But after the publication of those results, Viennese physicist Felix Ehrenhaft claimed to have conducted a similar experiment, measuring a much smaller value for the elementary charge. In 1910 Millikan published the first results from these experiments, which clearly showed that charges on the drops were all integer multiples of a fundamental unit of charge. After repeatedly timing the rise and fall of a drop, Millikan could calculate the charge on the drop. The experimenter could watch the drops through a specially designed telescope, and time how fast a drop falls or rises. The drops then fall or rise under the combined influence of gravity, viscosity of the air, and an electric field, which the experimenter can adjust. The oil droplets are injected into an air-filled chamber and pick up charge from the ionized air. He asked his graduate student, Harvey Fletcher, to figure out how to do the experiment using some substance that evaporated more slowly.įletcher quickly found that he could use droplets of oil, produced with a simple perfume atomizer. In 1909 he began the experiments, but soon found that droplets of water evaporated too quickly for accurate measurement. He realized that trying to determine the charge on individual droplets might work better than measuring charge on whole clouds of water. Millikan saw this opportunity to make a significant contribution by improving upon these measurements. The method did give a crude estimate of the electron’s charge. Thomson and others tried to measure the fundamental electric charge using clouds of charged water droplets by observing how fast they fell under the influence of gravity and an electric field. The next step was to determine the electron’s charge separately. Thomson had discovered the electron in 1897 and had measured its charge-to-mass ratio. Millikan attended Oberlin College, earned his PhD from Columbia University, and then spent a year in Germany before taking a position at the University of Chicago.īy about 1906, Millikan had become a successful educator and textbook writer, but he knew that he hadn’t done any research of real scientific significance, and was eager to make his mark as a researcher. Robert Millikan was born in 1868 and grew up in rural Iowa, the second son of a minister. The experiment, a great improvement over previous attempts to measure the charge of an electron, has been called one of the most beautiful in physics history, but is also the source of allegations of scientific misconduct on Millikan’s part. Robert Millikan’s famous oil drop experiment, reported in August 1913, elegantly measured the fundamental unit of electric charge. August, 1913: Robert Millikan Reports His Oil Drop Resultsĭiagram of Millikan's apparatus, from his Physical Review paper
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