Robert A. Millikan was the most famous American scientist of the 1920s, and the second American to receive the Nobel Prize in physics in 1923 for his study of the elementary electronic charge and the photoelectric effect.
Robert Andrews Millikan was born on the 22nd of March, 1868, in Morrison, Ill. (U.S.A.), as the second son of two proud parents. His father, Reverend Silas Franklin Millikan, was a congressional minister in a small town and his mother, Mary Jane (Andrews) Millikan, a former dean of women at Olviet College.
Robert’s grandparents were of Scotish-irish ancestry which had come to America before 1750, and were pioneer settlers in the Middle West. His family moved, when he was five, to Iowa. He led a rural existence in childhood, attending the Maquoketa High School (Iowa). He helped around the farm and at 14 he was working ten-hour days during the summers at a local barrelhead factory, earning only a dolar a day.
He attended the Maquoketa High School in Iowa. After high school he worked for a short time as a court reporter, and in 1886, he went to Oberlin College in Ohio. There he studied for five years. During his undergraduate course his favourite subjects were Greek and mathematics. During his second year there he took a 12-week physics course and thought it a complete loss.
However, at the end of that year (1891) Robert was asked to teach elementary physics. He only accepted the job because he was in a tight bind for money. Then Robert went to continue his education at Columbia University. In 1893, after obtaining his mastership in physics, he was appointed Fellow in Physics at Columbia University. It is interesting and worthwhile to note that Millikan for a time was the only graduate student in physics at Columbia. He afterwards received his Ph.D. (1895) for research on the polarization of light emitted by incandescent surfaces – using for this purpose molten gold and silver at the U.S. Mint.
Millikan was an enthusiastic tennis player, and golf was also one of his recreations. Professor Millikan married Greta Erwin Blanchard in 1902; they had three sons: Clark Blanchard, Glenn Allen, and Max Franklin.
Greta B. Millikan
On the instigation of his professors, Millikan spent a year (1895-1896) in Germany, at the Universities of Berlin and GÃ¶ttingen, as was the custom for young American scientists of his generation – during his summer in GÃ¶ttingen he found more Americans than Germans among the advanced students in the laboratory. While he was there he got a message from A. A. Michelson, offering him a teaching assistantship at the newly established Ryerson Laboratory at the University of Chicago.
Millikan jumped at it, although he could have had an appointment elsewhere at twice the salary, for Michelson promised that he could spend up to half his time doing his own research, a privilege not granted at most colleges. During his early years at Chicago he spent much time preparing textbooks and simplifying the teaching of physics, but he made little progress as a research scientist.
In collaboration he wrote elementary texts that educated a generation of Americans, and in the classroom he proved to be an outstanding educator. These qualities were valued at Chicago, but not as much as research. Millikan was appointed associate professor only at the age of 38, in a time when the median American physicist became a full professor at the age of 32.
He later recalled: “Although I had for ten years spent on research every hour I could spare from my other pressing duties, by 1906 I knew that I had not yet published results of outstanding importance, and certainly had not attained a position of much distinction as a research physicist.”
This was a period of crucial change in the history of physics. J. J. Thomson discovered the electron, Max Planck kicked off the quantum revolution, Albert Einstein produced his theories of relativity and the photo-electric effect, and Einsteinâ€™s theory and Perrinâ€™s experiments on Brownian motion established forever that matter was made of atoms. Professor Millikan made no contribution to these events.
Nearing 40 years of age, he became very anxious indeed to make his mark in the world of physics. He thought of devoting himself wholly to education. But instead he stopped writing textbooks and set out on one last try at a new line of research: the determination of the elementary unit of electric charge.
In 1909 Millikan began a series of experiments to determine the electric charge carried by a single electron. He began by measuring the course of charged water droplets in an electrical field. The results suggested that the charge on the droplets is a multiple of the elementary electric charge, but the experiment was not accurate enough to be convincing.
He obtained more precise results in 1910 with his famous oil-drop experiment in which he replaced water (which tended to evaporate too quickly) with oil. Shortly after the experiment’s publication in 1910, Millikan was rewarded with a full professorship.
