He is particularly well known for his highly accurate determination of the charge of an electron via his classic oil drop experiment. Along with his work on the photoelectric effect, the feat garnered him the Nobel Prize in Physics in 1923. Interestingly, Millikan’s research achievements promoted the general acceptance of both Niels Bohr's quantum theory of the atom and Albert Einstein’s photoelectric equation, an important step precipitating their recognition by the Nobel Foundation in 1922 and 1921, respectively, and, more importantly, placing modern physics on a firm foundation. During the latter part of his career, Millikan led the way toward establishing the California Institute of Technology as a world-class research institution.
Born in the small town of Morrison, Illinois, Millikan spent most of his childhood in a rural setting. His mother had been dean of women at a Michigan college before she relocated to Illinois and his father was a Congregational minister. The Millikan family, which included several other children in addition to Robert, moved to Maquoketa, Iowa, in 1875. At Maquoketa High School, Millikan was an excellent student. After he graduated in 1886, he was employed for a short period as a court reporter before enrolling in Oberlin College in Ohio. Initially Millikan had no intention of pursuing a scientific career, but before his junior year at Oberlin he was urged to teach an elementary physics class by his Greek professor, who was confident in Millikan's ability to quickly grasp the subject despite little formal instruction in it. Millikan spent a summer learning the subject by reading books and working out physics problems, and the next year he taught the class with great enthusiasm. Ironically, despite his own ability as a lecturer, he generally considered the lecture itself an outdated mode of teaching, perhaps because he had himself gained a thorough knowledge of a subject without its benefit.
Millikan’s first experience teaching physics spawned a lifelong love of the field, and he remained at Oberlin for two years after he received his undergraduate degree in 1891 to obtain his master’s and continue his role as an instructor. Subsequently, he embarked on a graduate career at Columbia University, where he received a fellowship and was the only graduate student majoring in physics at the time. He carried out much of his study under the guidance of renowned physicist and inventor Michael Pupin, who was teaching in the electrical engineering department. Pupin instilled in Millikan the importance of experimental precision and also encouraged him to spend a year abroad after he received his Ph.D. in physics in 1895, even going so far as to loan him the funds to make this possible. Pupin was correct in his estimation of what opportunities might come of such travel; the trip occasioned Millikan’s study with such influential physicists as Max Planck and Walther Nernst at a time when physics was being permanently transfigured by the discovery of X-rays and radioactivity.
Upon his return form Europe, Millikan accepted a position as assistant to Albert Michelson at the University of Chicago’s Ryerson Laboratory. He eventually became a full professor at the school in 1910, maintaining the appointment until 1921. Millikan spent a significant amount of time in his early years at Chicago trying to establish a solid physics curriculum, for the field of physics was then in its initial stages of development in America. As part of this endeavor, he authored and co-authored numerous texts on a range of topics, including physics, mechanics, optics, heat and electricity. His belief in the preeminence of problem solving and hands-on laboratory work as opposed to lectures is apparent in these books, some of which were used by several generations of students. Despite his considerable writing and teaching duties, Millikan found time to carry out important original research and to start a family, marrying in 1902 Greta Blanchard, with whom he would have three sons.
One of Millikan’s most significant research projects began around 1907, when he decided that there must be a method of more accurately determining the charge of an electron than those already in use. In particular, he sought a way to refine a technique developed by scientists at the Cavendish Laboratory in Cambridge, England, involving a falling mist of water droplets that resulted in significant variation in the charge value obtained. At first Millikan only made minor adjustments to the other method, but soon discovered a way of observing one water droplet at a time, a tremendous step forward. Later he further realized that the difficulty introduced in the experiment by the evaporation of the water could be completely eliminated by substituting oil droplets for water droplets. The end result was the incredibly precise Millikan oil drop experiment.
Millikan’s experiment featured a closed, see-through chamber with two parallel metal plates, one of which was placed toward the top of the chamber and the other at the bottom. The plates were connected to a battery so that a current could be used to render one plate positive and the other negative. To begin the experiment, he sprayed a mist of oil into the chamber above the top plate, which contained a pinhole that allowed only a few droplets at a time to pass through to the space between the plates. This space was ionized by radiation so that the falling droplets acquired electrons from the air, and was illuminated in such a way that the drops could be easily seen with a microscope set up for their observation. By allowing the droplets to fall, Millikan was able to calculate the mass of the droplets. Then, by adjusting the voltage applied to the plates, he could affect the downward motion of the droplets, stopping it altogether when the upward electric force was perfectly balanced against the force of gravity. Thus, he could suspend single droplets in the air. Once he determined the voltage needed to achieve this suspension for a particular droplet as well as the mass of the droplet, Millikan was capable of successfully calculating its overall electric charge. By carefully carrying out this process over and over again, he clearly demonstrated that the overall electric charge on a droplet was always a whole-number multiple of a particular value (the charge on a single electron), which he calculated to be about 1.592 • 10(E-19) coulomb. Today the elementary charge constant used is slightly larger (1.602 • 10(E-19) coulomb), but this difference is attributable to Millikan's use of a slightly inaccurate value for the viscosity of air rather than any shortcoming of his experiment.
Following his success in calculating the charge of an electron, Millikan applied similar precision to a study of the photoelectric effect. His work in this area enabled him to verify Einstein’s equation of the photoelectric effect, which established a linear relationship between energy and frequency, and to determine the value of Planck’s constant directly. Subsequent research by Millikan included important investigations of Brownian motion, ultraviolet spectra and cosmic rays, the latter of which is a term he coined for ionizing radiation of extraterrestrial origin.
Beginning in the 1920s, after he became the chair of the Executive Council of the California Institute of Technology and director of its Norman Bridge Laboratory, Millikan spent much of his time promoting science education, fundraising and carrying out administrative duties. His skills in these areas were as strong as his experimental skills, a fact that helped him make Caltech a haven for many of the top scientific minds in the world. Yet Millikan never stopped pursuing research even after he officially retired from Caltech in 1946. He died in Pasadena on December 19, 1953, having admirably spent his life attempting to abide by his own personal philosophy, which, as he described it, was “to start building on my own account that better world for which I pray.”