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David M. Dennison
LSA Minutes

1900 - 1976

David Mathias Dennison, Professor Emeritus of Physics died in Ann Arbor on April 3, 1976. He was 75 years of age.

Professor Dennison was born in Oberlin, Ohio, on April 26, 1900. After receiving the B.A. degree from Swarthmore College in 1921, he began graduate work at the University of Michigan and in 1924 received his Ph.D. During this same year he was married to Helen L. Johnson, of Luddington, Michigan. He, with Mrs. Dennison, spent the next three years in Copenhagen and in Zurich where, as a Fellow of the General Education Board, he studied with Professors Niels Bohr and Erwin Schrlidinger. In the fall of 1927, the Dennisons returned to Ann Arbor where Professor Dennison joined the staff of the Physics Department of the University as an Instructor. In 1935 he became professor of Physics and during the ten years from 1955-65 he was Chairman of the Department.

Professor Dennison was the recipient of numerous honors. In 1950 he received an honorary D.Sc. from Swarthmore College. He was appointed delegate to the Seventh General Assembly of the International Union for Pure and Applied Physics in Copenhagen, Denmark, in 1951, He was selected as the University of Michigan's Henry Russel Lecturer in 1952. The lecturer, who is chosen by the Research Club, is always a senior faculty member who has achieved distinction in his field of scholarship. In 1953, Professor Dennison was elected to membership in the National Academy of Science, which is regarded as one of the highest honors for an American scientist. He received a Distinguished Faculty Achievement Award in 1963, and was elected president of the Research Club, 1964-65. In 1966 he was appointed Harrison M. Randall University Professor. For his work during World War II he received the Certificate for Exceptional Service from the U.S. Navy. In September 1975, Professor Dennison was invited as the principal speaker at the Fourth International Conference on Molecular Spectroscopy, at Tours, France, and as a further high distinction, he was made an honorary citizen of the City of Tours. That was only the second time in the city's long history that the honor had been bestowed upon anyone.

Professor Dennison's scientific work ranged widely, including the fields of molecular structure, the optical properties of thin films and the theory of high energy accelerators. During his career he published about 100 scientific articles.

Molecular structure was the subject that engaged Professor Dennison's main effort throughout his career. He made his mark in this field very early: 1927. At that time the specific heat of hydrogen was a great puzzle because measurements were in striking disagreement with calculations. Following the discovery of the spin of the electron in 1925, by George E. Uhlenbeck and Samuel A. Goudsmit (both of whom later joined the faculty of the University of Michigan) there had been hope that the problem would be solved if a spin were assigned to the proton. But such attempts had failed. Professor Dennison solved the problem beautifully by adding one new postulate to that of a spin for the proton: that the ortho and para states of the molecule were so long lived that transitions did not occur appreciably in the time during which measurements were made. The agreement between theory and experiment then became quite exact, provided the spin of the prnton was taken to be one half unit of angular momentum, that is, 1/4ff times Planck's constant. That was the first solid, quantitative evidence for the spin of the proton.

In 1932 Professor Dennison, with Professor George E. Uhlenbeck, solved the two-minima problem for the position of the nitrogen atom in the ammonia molecule, the configuration sometimes called the "reversing umbrella." This work predicted an absorption level at a very long wavelength (about 1.6 cm), and Professor Dennison recognized that this might be observable by means of microwaves from a magnetron. He inspired a colleague in the Physics Department, Professor Neil Williams, and a graduate student, Claud E. Cleeton, to attempt the experiment. They succeeded by making magnetrons that were almost microscopic, and a spectrograph consisting of parabolic reflectors and a venetian-blind grating, with a large bag of ammonia in the radiation path. This experiment was successful and it was ahead of its time--it anticipated by a decade or more the post-war burst of interest in microwave molecular spectroscopy.

Throughout the 1920's and 30's, the Physics Department at Michigan was an active center for infrared spectroscopy, under Professor Harrison M. Randall. During most of this period Professor Dennison worked closely with the experimentalists, interpreting results and proposing new experiments. The molecules that intrigued him most were those having physical constants in just the right range to exhibit particular phenomena most clearly: carbon dioxide with its Fermi resonances, water vapor with the full complexity of an asymmetric rotator, ammonia with an inversion frequency accessible to measurement with microwaves, and methyl alcohol with all the features of rotational tunneling. His work on methyl alcohol, which began in the 1930's and resulted in the first spectroscopic determination of an internal rotation barrier in molecules, continued to the end of his life.

The onset of World Warr II caused Professor Dennison to turn his energy toward problems that were far from molecular structure. He worked at the University of Michigan on the development and analysis of the proximity fuse, a project for the U.S. Navy Bureau of Ordnance. His speciality was analyzing the performance of the fuse, at first from results of model experiments and later from battle reports from the Pacific. He was able to recommend changes in the design and use of the fuse, which had an important influence on its effectiveness in the war.

Following the war, Professor Dennison returned to his favorite subject, molecular structure, but also worked on the design of the race-track synchrotron that was built at Michigan. With the late Theodore H. Berlin, he worked out the theory of the stability of orbits in a synchrotron with straight sections, a feature that became standard in all the large synchrotrons.

Throughout his long career at Michigan, Professor Dennison gave inspiration and warm understanding to all who knew him and worked with him, or attended his beautiful lectures. The benefits of his wise guidance of the Physics Department while he was Chairman and at other times will extend far beyond his own life span. His place in the lives of his colleagues will not be filled.

The immediate members of the family surviving Professor Dennison are his widow Helen Dennison and two sons, Edwin W. Dennison who resides in California and David S. Dennison who resides in New Hampshire.

I move that this memorial be adopted by the faculty of the College of Literature, Science and the Arts and that copies be sent to the members of the family.

H. Richard Crane