The Footprints of Robert Millikan -- A Life Sketch of an Outstanding Scientist

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Features: While working on research related to temperature, Millikan and Rutherford were merely two among numerous researchers in the field.② Moreover, Millikan's research on photoelectric effects did not produce the impact he had expected. His work on the phenomenon later known as "cold emission of metals" — the release of current from metal surfaces with a strong negative charge — yielded no results at that time. He himself believed that his initial efforts to explore fundamental problems of modern physics had failed. In his Autobiography, he recalled: "Up to now, as an experimental physicist, I have achieved almost nothing."③ As the decade at the University of Chicago drew to a close, Millikan was in a low mood. To be precise, he was a successful textbook author, respected in the Ryerson Laboratory at the University, and several attractive positions awaited him elsewhere. However, his desire to be recognized as a researcher remained unfulfilled. Despite dedicating "every spare hour" to research, he did not achieve significant results, nor did he earn the reputation of an "outstanding physicist in scientific research." He still urgently desired to "make a mark in the field of research."④ How could a 40-year-old physicist make a difference in the field of modern physics? In late 1907 and most of 1908, Millikan began a series of studies that later determined the charge of the electron and earned him the Nobel Prize. But why? Why enter a field where many others were already working and choose this particular problem? With the materials at hand, we cannot provide a definitive answer. However, we can make some well-founded inferences. First, although Millikan was not entirely satisfied with his previous experimental research on modern physics, he persisted in studying key issues of fundamental importance to advanced physics — the cutting-edge topics he and his colleagues believed to be crucial. He later recalled: "Everyone was interested in the value of the electron's charge, as it might be the most fundamental and constant quantity in the universe, yet its measurement has not even reached 100% accuracy to this day."① The charge of the electron, e, is also a fundamental quantity in modern physical theory; without an accurate knowledge of it, modern physics would be like a house of cards. Second, Millikan wanted to undertake work that would earn the respect of Michelson and his colleagues and attract the attention of top physicists worldwide. This factor may have played an equally important role in his decision. We can recall that Michelson had long believed that the "major fundamental principles" of physics had been "firmly established," leaving only the issue of precision — that is, applying these principles precisely to all physical phenomena, or, to put it another way, physics precise to the sixth decimal place.② This topic may not have appealed to young innovators, but it remained ambitious. It set a standard of caution, dedication, and excellence that the physics community could not afford to ignore. Millikan's choice of research direction was subtle. On one hand, events since 1895 had convinced him that the foundations of physics were undergoing profound changes, and questions about atomic structure and evolution had captured his imagination; on the other hand, he deeply respected Michelson's research methods and his high standards for "good work." If Millikan wanted to make a mark, he had to follow what Michelson respected: rigor, caution, and his efforts to stabilize the rapidly evolving field of physics. In a sense, Millikan's choice to measure the charge of the electron reflected both of these objectives. Such a measurement was a modern counterpart to Michelson's precise determination of the speed of light. Through this work, Millikan could both become the scientist Michelson had foreseen — precise to the sixth decimal place — and serve the fundamental needs of the new physics. In his 1910 paper titled "A New Modification of the Cloud-Method for Determining the Elementary Charge and the Most Probable Value of the Elementary Charge," he explained: "It is generally agreed that among all physical constants, two are particularly important: one is the speed of light, which appears in many fundamental equations of theoretical physics; the other is the elementary charge, the knowledge of which is essential for determining many other important quantities. Although the speed of light is now known to an accuracy of one in twenty thousand (thanks to Michelson), the value of the elementary charge is still far from being determined." Although the experiments Millikan conducted earlier in this field and his more precise experiments are detailed elsewhere,① a brief review of them may help us better understand the nature of his contributions. Millikan had long and carefully observed the work of Thomson and his students at the Cavendish Laboratory in Cambridge. Thomson and others developed a method called the "cloud method" to measure the charge of the electron. Another researcher at the Cavendish Laboratory, C. T. R. Wilson, observed that charged atoms or ions could serve as condensation nuclei to form clouds in supersaturated water vapor. In 1898 and 1899, Thomson applied Wilson's method to ionize air with X-rays and then measure the resulting clouds. After measuring the total charge in the clouds with an electrometer, Thomson indirectly measured the "average" charge on the cloud droplets.② Another of Thomson's students, H. A. Wilson, improved the cloud method. He generated clouds between two metal plates and applied an electric field of known strength that could be turned on and off at will. Using known theories, Wilson could deduce that after the electric field was turned off, the droplets would fall at a rate proportional to their weight under gravity. When the electric field was applied, the additional pull of the field would increase the speed. Wilson measured the falling rate of the droplets in the clearly defined upper surface of the cloud. By comparing the results with and without the electric field, he calculated the average weight of the droplets using Stokes's law, which describes the relationship between the falling speed of a small sphere in a liquid and its radius, density, and the viscosity of the liquid.① Since the additional pull of the electric field on the droplets is proportional to their charge, he derived a relationship for the charge e: where g is a known constant, E is the known electric field strength, and v? and v? are the speeds measured with and without the electric field, respectively. In Wilson's words, "Measure v? and v?, and you can determine the value of e."② Compared to Thomson's method, Wilson's method had obvious advantages: he did not need to estimate the number of droplets or ions in the cloud or assume that each droplet contained only one ion, because the falling speed of the droplets he measured showed that they carried the smallest possible unit of charge.③ However, this method still had some serious drawbacks. The final falling speed was not measured in the same cloud but in different parts of the cloud chamber. The calculation assumed that the volume of the droplets remained the same during all expansions. In other words, it was assumed that the measurements were conducted in the same cloud. This was not the case. As Millikan pointed out, it was difficult to maintain the cloud constant during a continuous evaporation process.④ Wilson himself also noted other difficulties: "Because the cloud begins to evaporate as soon as it forms, it is particularly important to measure the falling speed as quickly as possible." At the same time, the strength of the applied electric field was limited by the available batteries. "If observations could be conducted over a wider range of potential differences, the results would naturally be more satisfactory, but given the current batteries, achieving this is impossible."① Wilson determined 11 sets of e values, ranging from 2.0 × 10?1? to 3.8 × 10?1? electrostatic units, with an average of 3.1 × 10?1?. He did not fully acknowledge the inconsistency in the measurements and the uncertainty of the experiment, but he provided Millikan with a clear opportunity.