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[459],,—ReadDecember8,.—INTRODUCTORY.(1),therefore,inreducingthesephenomenaintoscientificform,istoascertainthemagnitudeanddirectionoftheforceactingbetweenthebodies,andwhenitisfoundthatthisforcedependsinacertainwayupontherelativepositionofthebodiesandontheirelectricormagneticcondition,itseemsatfirstsightnaturaltoexplainthefactsbyassumingtheexistenceofsome-thingeitheratrestorinmotionineachbody,constitutingitselectricormagneticstate,,ofmagnetism,ofthemecha-nicalactionbetweenconductorscarryingcurrents,,,moreorlessexplicitly,theexistenceofsubstancestheparti-%*,,however,hehasfounditnecessarytoassumethattheforcebetweentwoelectricparticlesdependsontheirrelativevelocity,,,isexceedinglyingenious,andwonderfullycomprehensiveinitsapplicationtothephenomenaofstaticalelectricity,electromagneticattractions,inductionofcurrentsapddiamagneticphenomena;anditcomestouswiththemoreauthority,asithasservedtoguidethespeculationsofonewhohasmadesogreatanadvanceinthepracticalpartofelectricscience,bothbyintroducingaconsistentsystemofunitsinelectricalmeasurement,andbyactuallydeterminingelectricalquantitieswithanaccuracyhithertounknown.*,,andTaylor’sScientificMemoirs,..f"Explicaretentaturquomodofiatutlucisplanumpolarizationispervireselectricasvelmagneticasdecli-netur."—HalisSaxomim,
460PEOEESSOECLERKMAXWELLONTHEELECTROMAGNETICFIELD.(2)Themechanicaldifficulties,however,whichareinvolvedintheassumptionofparticlesactingatadistancewithforceswhichdependontheirvelocitiesaresuchastopreventmefromconsideringthistheoryasanultimateone,thoughitmayhavebeen,,bysupposingthemtobeproducedbyactionswhichgooninthesurroundingmediumaswellasintheexcitedbodies,andendeavouringtoexplaintheactionbetweendistantbodieswithoutassumingtheexistenceofforcescapableofactingdirectlyatsensibledistances.(3)ThetheoryIproposemaythereforebecalledatheoryoftheElectromagneticField),becauseithastodowiththespaceintheneighbourhoodoftheelectricormagneticbodies,anditmaybecalledaDynamicalTheory,becauseitassumesthatinthatspacethereismatterinmotion,bywhichtheobservedelectromagneticphenomenaareproduced.(4),orwemayendeavourtorenderitemptyofallgrossmatter,asinthecaseofGeisslbr’stubesandotherso-,however,enoughofmatterlefttoreceiveandtransmittheundulationsoflightandheat,anditisbecausethetransmissionoftheseradiationsisnotgreatlyalteredwhentransparentbodiesofmeasurabledensityaresubstitutedfortheso-calledvacuum,thatweareobligedtoadmitthattheundulationsarethoseofaneetherealsubstance,andnotofthegrossmatter,,fromthephenomenaoflightandheat,thatthereisansetherealmediumfillingspaceandpermeatingbodies,capableofbeingsetinmotionandoftransmittingthatmotionfromoneparttoanother,andofcom-municatingthatmotiontogrossmattersoastoheatitandaffectitinvariousways.(5)Nowtheenergycommunicatedtothebodyinheatingitmusthaveformerlyexistedinthemovingmedium,fortheundulationshadleftthesourceofheatsometimebeforetheyreachedthebody,*,thatthemediummusthaveadensitycapableofcomparisonwiththatofgrossmatter,andhasevenassignedaninfe-riorlimittothatdensity.(6)Wemaythereforereceive,asadatumderivedfromabranchofscienceinde-pendentofthatwithwhichwehavetodeal,theexistenceofapervadingmedium,ofsmallbutrealdensity,capableofbeingsetinmotion,andoftransmittingmotionfromoneparttoanotherwithgreat,butnotinfinite,*"OnthePossibleDensityoftheLuminiferousMedium,andontheMechanicalYalueofaCubicMileofSunlight/’TransactionsoftheRoyalSocietyofEdinburgh(1854),.
