Robert Andrews Millikan

UXL Newsmakers, (2005)

Robert Andrews Millikan

The American physicist Robert Andrews Millikan (1868-1953) measured the charge of the electron, proved the validity of Albert Einstein's photoelectric effect equation, and carried out pioneering cosmicray experiments.

The second son of a Congregational minister of Scotch-Irish ancestry, R. A. Millikan was born on March 22, 1868. He entered the preparatory department of Oberlin College in 1886.

The only physics Millikan studied during his first 2 years at Oberlin was in a 12-week course, which he later described as "a complete loss." It therefore came as a complete surprise when his Greek professor asked him to teach the elementary physics course. Encouraged by the professor's remark that "anyone who can do well in my Greek can teach physics," Millikan accepted the challenge and spent the summer reading an elementary textbook and working the problems in it. This was Millikan's real introduction to physics and the origin of a conviction he held throughout life: that the most effective way of learning physics is by problem solving and not by passively listening to lectures, which he regarded as "a stupid anachronism—a holdover from pre-printing-press days."

Millikan obtained his bachelor's degree in 1891 and his master's in 1893, at the same time continuing to teach elementary physics. He received his doctorate from Columbia in 1895 and then spent a year abroad, visiting the universities of Jena, Berlin, and Göttingen. He met many prominent physicists, who discussed with him the recent and startling discoveries of x-rays and radioactivity. In 1896 he became an assistant in physics at the University of Chicago.

Chicago: The First 12 Years

When Millikan assumed his duties in 1896, American physics was in its infancy. He therefore immediately found himself dividing his 12-hour work day equally between research and the writing of introductory textbooks and the organization of courses. He was convinced that lectures should be largely replaced by laboratory and problem-oriented activities, and between 1903 and 1908 he authored or coauthored several very influential textbooks compatible with that philosophy. In 1902 he married Greta Blanchard; they had three distinguished sons.

By 1907 Millikan decided to start working intensively on research. 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. Thinking that he could measure the rate of evaporation, he decided to apply the electric field in a direction opposite to the force of gravity, balance it, and suspend the electron-laden droplets in midair. When he turned on the electric field, however, the entire mist disappeared—with the exception of a few individual drops which remained within the field of view of his observing telescope. Millikan realized immediately that he had discovered the key to the entire problem: to make precision measurements, he should observe single droplets using this balancing-field technique. Repeated observations revealed that the charge carried by a given droplet was always a multiple of a definite, fundamental value—the charge of the electron. Millikan created a great stir when he reported these results in 1909 at a professional meeting in Canada.

On his return trip to Chicago, Millikan suddenly realized that he could discard the cloud chamber entirely, that he could replace the evaporating water droplets with non-evaporating oil droplets, which could pick up electrons by passing through air ionized by x-rays (or gamma rays). This was the refinement required to make Millikan's experiment extraordinarily precise, and for several years he made countless determinations of the electronic charge. The values he reported in 1913 and 1917 stood for two decades, until it became known that a slight error had been introduced owing to a slightly incorrect value Millikan had assumed for the viscosity of air.