Oil drop experimentNot to be confused with Pitch drop experiment.The oil drop experiment was an experiment performedMillikans setup for the oil drop experimentby Robert A. Millikan and Harvey Fletcher in 1909 tomeasure the elementary electric charge (the charge of theelectron).The experiment entailed observing tiny charged dropletsof oil between two horizontal metal electrodes. First,with zero applied electric eld, the terminal velocity ofa droplet was measured. At terminal velocity, the dragforce equals the gravitational force, and these depend onthe radius in dierent ways, so that the radius of thedroplet, and therefore the mass and gravitational force,could be determined (using the known density of the oil).Then an adjustable voltage was applied between the platesto induce an electric eld, and the voltage was adjusteduntil the drops were suspended in mechanical equilib-rium, indicating that the electrical force and the gravita-tional force were balanced. Nowusing the known electriceld, Millikan and Fletcher could determine the chargeon the oil droplet. By repeating the experiment for manydroplets, they conrmed that the charges were all smallinteger multiples of a certain base value, which was foundto be 1.5924(17)1019 C, within 1% of the currentlyaccepted value of 1.602176487(40)1019 C. They pro-posed that this was the (negative of the) charge of a singleelectron.1 BackgroundStartingin1908, whileaprofessorat theUniversityof Chicago, Millikan, with the signicant input ofFletcher,[1] and after improving his setup, published hisRobert A. Millikan in 1891seminal study in 1913.[2] This remains controversial sincepapers found after Fletchers death describe events inwhich Millikan coerced Fletcher into relinquishing au-thorship as a condition for receiving his PhD.[3][4] In re-turn, Millikan used his inuence in support of Fletcherscareer at Bell Labs.MillikanandFletchers experiment involvedmeasur-ing the force on oil droplets in a glass chamber sand-wiched between two electrodes, one above and one be-low. With the electrical eld calculated, he could measurethe droplets charge, the charge on a single electron being(1.5921019 C). At the time of Millikan and Fletchersoil drop experiments, the existence of subatomic parti-cles was not universally accepted. Experimenting withcathode rays in 1897, J. J. Thomson had discovered neg-atively charged "corpuscles", as he called them, with amass about 1840 times smaller than that of a hydrogenatom. Similar results had been found by George FitzGer-ald and Walter Kaufmann. Most of what was then knownabout electricity and magnetism, however, could be ex-plained on the basis that charge is a continuous variable;in much the same way that many of the properties of lightcan be explained by treating it as a continuous wave rather12 2 EXPERIMENTAL PROCEDUREthan as a stream of photons.Theelementarycharge e is oneof thefundamentalphysical constants and its accurate value is of great impor-tance. In 1923, Millikan won the Nobel Prize in physics,in part because of this experiment.Aside from the measurement, the beauty of the oil dropexperiment is that it is a simple, elegant hands-on demon-stration that charge is actually quantized. Thomas Edi-son, who had previously thought of charge as a continu-ous variable, became convinced after working with Mil-likan and Fletchers apparatus.[5]This experiment hassince been repeated by generations of physics students,although it is rather expensive and dicult to do prop-erly.In the last two decades, several computer-automated ex-periments have been conducted to search for isolatedfractionally charged particles. So far (2007), no evidencefor fractional charge particles was found after measuringover 100 million drops.