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MeasurementofPhysicalQuantity Thephysicalquantitieslistedbellowareoftenusedintestandmeasurementtechnology Thefirst2arecommonlycalledaselectricalquantitiesandtheotherarecommonlycalledaselectricalparameters Usuallythesensorsreceivetheinformationofquantitiesundermeasurement QUM andconvertitintovariationofelectricalparametersorelectricalpotential whichwillnormallybeconditioned andthenconvertedintotheelectricalquantities speciallythecurrent andsenttotheA Dconverters ElectricalpotentialdifferenceTheprimarystandardforelectricalpotentialdifferenceusedtobeprovidedbyanelectrochemicalstandardcell theWestonstandardcell ThevoltageofaWestoncellisapproximately1 01860Vat20 C withaninaccuracyof3X10 6underoptimalconditions Optimalconditionsmeansavariationoftemperaturelessthan10 3K noload novibrationsorjolting Thecellmustremaininanuprightposition AWestonstandardcellhasaverylongthermalafter effect Afterthecellhasbeenheatedto30 Citcantake6monthsbeforeitistotallystablewithin0 3uVoftheoriginalvalueat20degrees Furthermore aWestoncellwillage resultinginanincreaseintheinternalresistance R1 500 1000欧 andasmalldecreaseofafewuVintheoutputvoltage duringthefirstyears ElectricalcurrentElectricalcurrentisstandardizedbymeasurementwithaninstrumentcalleda currentbalance Thisdevicemeasurestheelectromagneticforcebetweentwocurrentcarryingcoils onefixed onemoving bybalancingitwiththeforceofgravity actingonaknownmass TheforcebetweenthecoilsisgivenbyF I 2dM dx inwhichMistheknownmutualinductionofthecoilsandxistheknowndistancebetweenthem ThedifferentialquotientdM dxisdeterminedbyknowngeometryofthecoils ElectricalresistanceStandardsofelectricalresistanceareresistorswoundofspecialalloywiregivingaminimaltemperatureeffect Anexampleofsuchanalloyisevanohm consistingof74 nickel 20 chromiumand6 aluminiumandiron Thisalloyisfrequentlyusedforhighresistancestandards 10kQ Forlowresistancevalues 1Q manganine 86 Cu 12 Mn 2 Ni orconstantan 54 Cu 45 Ni 1 Mn isoftenused Higher orderresistancestandardsarekeptatveryaccuratelystabilisedtemperaturebythermostats CapacitanceItispossibletoconstructacapacitorfromfourcoaxialcylinders whosecapacitanceonlydependsonthelengthofthecylinders Suchacapacitorisparticularlysuitableasastandardofcapacitance sinceonlythelengthhastobedeterminedaccurately Withtheaidofopticalinterferometrythiscanbedoneextremelyaccurately Theseso calledThompson Lampardcylindercapacitorscanachieveaninaccuracyoflessthan10 8 Adisadvantage however isthefactthatthecapacitanceissmall approximately1 9pFpermetre Forlower orderstandardsotherconfigurationsofelectrodesareused whichprovidelargercapacitancevalues 10 1000pF butalsocomelargeruncertainties InductanceAccuratestandardsofinductancearedifficulttorealise Thisiscausedbythemanyparametersthatdeterminetherelativelycomplexofacoil allofwhichinfluencetheaccuracyoftheinductor Furthermore powerlossesoccur duetowireresistance proximatelyeffectsandeddycurrents whichaddtoinaccuracy Currentlyaavailablestandardsofinductancehaveaninaccuracyofabout10 5 FrequencyThestandardoffrequencyisbasedonquantummechanicaleffectthatelectronsinanatomcanonlyoccupyoneofalimitednumberofenergylevels Ifanelectronjumpstoahigheroralowerenergylevel thedifferenceinenergyEofthephotonthatisabsorbedoraemitted respectively isrelatedtothefrequencyofthephotonbytheexpressionE hf0 Whenatomsareirradiatedwithelectromagneticenergyoffrequencyf0 manyelectronswillpasstohigherenergylevels 2Measurementofnon physicalquantities Non physicalquantitiesfoundinnon technicalfieldsareverydifficultorevenimpossibletomeasure mainlybecausetheyarepartofverycomplexsystems people organizations