Editing Oil drop experiment (section)

{{Short description|Experiment to measure elementary electric charge}}
{{Distinguish|Pitch drop experiment}}
[[Image:Millikan's setup for the oil drop experiment.jpg|right|thumb|Millikan's setup for the oil drop experiment|300x300px]]

The '''oil drop experiment''' was performed by [[Robert Andrews Millikan|Robert A. Millikan]] and [[Harvey Fletcher]] in 1909 to measure the [[Elementary charge|elementary electric charge]] (the charge of the [[electron]]).<ref name="Millikan1910">{{cite journal
 | author = Millikan, R. A.
 | title = The isolation of an ion, a precision measurement of its charge, and the correction of Stokes's law
 | journal = Science
 | volume = 32
 | number = 822
 | pages = 436–448
 | doi = 10.1126/science.32.822.436
 | year = 1910
| pmid = 17743310
 }}</ref><ref name="Fletcher1982"/> The [[experiment]] took place in the Ryerson Physical Laboratory at the [[University of Chicago]].<ref>{{Cite web |title=University of Chicago |url=https://www.aps.org/funding-recognition/historic-sites/university-of-chicago |access-date=2025-05-30 |website=www.aps.org |language=en}}</ref><ref>{{Cite web|url=https://www.uchicago.edu/breakthroughs/1910s/|title=UChicago Breakthroughs: 1910s|last1=AvenueChicago|first1=The University of ChicagoEdward H. Levi Hall5801 South Ellis|last2=Us|first2=Illinois 60637773 702 1234 Contact|website=The University of Chicago|language=en|access-date=2019-07-31}}</ref><ref>{{Cite web|url=http://chronicle.uchicago.edu/070104/millikan.shtml|title=Work of physicist Millikan continues to receive accolades|website=chronicle.uchicago.edu|date=4 January 2007|access-date=2023-10-15}}</ref> Millikan received the [[Nobel Prize in Physics]] in 1923.<ref name="Nobel1923">{{Cite web|url=https://www.nobelprize.org/prizes/physics/1923/summary/|title=The Nobel Prize in Physics 1923|website=NobelPrize.org|language=en-US|access-date=2023-10-15}}</ref>

The experiment observed tiny electrically charged [[droplet]]s of oil located between two parallel metal surfaces, forming the plates of a [[capacitor]]. The plates were oriented horizontally, with one plate above the other. A mist of [[atomizer nozzle|atomized]] oil drops was introduced through a small hole in the top plate; some would be [[ionization|ionized]] naturally.<ref name=Pekola-2013>{{Cite journal |last=Pekola |first=Jukka P. |last2=Saira |first2=Olli-Pentti |last3=Maisi |first3=Ville F. |last4=Kemppinen |first4=Antti |last5=Möttönen |first5=Mikko |last6=Pashkin |first6=Yuri A. |last7=Averin |first7=Dmitri V. |date=2013-10-02 |title=Single-electron current sources: Toward a refined definition of the ampere |url=https://link.aps.org/doi/10.1103/RevModPhys.85.1421 |journal=Reviews of Modern Physics |language=en |volume=85 |issue=4 |pages=1421–1472 |doi=10.1103/RevModPhys.85.1421 |issn=0034-6861}}</ref>
First, with zero applied [[electric field]], the velocity of a falling droplet was measured. At [[terminal velocity]], the [[drag (physics)|drag]] force equals the [[gravity|gravitational]] force. As both forces depend on the radius in different ways, the radius of the droplet, and therefore the [[mass]] and gravitational force, could be determined (using the known [[density]] of the oil).  Next, a [[voltage]] inducing an electric field was applied between the plates and adjusted until the drops were suspended in [[mechanical equilibrium]], indicating that the electrical force and the gravitational force were in balance. Using the known electric field, Millikan and Fletcher could determine the charge on the oil droplet. By repeating the experiment for many droplets, they confirmed that the charges were all small integer multiples of a certain base value, which was found to be {{val|1.5924|(17)|e=-19|u=[[coulomb|C]]}}, about 0.6% difference from the currently accepted value of {{physconst|e|after=.}} They proposed that this was the magnitude of the negative charge of a single electron.