Millikan Oil Drop Experiment
The apparatus associated with Millikan’s oil-drop experiment is shown in the figure (left). A closed chamber with transparent sides is fitted with two parallel metal plates, which acquire a positive or negative charge when an electric current is applied. At the start of the experiment, an atomizer sprays a fine mist of oil droplets into the upper portion of the chamber.
Under the influence of gravity and air resistance, some of the oil droplets fall through a small hole cut in the top metal plate. When the space between the metal plates is ionized by radiation (e.g., X rays), electrons from the air attach themselves to the falling oil droplets, causing them to acquire a negative charge. A light source, set at right angles to a viewing microscope, illuminates the oil droplets and makes them appear as bright stars while they fall.
Millikan’s setup for the oil drop experiment
The mass of a single charged droplet can be calculated by observing how fast it falls. By adjusting the potential difference, or voltage, between the metal plates, the speed of the droplet’s motion can be increased or decreased; when the amount of upward electric force equals the known downward gravitational force, the charged droplet remains stationary. The amount of voltage needed to suspend a droplet is used along with its mass to determine the overall electric charge on the droplet.
When a drop is suspended, its weight m Â· g is exactly equal to the electric force applied q Â· E:
The values of E, the applied electric field, m the mass of a drop, and g, the acceleration due to gravity, are all known values. So you can solve for q, the charge on the drop:
Through repeated application of this method, the values of the electric charge on individual oil drops are always whole-number multiples of a lowest value – that value being the elementary electric charge itself (about 1.602×10-19 coulomb). From the time of Millikan’s original experiment, this method offered convincing proof that electric charge exists in basic natural units. All subsequent distinct methods of measuring the basic unit of electric charge point to its having the same fundamental value.
Millikanâ€™s oil-drop apparatus
A diagram taken from Millikan’s 1913 paper shows that the chamber contained two metal plates (M and N) to which he applied a high voltage, generated by a bank of batteries (B). Fine droplets of oil produced by a perfume atomizer (A) were fed into the top of the chamber. A tiny hole in the upper plate allowed the occasional droplet (p) to fall through, at which point it was illuminated by an arc lamp (a) and could be seen in magnification through a “telescope.” A manometer (m) indicated internal pressure.
To eliminate differences in temperature (and associated convection currents), Millikan immersed the brass chamber in a container of motor oil (G), and he screened out the infrared component of illumination using an 80-centimeter-long glass vessel filled with water (w) and another glass cell filled with a cupric chloride solution (d). An x-ray tube (X) allowed him to ionize the air around the droplet.
With this equipment, Millikan could watch an oil drop that carried a small amount of charge rise when the applied electric field forced it upward and fall when only gravity tugged on it. By repeatedly timing the rate of rise and fall, he could determine precisely the electric charge on the drop.
Data acquisition was by telescopic observation of drops, timed by a stop watch as a manually operated knife switch was used to change the electric field. Drop generation was by generation of a mist via an atomizer. A single drop was selected from this mist by the human observer.
Millikan’s total mass throughput was about a hundred drops. Millikan also found that a charge always appears to be in exact integer multiples of plus or minus e; in other words, the charge is quantized. Other elementary particles discovered later were also found to have a charge of plus or minus e. For example, the positron, discovered in 1932 by Carl David Anderson of the California Institute of Technology, is exactly the same as the electron, except that it has a charge of +e.
With a fully established reputation he turned to other studies and in 1916 he confirmed experimentally Einstein’s photoelectric equation thus providing convincing proof of the concept of photons and determining directly the value of Planck’s constant. Robert A. Millikan’s 1916 paper on the measurement of Planck’s constant was dramatic in its time.
Today it lends itself to different, yet complementary, readings–the judgment by physicists that the work was worthy of the Nobel Prize, and the historical insight it offers into the struggles Millikan faced accepting the very quantum theory he was validating.
While it had been known for a long time that light falling on metal surfaces may eject electrons from them (the photoelectric effect), Millikan was the first to determine with great accuracy that the maximum kinetic energy of the ejected electrons obey the equation Einstein had proposed in 1905: namely, 1/2mv2 = hf – P, where h is Planck’s constant, f the frequency of the incident light, and P is, in Millikan’s words, “the work necessary to get the electron out of the metal.” Millikan determined h to have the value 6.57 x 10-27 erg-sec to “a precision of about 0.5 per cent,” a value far better than had been obtained in any previous attempt.