societies etc Cardinalmeasurementscanthereforeveryrarelybemadewhendealingwiththenon physicalcharacteristics statesorphenomenainsuchsystems Whenmeasuringcomplexsystemsorobjects themeasurandoftenalsodependsonallkindsofotherquantities Usually thenatureandmagnitudeofthesedependenciesareunknown Furthermore theseparasiticquantities whichinfluencethemeasurementresult arenotconstantduringthemeasurement makingitimpossibletocorrectanyerrorsinthemeasurement Thismeansthatthemeasurementisnolongerselective otherfactorsarealsobeingmeasured Non physicalquantitiesareusuallyapartofalivingorganismoranorganizationoflivingbeings Itisessentialtothenatureofanorganismoranorganizationthattheymaintain social cultural political interactionswiththeirenvironment Forthisreason itisusuallynotpossibletoperformanisolatedmeasurementinthesamewaywecanwithinanimatethings puttinginathermostat hookinguptoafixedsupplyvoltage etc Itis forexample notverypracticabletoisolateasectionofthepopulationfromtherestoftheworld forthesakeofaneconomicmeasurementconductedtoverifytherelationshipbetweenthescarcityandthepriceofconsumergoods Theobjectofthemeasurementcontinuestointeractwithitssurroundings toanextentandinamannerthatremainsunknown Theseinteractionsobscureandcorruptthemeasurementresultstosomeunknowndegree dependingonthesensitivitytotheseinterference Also therepetitionofexperimentisoftennotpossiblewhendealingwithsuchcomplexmeasurementobjects Withphysicalmeasurementsweoftenrepeatameasurementtoseehowreproducibleitisandtodeterminethemagnitudeofrandomerrors Withnon physicalmeasurements however theobjectoftenbehavesdifferentlythesecondtime becauseitnolongermotivatedtoco operate hasbecometired etc Think forexample oftakingthesameexaminationtwice Forethical politicalorfinancialreasonsitisalsousuallynotpossibletofreelymodifyvariablestodeterminehowtheyinfluencethemeasurementobject forexample increasingthescarcityoffoodinaneconomiccommunitytodeterminetheinfluenceontheconsumer sspendingbehavior Asopposedtoaphysicalmeasurement theobjectofanon physicalmeasurementisoftenconsciousofbeingmeasured Themerefactthatwemeasureinfluencesthesubject Thesubjectwillbehavedifferentlyinalaboratorythanindailylife Anexampleforanoftenused andabused testanimal therat Ameasurementcanevencauseirrevocableprocessesintheobject Certainpsychologicalexperiments forexample cancausecatatonicstatesincertainpatients resultingindisordersorevenchronicschizophrenia Theeffectsmeasurementabovewillgenerallymakeitdifferenttoreproducemeasurementsinthefieldsofhumanandsocialstudiesaccurately assumingthatthesemeasurementscanactuallybemadereasonablyselectivelyandobjectively 3 TheNatureofData Informationyougatheriscalleddata Datacanbeafactualstatementofphysicalphenomena Forexample thestatement thecopperwasremovedbythechemicalreactionwithferricchloride isdescriptivedata Whendataispurelydescriptive itissaidtobequalitativedata Whenaquantityismeasured weassociatenumericalvalueswithit andtheinformationismoreusefulinascientificwaybecausemoreinformationispresent Informationaboutthemagnitudeorintensityofaphysicalphenomenoniscalledquantitativedata Recallthatthequantitythatisbeingmeasurediscalledthemeasured Instrumentationextendsthehumansensesbyallowinganumericalvalueorvaluestobeassociatedwiththemeasurand Numericaldatacanbecategorizedinseveralways Itcanbeanisolatedvalueorcanbedependentontimeorlocation Valuesrecordeddirectlyfromanexperimentorobservationarecalledempiricaldata Priortoprocessing empiricaldataisoftenreferredtoasraw orunprocesseddata whereasdatathathasbeenanalyzediscalledprocesseddata Datacanalsobegeneratedbytheoretic
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