Robert Andrews Millikan

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(b. Morrison, Illinois, 22 March 1868; d. Pasadena, California, 19 December 1953)


Millikan was the son of Silas Franklin Millikan, a Congregational preacher, and Mary Jane Andrews, a graduate of Oberlin who had been dean of women at a small college in Michigan. Raised in Maquoketa, Iowa, where his family moved in 1875, young Millikan enjoyed a storybook Midwestern American boyhood, fishing, farming, fooling, and learning next to nothing about science. In 1886 he enrolled in the preparatory department of Oberlin College and, in 1887, in the classical course of the college itself. Mainly because he did quite well in Greek, at the end of his sophomore year he was asked to teach an introductory physics class. Glad to have the job, Millikan plunged into the subject, liked it, and soon decided to make it his career.

Millikan graduated from Oberlin in 1891 and continued to teach physics to the preparatory students while successfully pursuing a course of self-instruction in Silvanus P. Thomson’s Dynamic Electric Machinery. Awarded an M.A. for this achievement, in 1893 Millikan entered Columbia University on a fellowship as the sole graduate student in physics. He was impressed by the lectures of Michael I. Pupin, who emphasized the importance of mathematical techniques, and by the experimental deftness of Michelson, under whom he studied at the University of Chicago in the summer of 1894. Receiving his Ph.D. in 1895, Millikan went to Europe for postgraduate study, financed by a loan from Pupin. He heard Poincaré lecture at Paris, took a course from Planck at Berlin, and did research with Nernst at Göttingen. In 1896, the excitement of the discovery of X rays still fresh in his mind, Millikan joined the faculty of the University of Chicago as an assistant in physics.

There he soon met Greta Irvin Blanchard, the daughter of a successful manufacturer from Oak Park, Illinois. By the time the young couple was married in 1902, Millikan was pouring a large fraction of his considerable energies into the development of the physics curriculum, especially the introductory courses. In conjunction with this work, he wrote or coauthored a variety of textbooks and laboratory manuals which, like his First Course in Physics (1906), written with Henry Gale, quickly became standards and sold steadily through the years. In 1907, largely because of his outstanding pedagogical achievements, Millikan was promoted to an associate professorship.

But Millikan was acutely aware that at the University of Chicago the major rewards went to those who contributed to the advancement of knowledge. Although he had consistently done research, even his most recent investigation, on the photoelectric effect, had failed to yield significant results. Unaware of Einstein’s explanation of the effect, Millikan used a spark source of ultraviolet light to determine conclusively whether the photocurrent from various metals varied with temperature; as he found, it did not. Eager to earn a reputation in research, about 1908 he decided to shelve the writing of textbooks and concentrate on his work in the laboratory.

By 1909 Millikan was deeply involved in an attempt to measure the electronic charge. No one had yet obtained a reliable value for this fundamental constant, and some antiatomistic Continental physicists were insisting that it was not the constant of a unique particle but a statistical average of diverse electrical energies. Millikan launched his investigation with a technique developed by the British-born physicist H. A. Wilson; it consisted essentially of measuring, first, the rate at which a charged cloud of water vapor fell under the influence of gravity and then the modified rate under the counterforce of an electric field. Using Stokes’s law of fall to determine the mass of the cloud, one could in principle compute the ionic charge. Millikan quickly recognized the numerous uncertainties in this technique, including the fact that evaporation at the surface of the cloud confused the measure of its rate of fall. Hoping to correct for this effect, he decided to study the evaporation history of the cloud while a strong electric Held held it in a stationary position.

But when Millikan switched on the powerful field, the cloud disappeared; in its place were a few charged water drops moving slowly in response to the imposed electrical force. He quickly realized that it would be a good deal more accurate to determine the electronic charge by working with a single drop than with the swarm of particles in a cloud. Finding that he could make measurements on water drops for up to forty-five seconds before they evaporated. Millikan arrived at a value for e in 1909 which he considered accurate to within 2 percent. More important, he observed that the charge on any given water drop was always an integral multiple of an irreducible value. This result provided the most persuasive evidence yet that electrons were fundamental particles of identical charge and mass.