Millikan’s success was above all attributable to an ingenious device he termed “a machine shop in vacuo.” A rotating sharp knife, controlled from outside the evacuated glass container by electromagnetic means, would clean off the surface of the metal used before exposing it to the beam of monochromatic light.
The kinetic energy of the photoelectrons were found by measuring the potential energy of the electric field needed to stop them – here Millikan was able to confidently use the uniquely accurate value for the charge e of the electron he had established with his oil drop experiment.
Shining through it all are Millikan’s typical characteristics as experimenter and person: his penchant for experimenting in an area involving the hottest question of the day, his energetic persistence (this paper was the culmination of work he had begun in 1905), and his passion for obtaining results of great precision. In short, Millikan’s experiment was a triumphant work, of highest importance in its day, and richly deserving to be cited as part of his Nobel Prize award in 1923, given “for his work on the elementary charge of electricity and the photoelectric effect.”
To the historian, the volume in which Millikan’s paper appeared shows that physics in America was still a mixed bag. Other papers show that the main attention at that time is the experimental part of science, in which Americans were long regarded as most interested and most competent.
But the volume as a whole indicates that a good deal of the work going on in physics in this country in the early years of this century was still narrow and unambitious, even tending, for example, to descend to lengthy descriptions of improvements in basic equipment.
In an earlier paper (January 1916) in the same volume, Millikan writes in the very first sentence that “Einstein’s photoelectric equation… cannot in my judgment be looked upon at present as resting upon any sort of a satisfactory theoretical foundation,” even though “it actually represents very accurately the behavior” of photoelectricity.
Indeed, Millikan’s paper on Planck’s constant shows clearly that he is emphatically distancing himself throughout from Einstein’s 1905 attempt to couple photo effects with a form of quantum theory. What we now call the photon was, in Millikan’s view, “[the] bold, not to say the reckless, hypothesis” – reckless because it was contrary to such classical concepts as light being a wave propagation phenomenon. So Millikan’s paper is not at all, as we would now expect, an experimental proof of the quantum theory of light.
In 1912 Millikan gave a lecture at the Cleveland meeting of the American Association for the Advancement of Science, meeting jointly with the American Physical Society, in which he clearly regarded himself as the proper presenter of Planck’s theory of radiation.
With his usual self-confidence, Millikan confessed that a corpuscular theory of light was for him “quite unthinkable,” unreconcilable, as he saw it, with the phenomena of diffraction and interference. In short, Millikan’s classic 1916 paper was purely intended to be the verification of Einstein’s equation for the photoelectric effect and the determination of h, without accepting any of the “radical” implications which today seem so natural.
When Millikan’s Nobel Prize came to pass, his Nobel address contained passages that showed his continuing struggle with the meaning of his own achievement: “This work resulted, contrary to my own expectation, in the first direct experimental proof… of the Einstein equation and the first direct photo-electric determination of Planck’s h.” Yet it is difficult to find any published basis in Millikan’s experimental papers of that struggle with his own expectations. His internal conflict was of a somewhat different sort; while Millikan conceded that Einstein’s photoelectric equation was “experimentally established… the conception of localized light-quanta out of which Einstein got his equation must still be regarded as far from being established.” Ironically, it had been Millikan’s experiment which convinced the experimentalist-inclined committee in Stockholm to admit Einstein to that select circle in 1922.
One final irony: In 1950, at age 82, Millikan published his Autobiography, with Chapter 9 entitled simply “The Experimental Proof of the Existence of the Photon – Einstein’s Photoelectric Equation.” By then, Millikan had of course come to terms with the photon.
Moreover, he had evidently changed his mind about what he had done around 1916, for now he wrote that as the experimental data became clear in his lab, they “proved simply and irrefutably, I thought, that the emitted electron that escapes with the energy hf gets that energy by the direct transfer of hf units of energy from the light to the electron, and hence scarcely permits of any other interpretation than that which Einstein had originally suggested, namely that of the semi-corpuscular or photon theory of light itself.” In the end, Millikan re-imagined the complex personal history of his splendid experiment to fit the simple story told in so many of our physics textbooks.
