片断：
1.1.1Theinternationalsystemofunits(Sl)
History
Inthesecondhalfofthenineteenthcenturythecentimetre,gramandsecondwereinfairlygeneraluseas
baseunitsforscientificworkeveninsuchcountriesastheUKandtheUSAwherethefootandthepound
wereemployedforcommerceandengineering.Asaresult,theunitsrequiredbytherapidlyemergingscience
ofelectricitywerebasedonthecentimetre,gramandsecond,withwhichtheyformedacoherentsystem
knownastheCGSelectromagneticsystem.Asystemofunitsissaidtobecoherentwhenderivedunitsare
formedfromthebaseunitswithouttheinsertionoffactorsofproportionalityotherthanunity.Therewas
alsotheCGSelectrostaticsystem,buttheonlyquantitiesfrequentlyexpressedinelectrostaticunitswere
clectriccharge,electricpotential,andcapacitance.
Theyoungbutfast-growingelectricalindustrysoonfoundthatmanyCGSelectromagneticunitswere
ofanextremelyinconvenientsizeforitsneeds.Accordingly,in1881,internationalagreementwasreachedto
flxthepracticalunitofpotential,tobecalledthevolt,atlO8CGSunits(whichisapproximatelyequalto
thee.m.f.ofaprimarycell),andtheunitofresistance,theohm,at109CGSunits(whichisapproximately
theresistanceofacolumnofmercury1mlongand1mm2incross-section).Theunitofelectric
current,theampere,wasmadeatenthoftheCGSunit.Acoherentsystemofpracticalelectricunitswasthus
securedwhich,however,wasnotcoherentwiththemechanicalunitsbasedonthecentimetreandgram.The
practicalelectricunitssuitedtheneedsoftelegraphy,whichwasthenthemainelectricalindustry,andthey
alsohappentobeconvenientforheavyelectricalengineeringandforelectronics.
Themagneticunits,however,wereleftattheirCGSvalues,presumablybecausetheCGSunitofmagnetic
fluxdensity,subsequentlycalled'gauss',isoftheorderofthefluxdensityoftheEarthsfield,and,asitwas
suitableforgeomagnetism,thereseemednopointinchangingitforaunit104timeslarger.Coherencewas
therebylosttoelectromagnetismasithadalreadybeenlosttothesystemembracingthemechanicalunits
andthepracticalelectricunits.
Whereastheelectricunits,bytheagreementof1881,werechosentobeofsuitablemagnitudeforeveryday
use,andwhereasthecentimetreandthesecondhaveacceptablesizes,thegramistoosmallforthepractical
nccdsofman,whicharebetterservedbyaunitnearerthesizeofthepoundorthekilogram.Moreover,the
CGSunitofforce,thedyne,andtheunitofenergy,theerg,aremuchtoosmall.Ontheotherhand,theunit
ofenergyprovidedbythepracticalelectricunits,thevolt-ampere-second,calledthejoule-whichequals
107ergs--isofasatisfactorysize.
Theseconsiderations-theadvantagesofcoherenceandthefortuitouscircumstancethatamechanical
systcmbasedonthemetreandthekilogramhaspreciselythesameunitofenergyasisprovidedbythepractical
electricunits--ledG.Giorgiin1902toproposeasystembasedonthemetre,thekilogram,thesecond,and
oneofthepracticalelectricunits.Hepointedoutthatifmagneticfieldstrengthwereexpressedasamperes
permetreinsteadof4ntimesamperespermetre,whichisthedefinitioncorrespondingtothatoftheCGS
unit,thenumbernwoulddisappearfrommostelectricandmagneticformulaeinvolvingrectilineargeometry,
butwouldappear,asistobeexpected,inthoseinvolvingcylindersorspheres.
TheIntemationalElectrotechnicalCommissioneventuallychosetheampereasthefourthbaseunitof
theMKSAor'Giorgi'system,andin1948the9thGeneralConferenceofWeightsandMeasures
recommendeditforscienceandtechnology,aswellasforcommerceandindustry.Thissystemadmirably
TheGeneralConfcrenccofWeightsandMcasures(CGPM)istheauthoritysetupbytheMetreConventionof1875topromotcand
improvethemelricsystem,andtosecureinternationalunifonnityinmetricunitsandstandardsofmeasurement.Itconsistsofdclcgations
fromthenembernations(ofwhichtherewere46,includingtheUK,in1982),whichmeeteveryfewyears,the15th16thand17thConferences
hsvingbeenheldin1975,1979,and1979.TheInternationalBurcauofWeightsandMeasurcs(BIPM),Scvres(ncarPans)isthccentralofficc
andlaboraloryoftheorganization,andismanaged.undertheauthorityoftheGencralConference,bytheInternationalCommitteeof
WeightsandMeasures(CIPM)consis
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