Late in 1909 Millikan greatly improved the drop method by substituting oil for water. Because of the relatively low volatility of this liquid, he could measure the rise and fall of the drops for up to four and a half hours. Spraying the chamber with radium radiation, he could change the charge on a single drop at will. His overall results decisively confirmed the integral-multiple values of the total charge. As for the determination of e itself, Millikan found that Stokes’s law was inadequate for his experimental circumstances because the size of the drops was comparable with the mean free path of the air. Using the so-called Stokes-Cunningham version of the law, which took this condition into account, by late 1910 he had computed a charge for e of 4.891×10-10 e.s.u. Realizing that the accuracy of this figure was no better than that of the key constants involved in the computation, Millikan painstakingly reevaluated the coefficient of viscosity of air and the mean-free-path term in the Stokes-Cunningham law. In 1913 he published the value for the electronic charge, 4.774±.009×10-10 e.s.u., which would serve the world of science for a generation.

Off and on all the while, Millikan had continued his exploration of the photoelectric effect; about 1912, now aware of Einstein’s interpretation of it, he began an intensive experimental study of the phenomenon, with the aim of testing the formula relating the frequency of the incident light to the retarding potential which cut off the photocurrent. No experimentalist had yet succeeded in proving or disproving the validity of the equation. Millikan took great care to avoid the mistakes that he and other physicists had previously made. Since a spark source of ultraviolet light induced spurious voltages in the apparatus, he used a high-pressure mercury-quartz lamp arranged to suppress stray light, especially on the short wavelength side. To extend the range of test well into the visible region, he made targets of alkali metals which were photosensitive up to 6,000 Å. Where others had adulterated their results by using photosensitive materials as the reference for the cutoff voltage, Millikan employed a Faraday cage of welloxidized copper netting which was not photosensitive in the range of his incident radiation. Finally, he sought to reduce the inaccuracies introduced when the photocurrent near the cutoff point was too low to measure with precision. Having noticed that this current was highest when the metal was fresh, he fashioned his targets into thick cylinders and rigged up an electromagnetically operated knife to shave off the ends of the blocks.

By 1915, as the result of these meticulous investigations, Millikan had confirmed the validity of Einstein’s equation in every detail. He not only demonstrated the linear relationship between the cutoff potential and the frequency of the incident light but also showed that the intercept of the graphed data on the voltage axis equaled the contact electromotive force, or work potential, of the target metal, a quantity which he had measured independently, to within 0.5 percent. In addition Millikan proved that the slope of the line equaled the ratio of Planck’s constant to the electronic charge, and his work provided the best measure of h then available. Despite the conclusiveness of these results, Millikan did not believe that he had confirmed Einstein’s theory of light quanta but only his equation for the photoeffect. In the face of all the evidence for the wave nature of light, he was convinced, as were most other physicists of the day, that the equation had to be based on a false, albeit evidently quite fruitful, hypothesis.

By 1916, when Millikan completed his major work on the photoeffect, he had already assumed more than a mere professor’s role in the world of science. In 1913 he became a consultant to the research department of Western Electric, primarily to advise the company on vacuum tube problems. In 1914 he was elected to the American Philosophical Society and the American Academy of Arts and Sciences, in 1915 to the National Academy of Sciences, and in 1916 to the presidency of the American Physical Society, an office which he held for two years. Millikan also served as an associate editor of Physical Review from 1903 to 1916; and he was made an editor of Proceedings of the National Academy of Sciences…, which was started in the year of his election.

Early in 1917, after the United States broke diplomatic relations with Germany, Millikan went to Washington as a vice-chairman and director of research for the National Research Council, the organization which the National Academy of Sciences had recently created to help mobilize science for defense. Commissioned a lieutenant colonel in the Army Signal Corps, he served in his military capacity as the director of the Signal Corps Division of Science and Research and, in his National Research Council identity, as a member of the U.S. Navy’s Special Board on Antisubmarine Devices. After a brief postwar period back at Chicago, in 1921 Millikan accepted appointment as chairman of the executive council and director of the Norman Bridge Laboratory at the newly renamed California Institute of Technology in Pasadena. In effect the president of the school, he was an able fund raiser and its enthusiastic spokesman; and under his leadership it quickly developed into one of the most distinguished scientific centers in the world.

