregardedasdimensionallyindependent :themetre,thekilogram,thesecond,theamper e,thekelvin,themole,andthecandela( ,page3).

6 , , ( ,page6).AlthoughitmightbethoughtthatSIun itscanonlybebaseunitsorderivedunits,the1 1thCGPM(1960)admittedathirdclassofSIunit s,calledsupplementaryunits,forwhichitdec linedtostatewhethertheywerebaseunitsorde rivedunits( ,page11).TheSIunitsofthesethreeclassesfo rmacoherentsetinthesensenormallyattribut edtotheexpression"coherentsystemofunits" . ' "metre"and"kilogram" )Unitoflength. The11thCGPM(1960)replacedthedefinitionof themetrebasedontheinternationalprototype ofplatinum-iridium,inforcesince1889andam plifiedin1927, (11thCGPM(1960),Kesolution6). )Unitofmass. The1stCGPM(1889)legalizedtheinterna- "weight",the3rdCGPM(1901)de-clared:theki logramistheunitofmass[andnotofweightorof force]; )Unitoftime. Originallytheunitoftime,thesecond,wasde- finedasthefraction1 "meansolarday"waslefttoastronomers, (1960) ,basedonatransitionbetweentwoenergylevel sofanatomoramolecule, ,thesecond,isindispensablefortheneedsofa dvancedmetrology,the13thCGPM(1967)decide dtoreplacethedefinitionofthesecondbythef ollowing:Thesecondisthedurationof9192631 770periodsoftheradiationcorrespondingtot hetransitionbetweenthetwohyperfinelevels ofthegroundstateofthecesium-Y&Zatom.

7 (13thCGPM(1967),Reso-lution1).3550-419O- 74-2d)Unitofelectriccurrent. Electricunits,called" international ",forc urrentandresistance,hadbeenintroducedbyt heInternationalElectricalCongressheldinC hicagoin1893,andthedefinitionsofthe"inte rnational"ampereandthe" international " (1933)thattherewasaunanimousdesiretorepl acethose"inter-national"unitsbyso-called "absolute" units ,theofficialdecisiontoabo lishthemwasonlytakenbythe9thCGPM(1948),w hichadoptedfortheunitofelectriccurrent,t heampere,thefollowingdefinition:Theamper eisthatconstantcurrenttohich,ifmaintaine dintwostraightparallelconductorsofinfini telength,ofnegligiblecircularcrosssectio n,andplaced1metreapartinvacuum,wouldprod ucebetweentheseconductorsaforceequalto2Y 10~7newtonpermetreoflength.(CIPM(1946),R esolution2approvedbythe9thCGPM,1948)Thee xpression"MKSunitofforce"whichoccursinth eoriginaltexthasbeenreplacedhereby"newto n"adoptedbythe9thCGPM(1948,Resolution7).

8 E)Unitofthermodynamictemperature. Thedefinitionoftheunitofthermodynamictem peraturewasgiveninsubstancebythe10thCGPM (1954,Resolution3) (1967,Resolution3)adoptedthenamekelvin(s ymbolK)insteadof"degreeKelvin"(symbol K)andinitsResolution4definedtheunitofthe rmodynamictempera-tureasfollows:Thekelvi n,unitofthermodynamictemperature,isthefr action1 (13thCGPM(1967),Resolution4).The13thCGPM (1967,Resolution3) Inadditiontothethermodynamictemperature( symbolT),expressedinkelvins,useisalsomad eofCelsiustemperature(symbolt)definedbyt heequationt-T-To*whereTQ= (symbolCC).Theunit"degreeCelsius"isthuse qualtotheunit"kelvin" )Unitofamountofsubstance. Sincethediscoveryofthefundamentallawsofc hemistry,unitsofamountofsubstancecalled, forinstance,"gram-atom"and"gram-molecule ", "atomicweights"and"molecularweights",whi chwereinfactrelativemasses.

9 "Atomicweights"wereoriginallyreferredtot heatomicweightofoxygen(bygeneralagreemen ttakenas16).Butwhereasphysicistsseparate disotopesinthemassspec-trographandattrib utedthevalue16tooneoftheisotopesofoxygen ,chemistsattributedthatsamevaluetothe(sl ightlyvariable)mixtureofisotopes16,17,an d18, (IUPAP)andtheInternationalUnionofPureand AppliedChemistry(IUPAC)broughtthisdualit ytoanendin1959 ,Theunifiedscalethusobtainedgivesvalueso f"relativeatomicmass".Itremainedtodefine theunitofamountofsubstancebyfixingthecor respondingmassofcarbon12;byinternational agreement, ,andtheunitofthequantity,"amountofsubsta nce",3hasbeengiventhenamemole(symbolmol) .FollowingproposalsofIUPAP,IUPAC,andISO, theCIPM gavein1967,andconfirmedin1969,thefollowi ngdefinitionofthemole,adoptedbythe14thCG PM(1971,Resolution3) ,theelementaryentitiesmustbespeci-fiedan dmaybeatoms,molecules,ions,electrons,oth erparticles, )Unitofluminousintensity.

10 Theunitsofluminousintensitybasedonflameo rincandescentfilamentstandardsinuseinvar iouscountrieswerereplacedin1948bythe"new candle".Thisdecisionhadbeenpreparedbythe InternationalCommissiononIllumination(CI E)andbytheCIPM before1937, (1948)ratifiedthedecisionoftheCIPM andgaveanewinternationalname,candela(sym bolcd), ,asamendedin1967, ,intheperpendiculardirec-tion,ojasurface of1/600000squaremetreofablackbodyatthete m-3 Thenameofthisquantity,adoptedbyIUPAP,IUP AC,andISOisinFrench"quantitydematiere"an dinEnglish"amountofsubstance";(theGerman andRussiantranslationsare"Stoffmenge"and "kojiuhcctboBemecTBa") TheFrenchnamerecalls"quantitasmateriae"b ywhichinthepastthequantitynowcalledmassu sedtobeknown;wemustforgetthisoldmeaning, (13thCGPM(1967).Resolution5).2,SymbolsTh ebaseunitsoftheInternationalSystemarecol lectedintable1withtheirnamesandtheirsymb ols(10thCGPM(1954),Resolu-tion6;11thCGPM (I960),Resolution12;13thCGPM(1967),Reso- lution3;14thCGPM(1971), * Celsiustemperatureisingeneralexpressedin degreesCelsius(symbol C)(seeNote, ).)