MYTH 19

THAT THE MILLIKAN OIL-DROP EXPERIMENT WAS SIMPLE AND STRAIGHTFORWARD

Mansoor Niaz

By a brilliant method of investigation and by extraordinarily exact experimental technique Millikan reached his goal.

—Allvar Gullstrand, Presentation speech to award Nobel Prize to Robert A. Millikan (1924)

[The] Millikan oil-drop experiment [was the] first direct and compelling measurement of the electric charge of a single electron. It was performed originally in 1909 by the American physicist Robert A. Millikan, who devised a straightforward method of measuring the minute electric charge that is present on many of the droplets in an oil mist.… 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.

—“Millikan’s Oil-Drop Experiment,” Encyclopedia Britannica

What is electricity? This seems to be a simple question and may be discussed these days even in a primary-school classroom. The manifestation of electricity can be demonstrated by the movement of a pith ball when brought in contact with a glass rod rubbed with silk. Around 1750, Benjamin Franklin (1706–1790) was perhaps the first to suggest the concept of electrical particles or atoms. In 1881, George Johnstone Stoney (1826–1911) first made an estimate of the ultimate electrical unit and named it the electron. Joseph John Thomson (1856–1940) presented experimental evidence based on cathode-ray experiments that led to the determination of the charge to mass ratio of particles, which were later recognized as universal charged particles and finally led to the discovery of the electron.

The determination of the elementary electrical charge (e) aroused considerable interest in the scientific community and prompted Robert A. Millikan (1868–1953) to become deeply involved in its measurement. Millikan’s oil-drop experiment is generally considered to be a simple, beautiful, and straightforward experiment that unambiguously led to the determination of the elementary electrical charge. According to a poll conducted for Physics World, its readers considered the oil-drop experiment to be one of the ten “most beautiful” experiments of all time. In this chapter, I explain that the experiment was not only difficult to perform but also elusive with respect to its interpretation, leading to considerable controversy in the scientific community, which lasted for many years.1

From Pith Balls to Water Droplets to Oil Droplets

Physical scientists consider the elementary electrical charge (e) to be an important milestone in understanding the electrical nature of matter. However, some of the early experiments, such as those done by Thomson at the Cavendish Laboratory in Cambridge, were extremely difficult to design. Early researchers often conducted their experiments in cloud chambers in which they could observe clouds of charged water droplets moving in electrical and gravitational fields. These experiments were subject to various errors, such as the evaporation of the water droplets and the instability, distortion, and lack of sharpness of the surface of the cloud, which made measurements difficult and uncertain. In 1906, when Millikan began performing similar experiments at the University of Chicago, he faced the same difficulties. Thus, he innovated by using a 10,000 V battery instead of the 4,000 V battery used in previous experiments. This led to an entirely unexpected result: clouds of water droplets disappeared while a small number of individual drops remained, which could be easily observed as bright points by illumination with light. This brought the decade-long technique of measuring electrical charges by the formation of clouds to an abrupt end.2

Using his new technique, Millikan obtained results that he presented at the meeting of the British Association for the Advancement of Science held in Winnipeg (Canada) in August 1909. Despite his partial success, several sources of error—such as the gradual evaporation of the individual water droplets, the lack of uniformity of the electric field, and the difficulty of holding a drop under observation for more than a minute—remained. Thus, Millikan introduced various changes in his experiments, the most important of which was arguably the replacement of water with oil (thus avoiding rapid evaporation). Millikan later recalled that the idea of using oil instead of water occurred to him suddenly while he was riding the train back to Chicago from the Winnipeg meeting, where he had met Ernest Rutherford (1871–1937), a pioneer in nuclear physics, and others working on similar problems. With the use of oil, Millikan achieved a series of discoveries: (1) oil drops act essentially like solid spheres; (2) the density of the oil drops is the same as that of the oil in bulk; and (3) oil produces a better estimate of the frictional force exerted on spherical drops as they move in the surrounding fluid. He published these results in 1913.³

Controversies

Felix Ehrenhaft (1879–1952), working at the University of Vienna, conducted experiments based on metal drops (rather than oil), which were quite similar to those of Millikan. In fact, both scientists obtained experimental data that were quite similar. Millikan, however, postulated the existence of a universal charged particle (the electron), whereas Ehrenhaft postulated the existence of subelectrons based on fractional charges. The controversy started when Ehrenhaft recalculated Millikan’s data from oil drops and found a large spread of values of the electrical charge, quite similar to his own data. Ehrenhaft showed how Millikan’s method led to paradoxical situations. For example, Millikan considered two oil drops having very similar charges to have different numbers of electrons. Colleagues wondered how to explain these differences. The controversy between the two lasted for many years, from around 1910 to 1923, when Millikan was awarded the Nobel Prize. Recently opened archives for the Nobel Prize show that although Millikan was a nominee for the prize from 1916 on, the prize committee recommended that it be withheld as long as the controversy with Ehrenhaft continued.4

