作者	G.W.C.Kaye/等
出版社
出版时间	2003-01-01

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片断：1.1.1Theinternationalsystemofunits(Sl)HistoryInthesecondhalfofthenineteenthcenturythecentimetre,gramandsecondwereinfairlygeneraluseasbaseunitsforscientificworkeveninsuchcountriesastheUKandtheUSAwherethefootandthepoundwereemployedforcommerceandengineering.Asaresult,theunitsrequiredbytherapidlyemergingscienceofelectricitywerebasedonthecentimetre,gramandsecond,withwhichtheyformedacoherentsystemknownastheCGSelectromagneticsystem.Asystemofunitsissaidtobecoherentwhenderivedunitsareformedfromthebaseunitswithouttheinsertionoffactorsofproportionalityotherthanunity.TherewasalsotheCGSelectrostaticsystem,buttheonlyquantitiesfrequentlyexpressedinelectrostaticunitswereclectriccharge,electricpotential,andcapacitance.Theyoungbutfast-growingelectricalindustrysoonfoundthatmanyCGSelectromagneticunitswereofanextremelyinconvenientsizeforitsneeds.Accordingly,in1881,internationalagreementwasreachedtoflxthepracticalunitofpotential,tobecalledthevolt,atlO8CGSunits(whichisapproximatelyequaltothee.m.f.ofaprimarycell),andtheunitofresistance,theohm,at109CGSunits(whichisapproximatelytheresistanceofacolumnofmercury1mlongand1mm2incross-section).Theunitofelectriccurrent,theampere,wasmadeatenthoftheCGSunit.Acoherentsystemofpracticalelectricunitswasthussecuredwhich,however,wasnotcoherentwiththemechanicalunitsbasedonthecentimetreandgram.Thepracticalelectricunitssuitedtheneedsoftelegraphy,whichwasthenthemainelectricalindustry,andtheyalsohappentobeconvenientforheavyelectricalengineeringandforelectronics.Themagneticunits,however,wereleftattheirCGSvalues,presumablybecausetheCGSunitofmagneticfluxdensity,subsequentlycalled'gauss',isoftheorderofthefluxdensityoftheEarthsfield,and,asitwassuitableforgeomagnetism,thereseemednopointinchangingitforaunit104timeslarger.Coherencewastherebylosttoelectromagnetismasithadalreadybeenlosttothesystemembracingthemechanicalunitsandthepracticalelectricunits.Whereastheelectricunits,bytheagreementof1881,werechosentobeofsuitablemagnitudeforeverydayuse,andwhereasthecentimetreandthesecondhaveacceptablesizes,thegramistoosmallforthepracticalnccdsofman,whicharebetterservedbyaunitnearerthesizeofthepoundorthekilogram.Moreover,theCGSunitofforce,thedyne,andtheunitofenergy,theerg,aremuchtoosmall.Ontheotherhand,theunitofenergyprovidedbythepracticalelectricunits,thevolt-ampere-second,calledthejoule-whichequals107ergs--isofasatisfactorysize.Theseconsiderations-theadvantagesofcoherenceandthefortuitouscircumstancethatamechanicalsystcmbasedonthemetreandthekilogramhaspreciselythesameunitofenergyasisprovidedbythepracticalelectricunits--ledG.Giorgiin1902toproposeasystembasedonthemetre,thekilogram,thesecond,andoneofthepracticalelectricunits.Hepointedoutthatifmagneticfieldstrengthwereexpressedasamperespermetreinsteadof4ntimesamperespermetre,whichisthedefinitioncorrespondingtothatoftheCGSunit,thenumbernwoulddisappearfrommostelectricandmagneticformulaeinvolvingrectilineargeometry,butwouldappear,asistobeexpected,inthoseinvolvingcylindersorspheres.TheIntemationalElectrotechnicalCommissioneventuallychosetheampereasthefourthbaseunitoftheMKSAor'Giorgi'system,andin1948the9thGeneralConferenceofWeightsandMeasures recommendeditforscienceandtechnology,aswellasforcommerceandindustry.ThissystemadmirablyTheGeneralConfcrenccofWeightsandMcasures(CGPM)istheauthoritysetupbytheMetreConventionof1875topromotcandimprovethemelricsystem,andtosecureinternationalunifonnityinmetricunitsandstandardsofmeasurement.Itconsistsofdclcgationsfromthenembernations(ofwhichtherewere46,includingtheUK,in1982),whichmeeteveryfewyears,the15th16thand17thConferenceshsvingbeenheldin1975,1979,and1979.TheInternationalBurcauofWeightsandMeasurcs(BIPM),Scvres(ncarPans)isthccentralofficcandlaboraloryoftheorganization,andismanaged.undertheauthorityoftheGencralConference,bytheInternationalCommitteeofWeightsandMeasures(CIPM)consis