The problem he chose--the measurement of the charge of the electron--would gain him a full professorship (1910), the directorship of Chicago's Ryerson Physical Laboratory (1910), membership in the National Academy of Sciences (1914), and an international reputation.
Millikan intuitively sensed that the most fruitful approach to the problem would be to eliminate the sources of error in a method developed by J. S. E. Townsend (1897), J. J. Thomson (1903), and H. A. Wilson (1903) at the Cavendish Laboratory in Cambridge, England. In Wilson's experiments, air was compressed in a cloud chamber, ionized with x-rays, and then rapidly expanded, causing tiny water droplets to condense on the ions and form a mist. These droplets were allowed to fall, either under the influence of gravity alone or under the influence of gravity plus an electric field. By observing their velocities of fall in the first case, Wilson used Stokes' law to calculate their radii; by observing their velocities in the second case, he could then calculate the magnitude of the charge they carried--which Wilson found to vary between wide limits. The atomicity, or definiteness, of the charge of the electron was therefore still very much in doubt.
Millikan first attempted to eliminate the error introduced into Wilson's experiments by the gradual evaporation, and hence change in radii, of the water droplets.
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