;andatthesametimetheseconnexionsmustbecapableofacertainkindofelasticyielding,sincethecommunicationofmotionisnotinstantaneous,,namely,the"actual"energydependingonthemotionsofitsparts,and"potential"energy,consistingoftheworkwhichthemediumwilldoinrecoveringfromdisplace-mentinvirtueofitselasticity..Thepropagationofundulationsconsistsinthecontinualtransformationofoneoftheseformsofenergyintotheotheralternately,andatanyinstanttheamountofenergyinthewholemediumisequallydivided,sothathalfisenergyofmotion,andhalfiselasticresilience.(7)Amediumhavingsuchaconstitutionmaybecapableofotherkindsofmotionanddisplacementthanthosewhichproducethephenomenaoflightandheat,andsomeofthesemaybeofsuchakindthattheymaybeevidencedtooursensesbythepheno-menatheyproduce.(8)Nowweknowthattheluminiferousmediumisincertaincasesactedonbymagnetism;forFaradayfdiscoveredthatwhenaplanepolarizedraytraversesatrans-parentdiamagneticmediuminthedirectionofthelinesofmagneticforceproducedbymagnetsorcurrentsintheneighbourhood,,issufficienttoaccountforthephenomena,butthatwemustadmittheexistenceofamotioninthemediumdependingonthemagnetization,;butthepropertiesofthemagneticfieldarenotsomuchalteredbythesubstitutionofonemediumforanother,orforavacuum,,andwehavesomereasontosupposethatthismotionisoneofrotation,havingthedirectionofthemagneticforceasitsaxis.(9)Wemaynowconsideranotherphenomenonobservedintheelectromagnetic*ExperimentalBesearches,(1856,secondhalfyear,,and1857,firsthalfyear,).%ProceedingsoftheBoyalSociety,
;thetwoextremitiesofthebodytendtobecomeoppo-sitelyelectrified,,andismadetoactoncertaincompoundbodies,itdecomposesthem,andcausesoneoftheircomponentstopasstowardsoneextremityofthebody,-ance;electrifyingtheextremitiesofabodyinoppositeways,aconditionwhichissustainedonlybytheactionoftheelectromotiveforce,andwhich,assoonasthatforceisremoved,tends,withanequalandoppositeforce,toproduceacountercurrentthroughthebodyandtorestoretheoriginalelectricalstateofthebody;andfinally,ifstrongenough,tearingtopieceschemicalcompoundsandcarryingtheircomponentsinoppo-sitedirections,whiletheirnaturaltendencyistocombine,,then,isaforceactingonabodycausedbyitsmotionthroughtheelectro-magneticfield,orbychangesoccurringinthatfielditself;andtheeffectoftheforceiseithertoproduceacurrentandheatthebody,ortodecomposethebody,or,whenitcandoneither,toputthebodyinastateofelectricpolarization,—astateofconstraintinwhichoppositeextremitiesareoppositelyelectrified,andfromwhichthebodytendstorelieveitselfassoonasthedisturbingforceisremoved.(10)AccordingtothetheorywhichIproposetoexplain,this"electromotiveforce"istheforcecalledintoplayduringthecommunicationofmotionfromonepartofthemediumtoanother,,itpro-ducesacurrent,w7hich,asitmeetswithresistance,occasionsacontinualtransformationofelectricalenergyintoheat,whichisincapableofbeingrestoredagaintotheformofelectricalenergybyanyreversaloftheprocess.(11)Butwhenelectromotiveforceactsonadielectricitproducesastateofpolari-zationofitspartssimilarindistributiontothepolarityofthepartsofamassofironundertheinfluenceofamagnet,andlikethemagneticpolarization,capableofbeingdescribedasastateinwhicheveryparticlehasitsoppositepolesinoppositecon-ditions*.Inadielectricundertheactionofelectromotiveforce,wemayconceivethattheelectricityineachmoleculeissodisplacedthatonesideisrenderedpositivelyandtheothernegativelyelectrical,butthattheelectricityremainsentirelyconnectedwiththemolecule,-,becausewhenithasattainedtoacertainvalueitremainsconstant,butitisthecommencementofacurrent,anditsvariationsconstitutecurrentsinthepositiveorthenegativedirectionaccording*Faraday,.;Mossom,(Modena),.
,becausetheelectrificationofthesurfaceofanymoleculeisneutralizedbytheoppositeelectrificationofthesurfaceofthemoleculesincontactwithit;butattheboundingsurfaceofthedielectric,wheretheelectrificationisnotneutralized,,thesameelectromotiveforcepro-ducinggenerallyagreaterelectricdisplacementinsoliddielectrics,suchasglassorsulphur,thaninair.(12)Here,then,weperceiveanothereffectofelectromotiveforce,namely,electricdisplacement,whichaccordingtoourtheoryisakindofelasticyieldingtotheactionoftheforce,similartothatwhichtakesplaceinstructuresandmachinesowingtothewantofperfectrigidityoftheconnexions.(13),which,thoughinmanycasesexceedinglysmall,*,invirtueofwhich,whenthedielectricisexposedtoelectromotiveforce,theelectricdisplacementgraduallyincreases,andwhentheelectromotiveforceisremoved,thedielectricdoesnotinstantlyreturntoitsprimitivestate,butonlydischargesaportionofitselectrification,andwhenlefttoitselfgraduallyacquireselectrificationonitssurface,,whichgivesrisetotheresidualchargeintheLeydenjar,andtoseveral