Managing all the while to supervise many doctoral and postdoctoral fellows, Millikan maintained an active research career throughout the interwar years. One of the important subjects he investigated was the ability of electric fields on the order of a few hundred thousand volts per centimeter to draw electrons out of cold metals. By 1926, working in collaboration with Carl F. Eyring, a Caltech graduate student, Millikan had completed a thorough study of the phenomenon, using tungsten wires threaded along the axis of a hollow cylinder in high vacuum. The two men found that the field current, to use the term they introduced, depended only on the field gradient, not on the potential difference, between the wire and the walls of the cylinder. More important, within wide limitations the current was also entirely independent of temperature. Pointing out that these results violated Owen W. Richardson’s theory of thermionic emission, Millikan and Eyring speculated that at relatively low temperatures some metallic electrons must not obey the law of equipartition. But in 1928 Oppenheimer, R. H. Fowler, and their co-workers showed independently that cold emission was a quantum mechanical result of the leakage of electrons through a potential barrier. In 1929 Charles C. Lauritsen, who was completing his doctoral research under Millikan, derived an empirical formula from their data which related the field current to the field gradient; and this equation was ultimately found to be experimentally indistinguishable from the quantum mechanical expression.

During the 1920’s Millikan also did significant research in the “hot spark” spectra. As he knew, a high potential difference would maintain a spark source of ultraviolet radiation across two electrodes in a vacuum. The relative ease with which such radiation was absorbed had made its study difficult. In 1915 Millikan proposed that one could get around the problem of absorption by enclosing the path between the spark and a photographic plate entirely in a vacuum. To maximize the intensity of the spectrum, he had a grating ruled that would throw most of the light into the first order. Shortly after the war, with the apparatus now working reliably, Millikan and Ira S. Bowen, another Caltech graduate student, embarked upon a thorough study of the ultraviolet spectra of the lighter elements up to copper. By 1924 they had found and identified some 1,000 new lines. They had also extended the observable spectrum down to 136.6 A and had helped to close the last gap between the optical and the X-ray frequencies.

In the course of this work, Millikan and Bowen found that the strongest lines were produced by atoms which had been stripped of their valence electrons. Since the spectra of such hydrogen-like atoms ought to contain multiplets, they began, about the end of 1923, to study the fine spectra in the ultraviolet. By early 1924 they had found that the 2s, 2p1 and 2p2 terms of the ultraviolet doublets corresponded precisely to the LI LII, and LIII levels associated with the X-ray spectra of the heavier elements. Moreover, exactly the same relationship existed between the M and N X-ray levels and the higher ultraviolet multiplel terms. Millikan and Bowen concluded that the X-ray doublet laws based on Sommerfeld’s relativistic orbital analysis could account for the doublets in the whole field of optics.

Yet, as they also pointed out in 1924, independently of Alfred Landé, this conclusion raised a serious difficulty for the theory of spectra. On the one hand, Sommerfeld’s relativistic analysis of the X-ray doublets assigned a different azimuthal quantum number to the LII and LIII terms. On the other hand, Bohr’s spectral scheme accounted for the optical doublets by assuming different orientations for the same orbit; by definition, the p1 and p2 terms of the optical doublets possessed the same azimuthal quantum number. Since the results of Millikan and Bowen identified the LII and LIII terms with the p1 and p2 levels, it seemed that one had to give up either Sommerfeld’s relativistic explanation or the Bohr scheme of spectra. Millikan and Bowen could find no way out of the dilemma; but their forceful statement of it in 1924, coupled with Landés, contributed to the ultimate resolution of the difficulty through G. E. Uhlenbeck and S. A. Goudsmit’s postulation of electron spin in 1925.

In the 1920’s Millikan also began an increasingly intensive program of research into the penetrating radiation which in mid-decade he would name “cosmic rays,” In 1912 the Austrian-born physicist Victor Hess had found that atmospheric ionization increased with altitude up to 12,000 feet. But although Hess had argued that some kind of radiation was coming from the heavens, most physicists still attributed the phenomenon to some terrestrial cause, such as electrical discharges from thunderstorms or radioactivity. Millikan’s initial experiments in the field, done with an unmanned sounding balloon in 1922 to a height of fifteen kilometers and with lead-shielded electroscopes atop Pike’s Peak in 1923, failed to decide in favor of either interpretation. In the summer of 1925 Millikan proposed to settle the question by measuring the variation of ionization with depth in Muir Lake and Lake Arrowhead in the mountains of California. Snow-fed and separated by many miles, as well as 6,675 feet of atmosphere, each was likely to be free of both local radioactive disturbances and whatever atmospheric peculiarities might affect the ionization in the other.