[6]2 Experimental procedure2.1 Apparatusuniform electric eldmicroscopecoveroilsprayseveralthousandvoltsdSimplied scheme of Millikans oil drop experimentOil drop experiment apparatusMillikans and Fletchers apparatus incorporated a paral-lel pair of horizontal metal plates. By applying a potentialdierence across the plates, a uniform electric eld wascreated in the space between them.A ring of insulatingmaterial was used to hold the plates apart. Four holeswere cut into the ring, three for illumination by a brightlight, and another to allow viewing through a microscope.A ne mist of oil droplets was sprayed into a chamberabove the plates. The oil was of a type usually used invacuum apparatus and was chosen because it had an ex-tremely low vapour pressure. Ordinary oil would evapo-rate under the heat of the light source causing the mass ofthe oil drop to change over the course of the experiment.Some oil drops became electrically charged through fric-tion with the nozzle as they were sprayed. Alternatively,charging could be brought about by including an ionisingradiation source (such as an X-ray tube). The dropletsentered the space between the plates and, because theywere charged, could be made to rise and fall by changingthe voltage across the plates.2.2 MethodInitially the oil drops are allowed to fall between the plateswith the electric eld turned o. They very quickly reacha terminal velocity because of friction with the air in thechamber. The eld is then turned on and, if it is largeenough, some of the drops (the charged ones) will startto rise. (This is because the upwards electric force FE isgreater for them than the downwards gravitational forceF, in the same way bits of paper can be picked by acharged rubber rod). A likely looking drop is selectedand kept in the middle of the eld of view by alternatelyswitching o the voltage until all the other drops havefallen. The experiment is then continued with this onedrop.The drop is allowed to fall and its terminal velocity v1 inthe absence of an electric eld is calculated. The dragforce acting on the drop can then be worked out usingStokes law:Fd= 6rv1where v1 is the terminal velocity (i.e. velocity in the ab-sence of an electric eld) of the falling drop, is the3viscosity of the air, and r is the radius of the drop.The weight w is the volume D multiplied by the density and the acceleration due to gravity g. However, what isneeded is the apparent weight. The apparent weight in airis the true weight minus the upthrust (which equals theweight of air displaced by the oil drop). For a perfectlyspherical droplet the apparent weight can be written as:w =43r3( air)gAt terminal velocitytheoil dropisnot accelerating.Therefore the total force acting on it must be zero andthe two forces F and w must cancel one another out (thatis, F = w). This impliesr2=9v12g( air).Once r is calculated, w can easily be worked out.Now the eld is turned back on, and the electric force onthe drop isFE= qEwhere q is the charge on the oil drop and E is the electriceld between the plates. For parallel platesE=Vdwhere V is the potential dierence and d is the distancebetween the plates.One conceivable way to work out q would be to adjust Vuntil the oil drop remained steady. Then we could equateFE with w. Also, determining FE proves dicult becausethe mass of the oil drop is dicult to determine withoutreverting to the use of Stokes Law. A more practicalapproach is to turn V up slightly so that the oil drop riseswith a new terminal velocity v2. ThenqE w = 6(r v2) =

v2v1

w3 Fraud allegationsThere is some controversy raised by the historian GeraldHolton over the use of selectivity in the results of Mil-likans second experiment measuring the electron charge.Holton (1978) pointed out that Millikan disregarded thedata from a large set of the oil drops in his experimentswithout apparent reason. Allan Franklin, a former highenergy experimentalist and philosopher of science at theUniversity of Colorado has tried to rebut this point byHolton.