During World War I, Millikan was Vice-Chairman of the National Research Council, playing a major part in developing anti-submarine and meteorological devices.
Robert Millikan has long-term collaboration with many famous scientists. Pictures below show him with famous scientists George Hale and Arthur Noyes when he was working at the University of Chicago (left) and later in Caltech (right):
Early in 1917 Millikan went to Washington to be executive officer of the National Research Council of the National Academy of Sciences, charged with war research on the detection of submarines and other essential problems. This work threw him into contact with the astrophysicist George Ellery Hale, one of America’s chief organizers of science. After the war Hale bombarded Millikan with requests to join him at the new and still obscure California Institute of Technology. Since physics was to be the centerpiece of the Institute and since Millikan was promised lavish funds and a free hand, in 1921 he agreed to come.
He left the University of Chicago to become director of the Norman Bridge Laboratory of Physics at the California Institute of Technology (Caltech) in Pasadena, Calif.; he was also made Chairman of the Executive Council of that Institute. Under his guidance Caltech almost immediately entered the top rank of American research centers. Convinced by his wartime experience that physics must be organized and funded for the benefit of the nation, Millikan soon became well-known to the public as a vigorous spokesman for science and education and a busy moneyraiser; he was also a promoter of the reconciliation of science with religion.
In Caltech he undertook a major study of the radiation that the physicist Victor Hess had detected coming from outer space. Millikan proved that this radiation is indeed of extraterrestrial origin, and he named it “cosmic rays.” With his collaborator Ira Bowen he meanwhile opened up the field of vacuum ultraviolet spectroscopy.
At the same time he continued his outstanding contributions to education, helping administer Caltech and personally attracting and inspiring a constant stream of students. As chairman of the executive council of Caltech from 1921 until his retirement in 1945, Millikan turned that school into one of the leading research institutions in the United States.
Robert Millikan won the 1923 Nobel Prize in physics for his work on the elementary electric charge and on the photoelectric effect.
His studies of the Brownian movements in gases put an end to all opposition to the atomic and kinetic theories of matter. During 1920-1923, Millikan occupied himself with work concerning the hot-spark spectroscopy of the elements (which explored the region of the spectrum between the ultraviolet and X-radiation), thereby extending the ultraviolet spectrum downwards far beyond the then known limit.
The discovery of his law of motion of a particle falling towards the earth after entering the earth’s atmosphere, together with his other investigations on electrical phenomena, ultimately led him to his significant studies of cosmic radiation (particularly with ionization chambers). Millikan’s research in Caltech was focused on the nature and origin of cosmic rays – Millikan coined the term “cosmic ray”. These investigations helped demonstrate the extraterrestrial source of this radiation and its variation in intensity with latitude.
Millikan’s instrunents developed in Caltech for the cosmic-ray study
Millikan became interested in cosmic rays following his World War I work in meteorology. His earliest measuring devices were small, visually read electroscopes – also called ionization chambers – built in the Caltech physics shop in the early twenties.
Millikan, Pearson and Bowen (left to right)
When Robert Millikan came to Caltech, he brought with him from the University of Chicago a first-class instrument maker named Julius Pearson (center of photo), as well as his research assistant Ira (‘Ike’) Bowen (right of photo). Starting in 1921, Millikan designed and had Pearson build in the Caltech physics shop a series of instruments to be used in his cosmic-ray investigations of the 1920s and 1930s.
Electroscope of cosmic-ray apparatus used by Millikan and Ira Bowen in 1932. By this time Millikan and Arthur Compton were locked in a much-publicized debate over the nature of cosmic rays – whether they were photons, as Millikan claimed, or charged particles, which was Compton’s view. Compton would be proved correct.
Small cosmic-ray electroscope
This small electroscope was sent aloft by a hydrogen-filled balloon by Millikan and Bowen from a Texas airfield in 1922. Its job was to measure the levels of ionizing radiation at different altitudes. This was accomplished by photographically recording the deflection of two electrically charged quartz fibers within the chamber as the air around them became increasingly ionized during the balloon’s ascent through the atmosphere. The whole apparatus weighed approximately seven ounces.