Millikan’s electroscopie measurements showed that the intensity of ionization at any given depth in Lake Arrowhead was the same as the intensity six feet lower in Muir Lake. Since the layer of atmosphere between the surfaces of the two lakes had precisely the absorptive power of six feet of water, the results decisively confirmed that the radiation was coming from the cosmos. Moreover, since the intensity of the ionization showed no diurnal variation, the radiation was uniformly distributed over all directions in space. And, finally, since Millikan detected ionization as far below the top of the atmosphere as the combined air and water equivalent of six feet of lead, it was evident that cosmic rays were a good deal more energetic than even the hardest known gamma rays.

To penetrate six feet of lead, charged particles would have to possess stores of energy then considered impossibly large; accordingly, Millikan assumed that cosmic rays must consist of photons. In 1926 he tested this assumption experimentally with what he considered confirmatory results. If cosmic rays were charged particles, their trajectories would be affected by the earth’s magnetic field, so that more of them would strike the earth at higher than at lower latitudes. But Millikan could detect virtually no difference in cosmic ray flux at Lake Titieaea in South America from that at Muir Lake. And, although he ran his electroscope while sailing back from Mollendo, Peru, to Los Angeles, he found no variation of intensity with latitude at sea level.

Employing the photonic interpretation of cosmic rays, Millikan developed a theory of their origin in 1928. Combining the data from the balloon flight of 1922 with that of his terrestrial surveys, he graphed a curve of ionization versus depth which covered the range from sea level up to virtually the top of the atmosphere. Because no single coefficient of absorption could account for the curve, he inferred that cosmic rays were spread across a spectrum of energies. Going further, he argued that the experimental curve could be constructed from three different curves, each representing a different coefficient of absorption. According to this analysis, cosmic ray energies were not generally distributed but were clustered in three distinct bands.

To account for these bands, Millikan introduced what he called the “atom-building hypothesis.” Using Dirac’s formula for absorption through Compton scattering, Millikan computed the energy of the three bands from their absorption coefficients and found them equal to 26, 110, and 220 MEV. These figures equaled the mass defects of hydrogen, oxygen, and silicon, which were known to be three of the most abundant elements of the universe. Millikan concluded that the photons striking the earth must be produced when four atoms of hydrogen somehow fused to form helium, sixteen to form oxygen, and twenty-eight to form silicon. In his summary of the argument, cosmic rays were the “birth cries” of atoms, a phrase which quickly achieved a good deal of notoriety among both the scientific and the lay publics.

Although in the late 1920’s most physicists agreed with Millikan that cosmic rays were photons, few accepted his atom-building hypothesis. He had no proof of the uniqueness of his three absorption coefficients and could not convincingly explain away the kinetic difficulties involved in the spontaneous union of sixteen hydrogen atoms into oxygen, let alone twenty-eight into silicon. Moreover, some of his own experimental evidence cast doubt on the validity of using the Dirac formula to compute cosmic ray energies. Then, at the beginning of the 1930s, Millikan’s assumption that the primary radiation consisted of photons was refuted by the work of other experimentalists, especially by Arthur Compton’s conclusive detection of a latitude effect in 1932.

Millikan hotly contested Compton’s findings. He had repeated his search for a latitude effect in the late 1920’s, and in late 1932 he did so once more, again without success. But Millikan was the victim of experimental circumstance. In the longitudinal region of California, the dip in cosmic ray intensity began quite suddenly in the neighborhood of Los Angeles and quickly reached its maximum fall of some 7 percent less than two days’ sail south of the city. In Millikan’s initial search for the latitude effect—the voyage from Mollendo, Peru, to Los Angeles—his estimated error had been 6 percent. In most of his later searches, he went to the north of Pasadena, where the rise in intensity was too small to detect easily. In 1932 he sent H. Victor Neher, a young collaborator at Calteeh, on a voyage to the south; but Neher did not get his electroscope working before he had passed the region of the dip.

By 1933, with Neher having now found a latitude effect, Millikan had admitted that some percentage of cosmic radiation must consist of charged particles. By 1935 he had also rejected the atom-building hypothesis, mainly because it was now clear that the bulk of cosmic radiation possessed energies much higher than the mass defects of the abundant elements. All the same, despite a vast array of contrary evidence and the overwhelming body of professional opinion, Millikan clung tenaciously to the assumption that some fraction of the primary cosmic radiation could be photons. in the late 1930’s and early 1940’s he searched for evidence in support of this view, measuring cosmic ray intensities around the world at sea level, in airplanes at high altitudes, and with unmanned sounding balloons up to the top of the atmosphere. On the basis of this data, he also developed a theory that cosmic ray photons originated in the spontaneous annihilation of atoms in interstellar space. No more convincing than its predecessor, this hypothesis became completely untenable, as Millikan himself admitted a few years before his death, after the detection of the π-meson in 1947 made it clear that the primary cosmic radiation consisted almost entirely of protons.