Almost fifty-five years later, in 1978, Gerald Holton (b. 1922) added a new dimension to the controversy with his discovery of Millikan’s two laboratory notebooks at the California Institute of Technology. The Millikan notebooks have 175 pages of data from experiments conducted between October 28, 1911, and April 16, 1912, much of the content used in his Physical Review article in 1913. In these notebooks, Holton found data from 140 drops, but the published article reported results from only 58 drops. What happened to the other 82 drops? It seems that Millikan made a rough calculation for the value of e as soon as the data for the times of descent/ascent of the oil drops started coming in and ignored any experiment that did not give the value of e that he expected. More recently, after having read a preliminary version of this essay, Holton added, “So even if Millikan had included all drops and yet had come out with the same result, the error bar of Millikan’s final result would not have been remarkably small, but large—the very thing Millikan did not like.”⁵

This leads to the question: What was the warrant under which Millikan discarded more than half of his observations? Millikan’s guiding assumption, based on the atomic nature of electricity and the value suggested by the previous experiments of Ernest Rutherford at the University of Manchester, was a constant source of guidance. From his own experiments, Millikan had learned that all data could not be used because of difficulties associated with evaporation, sphericity, radius, change in the density of drops, and variation in experimental conditions (battery voltages, stopwatch errors, temperature, pressure, and convection). Like Millikan, Ehrenhaft had also obtained data that he interpreted as integral multiples of the elementary electrical charge (e)—as well as data for many drops that did not lead to an integral multiple of e. According to Holton, Ehrenhaft used data from all the drops that he studied, producing the impasse with Millikan. “It appeared,” Holton concluded, “that the same observational record could be used to demonstrate the plausibility of two diametrically opposite theories, held with great conviction by two well-equipped proponents and their collaborators.”6 Furthermore, Holton showed that (contrary to many commentators and textbook authors) Millikan had not measured the charge on the electron itself but rather the transfer of charge on drops as an integral multiple of the elementary electrical charge (e).⁷

The Oil-Drop Experiment in Textbooks and Laboratories

The oil-drop experiment is an important part of high school and introductory university physics and chemistry courses in almost all parts of the world. Thus, the Millikan-Ehrenhaft controversy can open a new window for students by demonstrating how two well-trained scientists could interpret the same data in two different ways. Studies based on thirty-one general chemistry textbooks and forty-three general physics textbooks published in the United States showed that none of the textbooks referred to the controversy; very few of them explained that Millikan did not measure the charge on the electron but rather the transfer of charge; and very few of them explained satisfactorily that the oil-drop experiment was extremely difficult to perform because of the incidence of various experimental variables.8

Most textbooks ignored one of the most important aspects of the oil-drop experiment—namely, the guiding assumptions of both Millikan and Ehrenhaft. Relying on previous research and historical antecedents, Millikan embraced the atomic nature of electricity. In contrast, Ehrenhaft adopted the anti-atomistic ideas of Ernst Mach (1838–1916) and hence advocated the existence of fractional charges (subelectrons). The textbook accounts similarly ignored various other essential elements of the oil-drop experiment, such as how scientists persevere in the face of difficulties, how they find new alternative interpretations, and how they deal with criticisms from their peers that lead to controversies. On the contrary, these textbooks endorse a vision of science that leads to the perpetuation of such myths as those about an accurate direct measurement of the charge on the electron, about developing a series of brilliant experiments which measured the elementary electrical charge of an electron, and about measuring the charge on the electron directly and forming the basis of measurements of great precision of this quantity. Interestingly, textbooks have continued to ignore the controversial nature of Millikan’s data-reduction procedures even after the publication of Holton’s study in 1978.⁹

In many parts of the world, the oil-drop experiment still forms an important part of laboratory instruction in undergraduate physics. One study based on eleven laboratory manuals published in the United States reported that they closely followed the portrayal of the experiment that appears in general physics and chemistry textbooks. However, some instructors and authors of manuals did refer to the difficulties involved in replicating the experiment—for example, in choosing which drops to observe. Probably many teachers would be surprised that Millikan himself faced the same dilemma, in one case discarding almost 59 percent of the drops studied.¹⁰

It seems that the oil-drop experiment continues to be considered in present-day textbooks as a simple, beautiful, precise, brilliant, and clever one, which provided convincing evidence for the determination of the elementary electrical charge (e). Interestingly, present-day undergraduate students do not consider the experiment either simple or beautiful but find it rather frustrating.