物理学史上最美的十个实验第六名Cavendish'storsion-barexperiment卡文迪许扭矩实验AnotherofNewton'scontributionswashistheoryofgravity,whichholdsthatthestrengthofattractionbetweentwoobjectsincreaseswiththesquareoftheirmassesanddecreaseswiththesquareofthedistancebetweenthem.Buthowstrongisgravityinthefirstplace?Inthelate1700'sanEnglishscientist,HenryCavendish,decidedtofindout.Hetookasix-footwoodenrodandattachedsmallmetalspherestoeachend,likeadumbbell,thensuspendeditfromawire.Two350-poundleadspheresplacednearbyexertedjustenoughgravitationalforcetotugatthesmallerballs,causingthedumbbelltomoveandthewiretotwist.Bymountingfinelyetchedpiecesofivoryontheendofeacharmandinthesidesofthecase,hecouldmeasurethesubtledisplacement.Toguardagainsttheinfluenceofaircurrents,theapparatus(calledatorsionbalance)wasenclosedinaroomandobservedwithtelescopesmountedoneachside.Theresultwasaremarkablyaccurateestimateofaparametercalledthegravitationalconstant,andfromthatCavendishwasabletocalculatethedensityandmassoftheearth.Cavendishhadweighedit:6.0×1024kilograms,orabout13trilliontrillionpounds.牛顿的另一伟大贡献是他的万有引力定律，但是万有引力到底多大？18世纪末，英国科学家亨利·卡文迪许决定要找出这个引力。他将两边系有小金属球的6英尺木棒用金属线悬吊起来，这个木棒就像哑铃一样。再将两个350磅重的铅球放在相当近的地方，以产生足够的引力让哑铃转动，并扭转金属线。然后用自制的仪器测量出微小的转动。测量结果惊人的准确，他测出了万有引力恒量的参数，在此基础上卡文迪许计算地球的密度和质量。卡文迪许的计算结果是：地球重6.0×1024公斤，或者说13万亿万亿磅。
