Millikan’s Oil Drop Experiment Dr. Frank Walmsley Department of Chemistry Trinity University San Antonio, TX 78212

Historical Setting Early 1900s Rutherford had reported his alpha scattering experiments. Structure of atom known but not accepted by everyone. Charge on an alpha particle known. Experiments with cathode rays known including fact that negatively charged.

Historical Setting Needed to know charge on an electron. Millikan modified experiments tried by others and did very careful work. Used nonvolatile oil rather than volatile water. Corrected equations for rate of fall of drop – determined this experimentally

Historical Setting Robert Millikan Received Nobel Prize for this work Work done at University of Chicago Became first president of Californa Institute of Technology

Design of the Experiment Sketch of apparatus used Blue plates charged – charge can be varied. Oil drops fall through hole in top plate. Xrays place electrons on drops. Microscope used for observing drops.

Design of the Experiment Photo of actual apparatus used in the experiments.

Design of the Experiments Time of fall of oil drop in the absence of field measured between two fixed points. Drop falls due to gravity. Electric field needed to hold drop steady between the two plates measured. When drop is suspended, force due to gravity equals force due to electrical attraction to + plate.

Design of the Experiment Information needed Density of air Viscosity of air Density of oil Equation for rate of fall Strength of electric field Temperature Air Pressure

Data Treatment From Millikan’s 1911 paper. q/Cq/1.64 x Round to whole number x x x

Data Treatment Notice that dividing by the smallest of the three numbers does not give whole number results Millikan had lots of measurements, not just 3 which is needed in order to find that common multiplier.

Experimental Results Showed that the charges were multiples of the minimum basic charge of one electron. Determined the value of that charge. This led to more accurate values for other constants such as Avogadro’s Number.

Was Millikan Dishonest? Millikan did not use all his data points in his published articles. Millikan said in his articles that he did use all points. How can both be true?

Was Millikan Dishonest? It is true that Millikan selectively chose points to report. He discarded drops that were too small because they were too affected by bombardment from air molecules. He discarded drops that were too large because they did not obey the equations for rate of fall. He discarded drops that had only one measurement – that is, did not have additional measurements with fewer or greater numbers of electrons.

Was Millikan Dishonest? It appears that what he meant (and did not say clearly) was: He used all drops that met his criteria for being reliable. If he had used all drops, the results would have been essentially the same.