Followingthisheundertookaninvestigationoncrystalsincollaborationwithhisbrother,whohadpassedhislicentiateandwaspreparatorforFriedelinthelaboratoryofmineralogyattheSorbonne.Theirexperimentsledthetwoyoungphysiciststoagreatsuccess:thediscoveryofthehithertounknownphenomenaofpiezo-electricity,whichconsistsofanelectricpolarizationproducedbythecompressionortheexpansionofcrystalsinthedirectionoftheaxisofsymmetry.Thiswasbynomeansachancediscovery.Itwastheresultofmuchreflectiononthesymmetryofcrystallinematter,whichenabledthebrotherstoforeseethepossibilitiesofsuchpolarization.ThefirstpartoftheinvestigationwasmadeinFriedel'slaboratory.Withanexperimentalskillrareattheirage,theyoungmensucceededinmakingacompletestudyofthenewphenomenon,establishedtheconditionsofsymmetrynecessarytoitsproductionincrystals,andstateditsremarkablysimplequantitativelaws,aswellasitsabsolutemagnitudeforcertaincrystals.Severalwell-knownscientistsofothernations(Roentgen,Kundt,Voigt,Riecke)havemadefurtherinvestigationsalongthisnewroadopenedbyJacquesandPierreCurie.
Thesecondpartofthework,andmuchmoredifficulttorealizeexperimentally,concernedthecompressionresultinginpiezo-electriccrystalswhentheyareexposedtotheactionofanelectricfield.Thisphenomenon,foreseenbyLippmann,wasdemonstratedbytheCuriebrothers.Thedifficultyoftheexperimentlayintheminutenessofthedeformationsthathadtobeobserved.FortunatelyDesainsandMoutonplacedasmallroomadjoiningthephysicslaboratoryatthedisposalofthebrotherssothattheymightproceedsuccessfullywiththeirdelicateoperations.
Fromtheseresearches,asmuchtheoreticalasexperimental,theyimmediatelydeducedapracticalapplication,intheformofanewapparatus,apiezo-electricquartzelectrometer,whichmeasuresinabsolutetermssmallquantitiesofelectricity,aswellaselectriccurrentsoflowintensity.Thisapparatushassincethenrenderedgreatserviceinexperimentsinradioactivity.
Duringthecourseoftheirexperimentsonpiezo-electricitytheCurieswereobligedtoemployelectrometricapparatus,and,notbeingabletousethequadrantelectrometerknownatthattime,theydevelopedanewformofthatinstrument,betteradaptedtotheirnecessities.ThisbecameknowninFranceastheCurieelectrometer.Thustheseyearsofcollaborationbetweenthetwobrothers,alwaysintimatelyunited,provedbothhappyandfruitful.Theirdevotionandtheircommoninterestinscienceweretothembothastimulantandasupport.DuringtheirworkthevivacityandenergyofJacqueswereofpreciousaidtoPierre,alwaysmoreeasilyabsorbedbyhisthoughts.
However,thisbeautifulandclosecollaborationlastedonlyafewyears.In,PierreandJacqueswereobligedtoseparate;JacquesleftfortheUniversityofMontpelierasHeadLecturerinMineralogy(Ma?tredeConferences).PierrewasmadeDirectorofLaboratoryWorkintheSchoolofIndustrialPhysicsandChemistryfoundedbythecityofParisatthesuggestionofFriedelandofSchutzenberger,whobecameitsfirstdirector.
Theirremarkableresearcheswithcrystalswonforthebrothersin——verylate,itistrue——thePlanteprize.