Weird Scientis Number 1: Clinton Davisson

This is the first in a series of blogs featuring the creators of Quantum Mechanics/Theory. These weird scientists (in the most respectable sense) are part of my not-yet-published book "Weird Scientists (not to be confuses with "Weird Science", a 1985 American teen fantasy comedy film).

Clinton Davisson

“We think we understand the regular reflection of light and X rays - and we should understand the reflections of electrons as well if electrons were only waves instead of particles ... It is rather as if one were to see a rabbit climbing a tree, and were to say ‘Well, that is rather a strange thing for a rabbit to be doing, but after all there is really nothing to get excited about. Cats climb trees—so that if the rabbit were only a cat, we would understand its behavior perfectly.’ Of course, the explanation might be that what we took to be a rabbit was not a rabbit at all but was actually a cat. Is it possible that we are mistaken all this time in supposing they are particles, and that actually they are waves?”

- Clinton Davisson

Clinton Joseph Davission (October 22, 1881 – February 1, 1958) studied the properties of subatomic particles, not cats or rabbits. Davisson was an American physicist who won the 1937 Nobel Prize in Physics for his discovery of electron diffraction. Davisson shared the Nobel Prize with George Paget Thomson, who independently discovered electron diffraction at about the same time as Davisson. Their results provided poof for Louis de Broglie's pioneering theory of wave-particle duality in quantum mechanics.

Biography

Early years

Davisson was born in Bloomington, Illinois, on 22 October 1881, the first of two children. His father, Joseph, who had settled in Bloomington after serving in the Civil War, was a contract painter and paperhanger by trade. His mother, Mary, occasionally taught in the Bloomington school system. Their home was, as Davisson's sister, Carrie, characterized it, "a happy congenial one—plenty of love but short on money."

Davisson, slight of frame and frail throughout his life, graduated from high school at age 20, in 1902. For his proficiency in mathematics and physics, he received a one-year scholarship to the University of Chicago; his six-year career there was interrupted several times for lack of funds. He acquired his love and respect for physics from Robert Millikan[i]; Davisson was "delighted to find that physics was the concise, orderly science [he] had imagined it to be, and that a physicist [Millikan] could be so openly and earnestly concerned about such matters as colliding bodies" (Gehrenbeck, 1978).

In 1905, upon the recommendation of Millikan, Davisson was hired by Princeton University as Instructor of Physics. He completed the requirements for his B.S. degree from Chicago in 1908, mainly by working in the summers. While teaching at Princeton, he did doctoral thesis research with Owen Richardson[ii]. He received his Ph.D. in physics from Princeton in 1911; in the same year he married Richardson's sister, Charlotte (Kelly, 1962) (Nobelprize.org, 1937).

Career

Before finishing his undergraduate degree at Chicago, he became a part-time instructor in physics at Princeton University, where he came under the influence of the British physicist Richardson, who was directing electronic research there. Davisson's PhD thesis at Princeton, in 1911, extended Richardson's research on the positive ions emitted from salts of alkaline metals. Davisson later credited his own success to having caught "the physicist's point of view—his habit of mind—his way of looking at things" from such men as Millikan and Richardson (Kelly, 1962).

After completing his degree, Davisson married Richardson's sister, Charlotte, who had come from England to visit her brother. After a honeymoon in Maine, Davisson joined the Carnegie Institute of Technology in Pittsburgh as an instructor in physics. The 18-hour-per-week teaching load left little time for research, and in six years there he published only three short theoretical notes. One notable break during this period was the summer of 1913, when Davisson worked with J. J. Thomson at the Cavendish laboratory in England.

Davisson was then appointed as an assistant professor at the Carnegie Institute of Technology. In April 1917, he was refused enlistment in the United States Army, because of his frailty (MacRae, 1972). In June of the same year he accepted war-time employment in the Engineering Department of the Western Electric Company (later Bell Telephone Laboratories), New York City—at first for summer, then, on leave of absence from Carnegie Tech., for the duration of the World War. His work was to develop and test oxide-coated nickel filaments to serve as substitutes for the oxide-coated platinum filaments then in use. At the end of the war, he resigned an assistant professorship to which he had been appointed at Carnegie Tech. to continue as a Member of the Technical Staff of the Telephone Laboratories (Nobelprize.org, 1937).

At the end of the war, Davisson accepted a permanent position at Western Electric after receiving assurances of his freedom there to do basic research. He had found that his teaching responsibilities at the Carnegie Institute largely precluded him from doing research (Kelly, 1962). The assignment that engaged Davisson and Lester Germer in their first joint effort reflects one of the chief interests of the parent company, AT&T, at this time: to conduct a fundamental investigation into the role of positive-ion bombardment in electron emission from oxide-coated cathodes. They published their results in the Physical Review in 1920, concluding that positive-ion bombardment has a negligible effect on the electron emission from oxide-coated cathodes (C. J. Davisson, 1920).

Davisson remained at Western Electric (and Bell Telephone) until his formal retirement in 1946. He then accepted a research professor appointment at the University of Virginia that continued until his second retirement in 1954 (Kelly, 1962).

Electron Diffraction and the Davisson-Germer Experiment

“Discoveries in physics are made when the time for making them is ripe, and not before.” (Davisson, 1965)

- Clinton Davisson

The Davisson–Germer experiment was a physics experiment conducted by American physicists Clinton Davisson and Lester Germer in 1927, which confirmed the de Broglie hypothesis. The de Broglie hypothesis says that particles of matter (such as electrons) have wave properties. This demonstration of wave–particle duality was important historically in the establishment of quantum mechanics and of the Schrödinger equation.