But however wrongheaded Millikan had been, his cosmic ray research yielded a valuable fund of experimental data. Moreover, in 1934, independently of Jacob Clay, he detected the variation of the latitude effect with longitude because of the dissymmetry of the earth’s magnetic field. In a roundabout way even the atom-building hypothesis strikingly benefited the progress of science. In the late 1920’s, troubled by the discrepancy between his experimental data and the predictions of both the Dirac absorption formula and its successor, the Klein-Nishina formula, Millikan recognized that he needed a measure of cosmic ray energies that was not based on absorption coefficients. To obtain a direct determination, he put Carl Anderson, a young research fellow at Caltech, to work with a cloud chamber set in a powerful magnetic field. In 1931 Anderson’s studies of the trajectories of charged particles showed conclusively that the absorption of cosmic rays resulted from nuclear encounters as well as from Compton scattering. They also led to his detection of the positron in 1932.

Between the wars Millikan played a prominent role in the affairs of his profession. The president of the American Association for the Advancement of Science in 1929 and the holder of various offices in the National Academy of Sciences and the National Research Council, he was especially active as a member of the NRC fellowship board and as foreign secretary of the Academy. From 1922 to 1932 Millikan served as the American representative to the Committee on Intellectual Cooperation of the League of Nations. Throughout the interwar period he participated in the International Research Council; its successor, the International Council of Scientific Unions; and the affiliate of both, the International Union of Pure and Applied Physics. In 1933 Millikan was appointed by President Franklin D. Roosevelt to the Science Advisory Board, a joint venture of the Academy and the federal government to find ways to use science for economic recovery.

Millikan was an able popularizer and lecturer, and after he won the Nobel Prize in 1923 he became perhaps the most famous American scientist of his day. An outspoken religious modernist, he was a leading exponent of the reconcilability of science and religion in the l920’s, the decade of the Scopes trial. Politically, Millikan was a conservative Republican. During the 1930’s he vigorously opposed the New Deal, repeatedly denounced governmental intervention in the economy, and argued that the promotion of science, because it led to new industries and new jobs, was a much sounder way to achieve economic recovery. Always an internationalist, Millikan believed firmly in collective security. In the late 1930’s, unlike many conservative Republicans at the time, including his good friend Herbert Hoover, he helped propagandize in favor of aid to the Allies; by early 1941 he was encouraging the conversion of Caltech from academic to military purposes.

During the war Millikan turned over an increasing fraction of his administrative responsibilities at Caltech to the younger staff members who were running the various defense projects. In 1946 he retired from his professorship and the chairmanship of the executive council. He remained active as a public lecturer and spoke frequently on the subject of science and religion. He was cool to the creation of the National Science Foundation and spoke often against federal aid to education. By the time of his death, Millikan had been awarded numerous medals, even more honorary degrees, and membership in twenty-one foreign scientific societies, including the Royal Society of London and the Institut de France.


I. Original Works. A complete bibliography of Millikan’s published work, which includes close to 300 scientific papers, is in Lee A. DuBridge and Paul S. Epstein, “Robert Andrews Millikan,” in Biographical Memoirs. National Academy of Sciences, 33 (1959), 241–282. In The Autobiography of Robert A. Millikan (New York, 1950) Millikan provided valuable accounts of his childhood and education, work on the electronic charge and the photoeffect, and involvement in the mobilization of science during World War I; curiously, he devoted little space to his research in hot spark spectra or cosmic rays, and his account of the development of the California Institute of Technology must be used with special care. Millikan left a voluminous body of correspondence, which is now in the Caltech archives. Dating in the main from 1921, the collection contains substantial materials on the National Academy of Sciences-National Research Council and the administration of the California Institute of Technology, as well as a sizable amount of family and scientific letters. Another important batch of Millikan’s letters is in the papers of George Ellery Hale in the Caltech archives, which were also published in a microfilm edition (Pasadena, 1968) under the editorship of Daniel J. Kevles. The locations of other letters to and from Millikan are given in Thomas S. Kuhn, et al., Sources for the History of Quantum Mechanics (Philadelphia, 1967), 68.

