Robert Millikan Oil 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

Oil Drop Experiment

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