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.
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.
In 1912 Millikan went to Europe for six months to be able to analyze a mass of data uninterrupted by his many duties at the university. As on all of his many trips abroad, he visited a host of physicists and exchanged ideas with them. In Berlin he was forcefully reminded of the chaotic experimental situation regarding Einstein's famous 1905 equation of the photoelectric effect. Millikan was familiar with the great experimental difficulties from some work he had carried out in 1907. He also knew that subsequent work by other physicists had been extremely inconclusive. Once again he succeeded but it took him three years (1912-1915) of intensive work.
Capitalizing on an accidental observation, Millikan discovered that the alkali metals are sensitive to a very wide range of radiant frequencies. That was the key to the problem, but it was only the beginning: numerous ingenious experimental techniques, for example, a rotating knife inside the apparatus to clean the metal surface, had to be invented. By the time he was finished he considered it "not inappropriate to describe the experimental arrangement as a machine shop in vacuo." His efforts were rewarded: he established beyond doubt the validity of Einstein's linear relationship between energy and frequency, as well as all other predictions of Einstein's equation. This work, together with his measurement of the charge of the electron, won for Millikan the presidency of the American Physical Society (1916-1918) as well as many other honors, medals, and prizes, the highest of which was the Nobel Prize in 1923.
Millikan participated in the war effort in Washington (1917-1918) as third vice-chairman, director of research, and executive officer of the recently formed National Research Council. Most of his activities centered on the development of submarine detection and destruction devices: few goals were as urgent as that of breaking the back of the German U-boat menace.
One of Millikan's greatest services to the nation during this period was the role he played in establishing the National Research Council fellowships. He recommended the establishment of a fellowship program capable of supporting for two to three years the top 5 percent of recent American recipients of doctoral degrees in physics and chemistry. Millikan, who believed passionately in a decentralized university structure, hoped that the net result of this program would be not only to provide America with highly competent scientists but also to stimulate American universities to develop programs sufficiently competent to attract these very able students. From the start the program was a huge success, and it was soon extended to mathematics and the biological sciences.
After the war Millikan returned to the University of Chicago, where he immediately began several research projects. In 1921, however, he went to the California Institute of Technology (Caltech) as chairman of its Executive Council and director of the recently established Norman Bridge Laboratory of Physics.
At Caltech, Millikan soon fostered a wide variety of research, on everything from earthquakes to pure mathematics, but he himself took the greatest interest in the phenomenon known as "field emission" and particularly in cosmic rays. These radiations had been discovered in 1912 by V. F. Hess, who argued that they came from outer space. At first, Millikan was skeptical of this conclusion, but by the mid-1920s he was convinced of its accuracy, mostly as a result of high-altitude measurements. He coined the term "cosmic rays," a name retained to this day.
Millikan's convictions regarding the nature of the primary cosmic radiation—that which is incident on the earth's atmosphere—produced some of his stormiest days as a physicist. He argued convincingly that, in the vast hydrogen clouds in interstellar space, hydrogen atoms were being continually fused together to produce helium and heavier elements, thereby releasing a large amount of energy in the form of photons (light quanta). He concluded that these photons were the cosmic rays. This hypothesis, which was widely accepted, met its first serious challenge in 1929, and eventually Millikan was forced to abandon his photon hypothesis. It is now known that primary cosmic rays consist mostly of hydrogen and helium nuclei.
At Caltech, Millikan found a unique opportunity to implement his educational philosophy and, in general, influence American education. Under his guidance, Caltech grew from obscurity to a position of preeminence. The major educational policies he implemented were twofold: first, substantial emphasis on the humanities; and second, close ties between "pure sciences" such as physics and chemistry and the engineering disciplines.
"The secret of his success," wrote a friend about Millikan, "lay to a large extent in the simple virtues instilled in his upbringing. He had a single minded devotion to all that he was doing, and he put his work above his personal desires and aspirations." At the zenith of his powers, he was America's foremost experimentalist. He attracted and inspired a large number of exceptionally capable students, many of whom subsequently became his colleagues. Millikan, who died in Pasadena on Dec. 19, 1953, had a personal credo of great simplicity—and great beauty: "It is so to shape my own conduct at all times as, in my own carefully considered judgment, to promote best the well-being of mankind as a whole; in other words, to start building on my own account that better world for which I pray. The sum of all such efforts will constitute at least a first big step toward the attainment of that better world."
The most complete source of information on Millikan is his Autobiography (1950). A brief account of his life and work by L. A. DuBridge and Paul S. Epstein is in the Biographical Memoirs of the National Academy of Sciences, vol. 33 (1959). For information on various aspects of Millikan's work see David L. Anderson, The Discovery of the Electron (1964); Bruno Rossi, Cosmic Rays (1964); and Max Jammer, The Conceptual Development of Quantum Mechanics (1966). □