II. Secondary Literature. Paul S. Epstein wrote an excellent résumé of Milikan’s scientific work in “Robert A. Millikan as Physicist and Teacher,” in Reviews of Modern Physics, 20 (Jan. 1948), 10–25, a volume published in honor of Millikan’s eightieth birthday. A condensed version of Epstein’s essay occupies part of the memoir written with DuBridge (see above), which is on the whole a useful introduction to Millikan’s life. Millikan the famous scientist is treated in Daniel J. Kevles, “Millikan: Spokesman for Science in the Twenties,” in Engineering and Science, 32 (Apr. 1969), 17–22.

Daniel J. Kevles

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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.

Einstein's Photoelectric Effect Equation

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.

War Work; National Research Council

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.

Transfer to Caltech; Cosmic-Ray Researches

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.

Millikan: The Educator and Man

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."

Further Reading

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). □

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Robert Andrews Millikan


American Physicist

Robert Andrews Millikan, in his famous oildrop experiment of 1910, measured the charge of the electron. In another important experiment, he verified Einstein's equation for the photoelectric effect and determined the exact value of Planck's constant. For these accomplishments, Millikan was awarded the Nobel Prize in physics in 1923. He also provided a role model for the American-born scientist in the early twentieth century, at a time when most of the great physicists were European.

Millikan was born in Morrison, Illinois, on March 22, 1868, the second son of the Reverend Silas Millikan and his wife, Mary Jane. Nothing about his childhood, spent in the rural Midwest, particularly hinted at a future career as an eminent scientist. After graduating from high school, he worked briefly as a court reporter. His involvement in science began after his sophomore year at Oberlin College in Ohio, when his Greek professor recruited him to teach an elementary physics course. Millikan objected that he had no real knowledge of the subject, but the professor countered that it could not be any harder than Greek. He received his Ph.D. from Columbia University in 1895 and was the only physics graduate student at the time.

After a brief stint in Germany studying with Max Planck (1858-1947), Albert Michelson (1852-1931) invited him to join the physics faculty at the University of Chicago. He remained there for 25 years, attaining the status of full professor in 1910. Millikan had little patience with lecturing, believing that students learned more effectively through solving problems and performing experiments. He attempted to put this philosophy into practice by writing better physics textbooks. It was so important to him that he was checking page proofs on the morning of his wedding to Greta Erwin Blanchard in 1902.

Millikan began attempting to measure the charge of the electron by measuring how tiny charged water droplets moved in an electric field. The results suggested that the charge on the water droplets was always a multiple of a single elementary value. However, since water evaporates quickly, he had trouble getting sufficiently reliable data. He solved this problem by using oil instead. Spraying oil droplets into his small experimental chamber, he measured how the charge they carried affected their fall.

In the photoelectric effect, light hitting metal causes electrons to be emitted. Millikan showed that their energy varied with the frequency of the incident light. From this data he was able to obtain an experimental value for Planck's constant of proportionality relating frequency to energy. During World War I Millikan played a role in developing anti-submarine devices as Vice-Chairman of the National Research Council.

In 1921 Millikan became head of the California Institute of Technology (Caltech), where he remained until 1945. He was a skilled administrator as well as an experimentalist and was known for regularly putting in 12-hour work days. Under his leadership, the school became one of the United State's leading institutions for scientific research. He also served as professor and chairman of the Norman Bridge Laboratory of Physics there.

At Caltech, Millikan's research interests involved the radiation that Victor Hess (1883-1964) had detected impinging upon Earth from outer space. Millikan confirmed their extraterrestrial origin, and coined the term cosmic rays. By comparing their intensity at various latitudes on Earth, and at different altitudes, he gathered data that helped in the understanding of Earth's magnetic field and the way in which it deflects the incoming radiation.

Millikan was a prolific author. In addition to publishing more than a half-dozen physics textbooks, he wrote and lectured on the reconciliation of science and religion and the proper relationship of science and society. His autobiography was published in 1950. He was active in the larger scientific community, serving as president of the American Physical Society, Vice-President of the American Association for the Advancement of Science, and member of the Committee on Intellectual Cooperation of the League of Nations. He also enjoyed playing golf and tennis. Millikan died on December 19, 1953, in San Marino, California.


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Millikan, Robert Andrews (1868–1953) US physicist. His oil drop experiment enabled him to determine the electric charge of an electron. Millikan went on to study the photoelectric effect, verifying the equation of Albert Einstein and gaining a precise value for Max Planck's constant. In 1923 he received the Nobel Prize in physics.

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