Millikan Oil Drop Experiment
Millikan’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.
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.
Millikan in lab in Liege, Belgium, June, 1922 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.
Robert Andrews Millikan was born on the 22nd of March, 1868, in Morrison, Ill. (U.S.A.), as the second son of the Reverend Silas Franklin Millikan and Mary Jane Andrews. His grandparents were of the Old New England stock which had come to America before 1750, and were pioneer settlers in the Middle West. He led a rural existence in childhood, attending the Maquoketa High School (Iowa). After working for a short time as a court reporter, he entered Oberlin College (Ohio) in 1886. During his undergraduate course his favourite subjects were Greek and mathematics; but after his graduation in 1891 he took, for two years, a teaching post in elementary physics. It was during this period that he developed his interest in the subject in which he was later to excel. In 1893, after obtaining his mastership in physics, he was appointed Fellow in Physics at Columbia University. 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.
On the instigation of his professors, Millikan spent a year (1895-1896) in Germany, at the Universities of Berlin and Göttingen. He returned at the invitation of A. A. Michelson, to become assistant at the newly established Ryerson Laboratory at the University of Chicago (1896). Millikan was an eminent teacher, and passing through the customary grades he became professor at that university in 1910, a post which he retained till 1921. During his early years at Chicago he spent much time preparing textbooks and simplifying the teaching of physics. 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).