Footprints of Robert Millikan (A Life Sketch of an Outstanding Scientist)

Author: Robert H. Millikan (USA), translated by Fang Zaqing
Translator: Fang Zaqing / Country:
Publisher:
Publish Date: 1998-01-01
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 expected impact, and his work on what was later called the "cold emission of metals" — the phenomenon of electrically charged metal surfaces releasing current — yielded no results at that time. He himself believed that his initial efforts to explore the fundamental problems of modern physics had failed. In his Autobiography, he recalled: "So far, 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. Although he dedicated every spare hour to research, he did not achieve any significant results, nor did he earn the reputation of an "outstanding physicist in scientific research." He still desperately wanted 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 were already working and choose this particular problem? With the materials available to us, 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 in modern physics, he persisted in studying problems of critical importance to advanced physics — the cutting-edge topics he and his colleagues believed in. He later recalled: "Everyone was interested in the value of the electron 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 physics theory; without an accurate knowledge of it, modern physics would be like a house of cards. Second, Millikan wanted to do work that would earn the respect of Michelson and his colleagues and attract the attention of top physicists worldwide. This factor may have been equally important in his decision. We can recall that Michelson had long believed that the "major fundamental principles" of physics had been "firmly established," and what was needed was precision — that is, applying these principles precisely to all physical phenomena, or, to say, physics precise to the sixth decimal place. ② This topic may not have been appealing to young innovators, but it was still grand. It set a cautious, dedicated, and excellent standard 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 sparked 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 maintain stability in 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 replication of Michelson's precise determination of the speed of light. Through this work, Millikan could both become the scientist predicted by Michelson to achieve precision 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 recorded elsewhere, a brief review of them can 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 student of Thomson, H. A. Wilson, improved the cloud method. He created clouds between two metal plates and applied a known, variable electric field. 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 due to gravity. When the electric field was applied, the additional force from the field would increase the speed. Wilson measured the falling rate of the droplets with a 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 ball in a liquid and its radius, density, and the viscosity of the liquid. ① Since the additional force from 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 v1 and v2 are the measured speeds with and without the electric field, respectively. In Wilson's words, "By measuring v1 and v2, the value of e can be determined." ② Compared to Thomson's method, Wilson's method had clear 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 indicated 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 pointed out other difficulties: "Since 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 certainly be more satisfactory, but with the batteries available at present, 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 experiments, but he provided Millikan with a clear opportunity.