Webster, David Locke
WEBSTER, DAVID LOCKE
(b. Boston, Massachusetts, 6 November 1888; d. Palo Alto, California, 17 December 1976)
David Locke Webster exemplified the generation of experimental physicists trained in the United States before World War I. He developed X-ray techniques as tools for atomic physics, beginning while a Harvard graduate student and continuing while chairman of the Stanford physics department. Although Webster’s most important research was done before World War II, his scientific publications appeared from 1912 to 1973. He was a member of the National Academy of Sciences (elected in 1923), the American Association for the Advancement of Science (vice president and chairman of the Section of Physics, 1932), and the American Association of Physics Teachers (vice president 1933–1934 and president 1935–1936), and an editorial board member for the first twenty volumes of Reviews of Modern Physics (1929–1948).
Webster was the younger son of Andrew Gerrish Webster, a leather manufacturer, and of Lizzie Florence Briggs, the daughter of a banker. In their upper-class household, he was exposed to Episcopalianism, Swedenborgianism, and Irish Catholicism, and became an avid sailor. As an adult he professed agnosticism, remained devoted to sailing, and took up flying. He was married twice: to Anna Cutler Woodman on 12 June 1912—they had two sons and two daughters and were divorced in 1951—and to Olive Ross on 18 September 1951. In addition to his academic activities, service as an army researcher in both world wars influenced his life and scientific work.
Webster entered Harvard College in 1906, received his S.B. degree in 1910, and continued his studies under the spectroscopist Theodore Lyman. Since Harvard would not accept a dissertation in theoretical physics, he did experimental work on the pressure dependence of light absorption in chlorine gas. Webster’s dissertation, completed in 1913, included theoretical sections on gravitation, the ether, and X-ray scattering, the latter a topic he pursued as a Harvard instructor. Also in 1913, Henry G. J. Moseley used an X-ray spectrometer to link X-ray spectra to atomic numbers, demonstrating the kinds of techniques that physicists might exploit to learn about atomic structure. In 1915 Webster, drawing on descriptions of William H. Bragg’s crystal spectrometer and his own training in conventional optics, built an X-ray spectrometer on an optical mount from the freshman laboratory and began to examine the tungsten spectrum.
Harvard’s leader in X-ray research was the physicist William Duane. Out of his laboratory came several experiments critical for the development of X-ray physics and quantum theory; Webster’s contributions were vital but somewhat clouded by his relationship to Duane. The senior scientist had already begun to study the relationship between incident cathode-ray energy and the frequency of an X-ray tube’s output when Webster began measuring spectra. Using Webster’s apparatus, Duane and Franklin L. Hunt performed the experiments codified as the Duane-Hunt law: There is a definite limiting frequency for X rays produced by electrons of a given energy, the product of energy and maximum frequency being very nearly Planck’s constant.
According to his notebooks and his recollections in a 1964 oral history interview, Webster made the same tests before turning his spectrometer over to Duane and Hunt, found the frequency limit, and experimentally determined the value of h. Webster realized that his results amounted to an effective argument for the quantum theory of X rays and against the pulse theory. A pulse treated by Fourier analysis, he noted, would have no maximum frequency, yet the experiments showed a sharp cutoff, which pointed to the quantum nature of the radiation.
In September 1917, Webster began a brief tenure as physics professor at the University of Michigan. Already at work on submarine detection for the Naval Consulting Board, in November he left Ann Arbor to join the Science and Research Division of the Army Signal Corps Air Service Reserve. In February 1918, he was the first man nominated for training as a “scientific pilot” to help design and test flying instruments. He flew over 215 hours on active duty, most of them at Langley Field, Virginia, and remained a member of the Air Service Reserve until 1924.
Webster returned to Michigan for only a semester, then was assistant professor in 1919 and 1920 at MIT. Dissatisfied there, he weighed an offer from General Electric but accepted the post of professor and chairman of the Stanford University physics department. At Stanford he replaced retiring professors with younger men, who joined him in X-ray studies and improved and expanded graduate education. To secure support within the university, Webster’s department emphasized physics education for engineering and premedical students. But heavy teaching loads and and a paucity of funds impeded research and discouraged prominent physicists from joining the department. Professor George R. Harrison, an alumnus, left for well-heeled MIT, and Webster was unable to secure a leading theorist for the staff until 1934, when Rockefeller Foundation support brought the émigré Felix Bloch.
Still, Webster was able to make progress with the apparatus he could afford. He and P. A. Ross collaborated in 1924 and 1925 to test the possible sources of the “tertiary radiation” reported by Duane and George L. Clark, who disputed the existence of the Compton effect. Webster and Ross verified Compton’s position—an important result because it was the first to come from neither Compton nor Duane.