Compass used by Robert Millikan in his cosmic-ray work.
High-pressure electroscope (30 atmospheres) with eyepiece, used more than any other by Millikan in his underwater cosmic-ray experiments. The instrument was made in Caltech’s physics shop around 1929 and was used to calibrate other instruments for many years.
GM Detector of Robert Millikan, ca. 1939
Several hundred of these geiger mueller (GM) detectors were built in 1939 at the Caltech physics laboratory for use in cosmic ray studies. The above example is approximately 12 inches long and is made of copper. The paper label identifies three dates: August 2, 1947; January 25, 1948; and July 8, 1950.
The 1947 date refers to balloon flights performed at different latitudes from Texas to Saskatoon. A typical flight would carry the instruments to 70,000 to 80,000 feet. The 1948 date refers to experiments performed in a B-29 bomber flying at 30,000 feet from Hudson Bay to Lima, Peru. Robert Millikan and Victor Neher were among the personnel on this flight.
Professor Millikan has been President of the American Physical Society, Vice-President of the American Association for the Advancement of Science, and was the American member of the Committee on Intellectual Cooperation of the League of Nations, and the American representative at the International Congress of Physics, known as the Solvay Congress, at Brussels in 1921.
He held honorary doctor’s degrees of some twenty-five universities, and was a member or honorary member of many learned institutions in his country and abroad. He has been the recipient of the Comstock Prize of the National Academy of Sciences, of the Edison Medal of the American Institute of Electrical Engineers, of the Hughes Medal of the Royal Society of Great Britain, and of the Nobel Prize for Physics 1923.
He was also made Commander of the Legion of Honour, and received the Chinese Order of Jade. Millikan helped establish a program to support American students recieve their doctoral degrees in physics and chemistry. American universities developed programs to attract such able students. The program was a success and it was soon extended to mathematics and the biological sciences.
Robert Millikan was closed to many famous scientists of his time including Einstein, Nernst, Planck, Sklodowska-Curie:
Five Nobel Prize laureates are talking: Walter Nernst, Albert Einstein, Max Planck, Robert Millikan
and Max von Laue (left to right), Berlin 1931
Maria Sklodowska-Curie talking with Robert A. Millikan
Nobel laureates on Cal Tech’s faculty are (left to right) Robert A. Millikan, Thomas Hunt Morgan and Carl D. Anderson
Portraits of Professor Robert A. Millikan (drawings and paintings)
Later Professor Millikan was accused for scientific misconduct of his famous oil drop experiment, chauvinism and even anti-Semitism, however it is not clear how right are these accusations (see the details of the dispute).
He was author or co-author of the following books: A College Course in Physics, with S.W. Stratton (1898); Mechanics, Molecular Physics, and Heat (1902); The Theory of Optics,with C.R. Mann translated from the German (1903); A First Course in Physics, with H.G. Gale (1906); A Laboratory Course in Physics for Secondary Schools,with H.G. Gale (1907); Electricity, Sound, and Light, with J. Mills (1908); Practical Physics – revision of A First Course(1920); The Electron (1917; rev. eds. 1924, 1935).
Throughout his life Millikan remained a prolific author, making numerous contributions to scientific journals. He was not only a foremost scientist, but his religious and philosophic nature was evident from his lectures on the reconciliation of science and religion, and from his books: Science and Life(1924); Evolution in Science and Religion (1927); Science and the New Civilization (1930); Time, Matter, and Values (1932). Shortly before his death he published Electrons (+ and â€“), Protons, Photons, Neutrons, Mesotrons, and Cosmic Rays (1947; another rev. ed. of The Electron, previously mentioned,) and his Autobiography (1950).
Some of the Millikan’s papers are available on-line:
Robert Andrews Millikan
The electron and the light-quant from the experimental point of view
Nobel Lecture, May 23, 1924
Robert Andrews Millikan
On the Elementary Electrical Charge and the Avogadro Constant
Phys. Rev. 1913, 11, 109-143.
Robert and Greta Millikan, 1950
Robert and Greta Millikan in their later years
Robert Millikan passed away on the 19th of December 1953 in San Marino, California, at the age of 85. He is buried in Forest Lawn Memorial Park, Glendale, Los Angeles County, California, USA