The experiment consisted of firing an electron beam from an electron gun on a nickel crystal at normal incidence (i.e. perpendicular to the surface of the crystal). The electron gun consisted of a heated filament that released thermally excited electrons, which were then accelerated through a potential difference of 54 V, giving them a kinetic energy of 54 eV. An electron detector was placed at an angle to obtain a maximum reading, and measured the number of electrons that were scattered at that particular angle (Germer, 1964) (Eisberg & Resnick, 1985).

Davisson, Germer and Calbick in 1927, the year they demonstrated electron diffraction. In their New York City laboratory are Clinton Davisson, age 46; Lester Germer, age 31, and their assistant Chester Calbick, age 23. Germer, seated at the observer's desk, appears ready to read and record electron current from the galvanometer[iii] (seen beside his head); the banks of dry cells behind Davisson supplied the current for the experiments.

Diffraction is a characteristic effect when a wave is incident upon an aperture or a grating, and is closely associated with the meaning of wave motion itself. In the 19th Century, diffraction was well-established for light and for ripples on the surfaces of fluids. In 1927, while working for Bell Labs, Davisson and Germer performed their famous experiment showing that electrons were diffracted at the surface of a crystal of nickel. This celebrated Davisson-Germer experiment confirmed the de Broglie hypothesis that particles of matter have a wave-like nature, which is a central tenet of quantum mechanics. In particular, their observation of diffraction allowed the first measurement of a wavelength for electrons. The measured wavelength agreed well with de Broglie's equation , where is Planck's constant and is the electron's momentum (Germer, 1964) (Davisson, 1965).

The sixth of January 1927 might well be regarded as the birthday of electron waves, for it was the day that data directly supporting the de Broglie hypothesis of electron waves were first observed. Note the peak deflection at 65 volts, and the detailed study of the region directly below. Calbick's handwriting is neat and cautious; Germer's is bold and expansive. Davisson made no entries in any of the research notebooks kept in the Bell Labs files.

Davisson and Germer succeeded where others had failed. In fact, the others (Walter Elsasser, E. G. Dymond, Patrick Blackett, James Chadwick and Charles Ellis), who had the idea of electron diffraction considerably ahead of Davisson and Germer, were not able to produce the desired experimental evidence for it. George Paget Thomson, who did find that evidence by a very different method, testified to the magnitude of the technical achievement as follows (Thomson, 1961):

"[Davisson and Germer's work] was indeed a triumph of experimental skill. The relatively slow electrons [they] used are most difficult to handle. If the results are to be of any value the vacuum has to be quite outstandingly good. Even now [1961] ... it would be a very difficult experiment. In those days it was a veritable triumph. It is a tribute to Davisson's experimental skill that only two or three other workers have used slow electrons successfully for this purpose."

From 1930-1937, Davisson devoted himself to the study of the theory of electron optics[iv] and to applications of this theory to engineering problems. He then investigated the scattering and reflection of very slow electrons by metals. During World War II he worked on the theory of electronic devices and on a variety of crystal physics[v] problems.

In 1946 he retired from Bell Telephone Laboratories after 29 years of service. From 1947 to 1949, he was Visiting Professor of Physics at the University of Virginia, Charlottesville, Va.
The National Academy of Sciences awarded Davisson the Comstock Prize in 1928. In 1931 Franklin Institute awarded him the Elliott Cresson Medal, and in 1935 the Royal Society (London) awared him the Hughes Medal. In 1941, the University of Chicago awarded him the Alumni Medal. He held honorary doctorates from Purdue University, Princeton University, the University of Lyon and Colby College.

Personal life

In 1911, Clinton married Charlotte Sara Richardson, a sister of Professor Richardson. Clinton and Charlotte Davisson had four children, including the American physicist Richard Davisson. Clinton died in Charlottesville on February 1, 1958, at the age of 76, and was survived by his wife, three sons and one daughter. The crater Davisson on the Moon is named after him.

Notes

[i] Robert A. Millikan (22 March 1868 – 19 December 1953) was an American experimental physicist, and Nobel laureate in physics for his measurement of the charge on the electron and for his work on the photoelectric effect. He served as president of Caltech from 1921 to 1945. He also served on the board of trustees for Science Service, now known as Society for Science & the Public, from 1921-1953.
[ii] Sir Owen Willans Richardson, FRS (26 April 1879 - 15 February 1959) was a British physicist who won the Nobel Prize in Physics in 1928 for his work on thermionic emission, which led to Richardson's Law—the current from a heated wire seemed to depend exponentially on the temperature of the wire.
[iii] A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil. The term has expanded to include uses of the same mechanism in recording, positioning, and servomechanism equipment.
[iv] Electron optics deals with the focusing and deflection of electrons using magnetic and/or electrostatic fields.
[v] Chystal physics (physical crystallography), is the study of the physical properties of crystals and crystalline aggregates and changes in the properties under the influence of various factors.

Works Cited

C. J. Davisson, L. H. (1920). Physics Review, 15, p. 330.

Davisson, C. (1965). The Discovery of Electron Waves. In P. 1.-1. Nobel Lectures. Amsterdam: Elsevier Publishing Company.

Eisberg, R., & Resnick, R. (1985). Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles (2nd ed.). New York: John Wiley & Sons.

Gehrenbeck, R. K. (1978, January). Electron diffraction:fifty years ago. Physics Today.Germer, L. H. (1964, July). Low-Energy Electron Diffraction. Physics Today, pp. 19-23.

Kelly, M. J. (1962). Clinton Joseph Davisson. In Biographical Memoirs Vol. XXXVI (pp. 52-79). New York: Columbia University Press.MacRae, A. U. (1972, January).

Lester H. Germer - Obituary. Physics Today, pp. 93-97.Nobelprize.org. (1937). Clinton Davisson - Biography. The Nobel Prize in Physics 1937.

Thomson, G. P. (1961). The Inspiration of Science. London: Oxford U.P.