During the late 1920’s and early 1930’s, Webster and his students concentrated on measuring K-lines in X-ray spectra (the discrete radiations emitted by atoms when an electron is removed from an interior orbital), using them as clues to atomic structure and processes. They specialized in observing the characteristic radiations of moderately heavy elements, such as gold and silver, produced by electron bombardment of thin foils. Without intervening matter, the energy of the cathode ray as it reached the affected atom could be known precisely, and in 1933 the Stanford group published a nonrelativistic quantum theory treatment of the ionization cross section of interior electrons that took into account nuclear attraction.
With this success behind him, and Bloch at Stanford, in 1935 Webster hoped to go on to heavier elements and higher energies. He and the physics faculty considered building a 102-foot X-ray tube; unable to find support for it, they turned their attention to the resonance methods for accelerating electrons in a conducting sphere developed by their Stanford colleague William W. Hansen, Webster’s former student. With war on the horizon, though, Webster and Hansen soon joined Sigurd and Russell Varian to develop the klystron, a source of microwave power for aircraft detection and instrument landing systems.
Klystron research meant sharing laboratory space with engineers from the sponsoring Sperry Gyroscope Company, an arrangement Webster found intolerable. He left the project, after contributing a mathematical analysis of the klystron’s workings, and in 1941 found a new role in the preparedness effort as an author of pilot training manuals for the Civil Aeronautics Authority. From 1942 to 1945, Webster worked on bazookas and aircraft rockets at Aberdeen Proving Ground. During his absence, plans were laid at Stanford for a special laboratory where microwave research could continue under Hansen after the war. After his experience with Sperry, Webster opposed the project, and it was finally agreed that he would give up the department chair for his final years before retirement.
Physics education took up most of Webster’s time after World War II. He served on the Coulomb’s Law Committee of the American Association of Physics Teachers, charged with rationalizing instruction in electromagnetism, and wrote a long entry on electricity for the Encyclopaedia Britannica. After retirement in 1954, he joined the University of Hawaii’s tsunami research project, helping to model the diffraction of tidal waves around small islands as he had once contemplated the dispersion of X rays by crystals. In 1962 he became a consulting physicist at NASA’s Ames Research Center, where he contributed to the calculation of electromagnetic fields in space, Suffering from uremic poisoning, Webster finally left NASA in 1975, and died the following year.
I. Original Works. Webster’s papers are held in the Stanford University Archives; they include laboratory notebooks and teaching notes from his Harvard days until his death, journals and records saved during army service in World War I, records and correspondence from twenty years as chairman of the Stanford physics department, an autobiographical essay, and materials from postwar activities on behalf of the AAPT Coulomb’s Law Committee, Encyclopaedia Britannica, tsunami research, and NASA. In addition, two oral history interviews with Webster are on file at the Center for the History of Physics, American Physical Society. His publications are listed in the memoir by Kirkpatrick (see below), including his General Physics for Colleges, (New York, 1923), with Herman W. Farwell and Elmer Reginald Drew.
II. Secondary Literature. Paul Kirkpatrick, “David Locke Webster,” in Biographical Memoirs. National Academy of Sciences, 53 (1982), 367–400, gives much information on Webster’s personality and family background, as well as his scientific work. Kirkpatrick also makes the case for Webster’s anticipation of Duane and Hunt, based on Webster’s research notebooks, in “Confirming the Plank-Einstein Equation hv = (½12)mv2,” in American Journal of Physics, 48 (1980), 803–806. For Webster’s Harvard education and role in the elaboration of the old quantum theory, see Katherine Russell Sopka, Quantum Physics in America, 1920–1935 (New York, 1980; repr. Los Angeles, 1988). Arthur Holly Compton and Samuel K. Allison, X-Rays in Theory and Experiment (New York, 1935), summarizes Webster’s most important work and integrates it with contemporary X-ray physics. On the dispute between pulse and wave theories in the early years of X-ray physics, and Webster’s argument against the pulse theory, see Bruce R. Wheaton, The Tiger and the Shark (Cambridge, 1983); and on his involvement in the Duane-Compton controversy of the 1920’s, see Roger H. Stuewer, The Compton Effect: Turning Point in Physics (New York, 1975). On building up the Stanford physics department, in competition with Berkeley and CalTech, see Robert Wayne Seidel, “Physics Research in California: The Rise of a Leading Sector in American Physics” (Ph.D. diss., University of California, Berkeley, 1978); and on the difficult transition to postwar research, see Stuart W. Leslie and Bruce Hevly, “Steeple Building at Stanford: Electrical Engineering, Physics, and Microwave Research” in Proceedings of the IEEE, 73 (1985), 1169–1180.