[7] Franklin contends that Millikans exclusions ofdata did not aect his nal value of e but concedes thatthere was substantial cosmetic surgery that Millikanperformed which had the eect of reducing the statisticalerror on e. This enabled Millikan to claimthat he had cal-culated e to better than one half of one percent; in fact,if Millikan had included all of the data he threw out, itwould have been to within 2%. While this would still haveresulted in Millikan having measured e better than any-one else at the time, the slightly larger uncertainty mighthave allowed more disagreement with his results withinthe physics community. David Goodstein counters thatMillikan plainly states that he only included drops whichhad undergone a complete series of observations andexcluded no drops from this group.[8]4 Millikansexperimentasanex-ample of psychological eects inscientic methodologySee also: Conrmation bias In scienceIn a commencement address given at the California In-stitute of Technology (Caltech) in 1974 (and reprinted inSurely You're Joking, Mr. Feynman! in 1985 as well asin The Pleasure of Finding Things Out in 1999), physicistRichard Feynman noted:Wehavelearnedalot fromexperienceabout how to handle some of the ways we foolourselves. One example: Millikan measuredthe charge on an electron by an experimentwith falling oil drops, and got an answer whichwe now know not to be quite right. Its a littlebit o because he had the incorrect value forthe viscosity of air. Its interesting to look atthe history of measurements of the charge ofan electron, after Millikan. If you plot them asa function of time, you nd that one is a littlebit bigger than Millikans, and the next ones alittle bit bigger than that, and the next ones alittle bit bigger than that, until nally they settledown to a number which is higher.Why didn't they discover the new numberwas higher right away? Its a thing that sci-entists are ashamed ofthis historybecauseits apparent that people did things like this:When they got a number that was too highabove Millikans, they thought something mustbe wrongand they would look for and nd areason why something might be wrong. Whenthey got a number close to Millikans value they4 7 EXTERNAL LINKSdidn't look so hard. And so they eliminated thenumbers that were too far o, and did otherthings like that...[9][10]As of 2014, the accepted value for the elemen-tary charge is 1.602176565(35)1019 C,[11] where the(35) indicates the uncertainty of the last twodeci-mal places. InhisNobel lecture, Millikangavehismeasurement as 4.774(5)1010 statC,[12] which equals1.5924(17)1019 C. The dierence is less than one per-cent, but it is more than ve times greater than Millikansstandard error, so the disagreement is signicant.5 References[1] Elektrizittsmengen. Phys. Zeit. 10: 308. 1910.[2] Millikan, R. A. (1913). OntheElementaryElec-tric charge and the Avogadro Constant. Phys.Rev. 2 (2): 109143. Bibcode:1913PhRv....2..109M.doi:10.1103/PhysRev.2.109.[3] Perry, Michael F. (May 2007). Remembering The OilDrop Experiment. Physics Today 60 (5): 56.[4] Fletcher, Harvey (June 1982). My Work with Millikanon the Oil-drop Experiment. Physics Today 43.[5] Bandrawal, Praveen Kumar (11March 2009). NobelAwards Winner Physics. Pinnacle Technology. pp. 169. ISBN 978-1-61820-254-3. Retrieved 14 December2012.[6] SLAC Fractional Charge Search Results.StanfordLinear Accelerator Center. January 2007. Retrieved 8April 2015.[7] Franklin, A. (1997). Millikans Oil-Drop Exper-iments. The Chemical Educator 2 (1): 114.doi:10.1007/s00897970102a.[8] Goodstein, D. (2000). In defense of Robert AndrewsMillikan. Engineering and Science (Pasadena, California:Caltech Oce of Public Relations) 63 (4): 3038.[9] Feynman, Richard, Cargo Cult Science (adapted from1974 California Institute of Technology commencementaddress),Donald Simaneks Pages, Lock Haven Univer-sity, rev. August 2008.[10] Feynman, Richard Phillips; Leighton, Ralph; Hutchings,Edward (1997-04-01). Surely you're joking, Mr. Feyn-man!": adventuresofacuriouscharacter. New York:W. W. Norton & Company. p. 342. ISBN 978-0-393-31604-9. Retrieved 10 July 2010.[11] elementary change e. NISTReferenceonConstants,Units and Uncertainty. National Institute of Standards andTechnology. Retrieved 8 April 2015.