Robert MillikanOil drop Experiment ~1906-1913
Problems with the previous model (Unknowns) • Scientists at this time believed the atom contains minute bodies that carry a charge called electrons. • Thomson’s cathode ray tube experiments found that the stream of energy had a negative charge and the mass to charge ratio. • Scientists at this time wanted to know: • Magnitude of charge of an electron • If all electrons carry the same charge • Discrete particle
Experimental Design • Atomizer would produce small drops of oil in which some would fall through a hole on the upper floor • He would allow to the drop to free fall until it reached its terminal velocity • He would then apply an electrical current to see which drops had a negative charge • The drop would then be analyzed to see how long it took the drop to move up some distance or the energy required to stop it.
Oversimplified formula Charge x Electrical field Mass x Gravity
Evidence • All electrons carry the same charge “quantized”
New atomic model • Thomson experiments did not provide any direct evidence of individual negative particles versus a stream of energy with a negative charge. • Proved electrons are discrete particles with a finite charge (within 2% currently accepted value) • Mass of the electron could be calculated combining Thomson’s and Millikan’s experiments
Interesting Facts • Graduate students may have had a greater role in designing the experiment • Excluded outliers from data
Websites and other Resources History The Prism and the Pendulum by Robert P. Crease http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Ryan_Mcallister/Slide3.htm http://nobelprize.org/nobel_prizes/physics/laureates/1923/millikan-bio.html http://www.aip.org/history/gap/Millikan/Millikan.html http://www.springerlink.com/content/u7t7551523087683/fulltext.pdf Lab write up for physics http://www.stkate.edu/physics/phys112/curric/millikan.html http://web.mit.edu/8.13/www/JLExperiments/JLExp_02.pdf Movie: Interview with Fletcher • http://auditorymodels.org/jba/BOOKS_Historical/FletcherVideo/ Applet http://physics.wku.edu/~womble/phys260/millikan.html