[12] Millikan, Robert A. (May 23, 1924). The electron and thelight-quant from the experimental point of view (Speech).Stockholm. Retrieved 2006-11-12.6 Further readingSerway, RaymondA.; Faughn, JerryS. (2006).Holt: Physics. Holt, Rinehart and Winston. ISBN0-03-073548-3.Thornton, Stephen T.; Rex, Andrew (2006). Mod-ernPhysicsforScientistsandEngineers(3rded.).Brooks/Cole. ISBN 0-495-12514-8.Serway, RaymondA.; Jewett, JohnW. (2004).Physics for Scientists and Engineers (6th ed.).Brooks/Cole. ISBN 0-534-40842-7.7 External linksSimulationof theoil dropexperiment (requiresJavaScript)Thomsen, Marshall, "Good to the Last Drop". Mil-likan Stories as Canned Pedagogy. Eastern Michi-gan University.CSR/TSGC Team, "Quark search experiment". TheUniversity of Texas at Austin.The oil drop experiment appears in a list of Sciences10 Most Beautiful Experiments originally publishedin the New York Times.Engeness, T.E., "The MillikanOil DropExperi-ment". 25 April 2005.Millikan R. A. (1913). On the elemen-tary electrical charge and the Avogadro con-stant (PDF). Physical Review. Series II2: 109143. Bibcode:1913PhRv....2..109M.doi:10.1103/PhysRev.2.109., Paper by Millikandiscussing modications to his original experimentto improve its accuracy.Hudspeth,P. and Klingler,R.,A search for freequarks in the micro gravity environment of the In-ternational Space Station , AIP Conf. Proc. 504, 715(2000). Avariation of this experiment has been sug-gested for the International Space Station.Perry, M. F., "Remembering the oil-drop experi-ment", Physics Today, May 2007, 56-60.58 Text and image sources, contributors, and licenses8.1 Text Oil drop experiment Source: https://en.wikipedia.org/wiki/Oil_drop_experiment?oldid=669990877 Contributors: Heron, Michael Hardy,Dominus, Theresa knott, Jll, Mulad, Charles Matthews, Reddi, Marshman, Raul654, Mjmcb1, Boy b, Auric, Cutler, DocWatson42,Awolf002, Pretzelpaws, Tom harrison, Alison, Andycjp, LucasVB, GNU, Nicobn~enwiki, Discospinster, Rich Farmbrough, Pavel Voze-nilek, MisterSheik, Kaszeta, Pt, Shanes, Kghose, Redit, 3mta3, Hooperbloob, I-hunter, Alansohn, Yhr, Keenan Pepper, Batmanand,Dave.Dunford, Gene Nygaard, Drbreznjev, Popefelix, Angr, Linas, Camw, Stolee, StradivariusTV, Spiritllama, Christianjb, Grammar-bot, Nanite, Robinspw, Chobot, Jaraalbe, The Rambling Man, YurikBot, Sceptre, Phantomsteve, JabberWok, Gravecat, Gaius Cornelius,The Hokkaido Crow, WhiteInk, Ragesoss, Syrthiss, Elkman, Kkmurray, Tevildo, Mejor Los Indios, Itub, Tttrung, SmackBot, Inverse-Hypercube, KnowledgeOfSelf, Bluebot, Sixsous~enwiki, Thumperward, PrimeHunter, DHN-bot~enwiki, Worthawholebean, Fotoguzzi,OrphanBot, Electrolite, Cybercobra, Daniel.Cardenas, Xandi, Waggers, JdH, Levineps, Villahj Ideeut, Thedemonhog, Skoch3, Skittleys,Thijs!bot, Headbomb, D.H, OrenBochman, Mumby, Escarbot, LachlanA, AntiVandalBot, Twbeals31, MECU, Rico402, Lorenj, Husond,Seddon, Mkdw, CommonsDelinker, J.delanoy, Phelonius Friar, MrBell, Jerry, FMCTandP, Am00nz0r5, TottyBot, KylieTastic, Jkeohane,DavidCBryant, Ashcroftgm, Hugo999, Wrev, Fences and windows, Philip Trueman, TXiKiBoT, GcSwRhIc, Nickipedia 008, Astrogrikor,Guthrie509, AlleborgoBot, SieBot, Ireas, Pinkadelica, ClueBot, WurmWoode, The Thing That Should Not Be, Kendo70133, Puchiko,Bchcky77, NuclearWarfare, Eebster the Great, Ginbot86, Chhe, Kwjbot, Asdf1234567890000000000099, Addbot, Fgnievinski, Elec-tron, Aboctok, Download, Dumbelephants, Lightbot, Luckas-bot, Yobot, AnomieBOT, AmritasyaPutra, Dbln, Materialscientist, Citationbot, GrouchoBot, Stephen.G.McAteer, Pinethicket, I dream of horses, Vicenarian, Primalbeing, Full-date unlinking bot, Dude1818, AleAnd Quail, Maja3141, JSquish, Dagko, Bomazi, DASHBotAV, ClueBot NG, Iiii I I I, Hon-3s-T, Homk, Widr, Schneider anc, HelpfulPixie Bot, Bibcode Bot, Vivek Verma 38, Thedeathlord, Cweisbecker, Tsquared2215, ChrisGualtieri, Billzweig, Youwillalwaysbelovely,Anrnusna, W.Greensleeves, GinAndChronically, Your Dad 243, Craighurley88, NepaliBiebs and Anonymous: 1998.2 Images File:Commons-logo.svg Source: https://upload.wikimedia.org/wikipedia/en/4/4a/Commons-logo.svg License: ? 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