Seventy-five years Ago
"Lucky breaks" and Second Honeymoons.
5th February 1925 is the date in Lester Germer's notebook which records the discovery of a crack in the vacuum trap in the electron scattering apparatus he was working on with Clinton Davisson at Western Electric, New York.
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 | This was not the first time their equipment had broken, and not the first time they had "resurrected" their precious nickel crystal by heating it in vacuum and hydrogen.
This particular break and the subsequent method of repair, however, had a crucial role to play in the later discovery of electron diffraction. By 6 April 1925 the repairs had been completed and the tube put back into operation. During the following weeks, as the tube was run through the usual series of tests, results very similar to those obtained four years earlier were obtained. Then suddenly, in the middle of May, unprecedented results began to appear. These so puzzled Davisson and Germer that they halted the experiments a few days later, cut open the tube, and examined the target (with the assistance of the microscopist F. F. Lucas) to see if they could detect the cause of the new observations. What they found was this: The polycrystalline form of the nickel target had been changed by the extreme heating until it had formed about ten crystal facets in the area from which the incident electron beam was scattered. Davisson and Germer surmised that the new scattering pattern must have been caused by the new crystal arrangement of the target. In other words, they concluded that it was the arrangement of the atoms in the crystals, not the structure of the atoms, that was responsible for the new intensity pattern of the scattered electrons. |
Davisson (left) and Germer
in their New York City Laboratory, 1927
The notice on the equipment upon which Davisson
appears to be leaning reads "Do not touch this apparatus"
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-oOo-
Not all was to go well immediately, however.
" After an entire year spent in preparation, and with a new tube and a new theory in hand, they obtained experimental results that were even less interesting than those from the earliest experiments. The new colatitude curves showed essentially nothing, and even the new azimuth curves gave at best only a weak indication of the expected three-fold symmetry of the nickel crystal about the incident beam. Davisson must have been quite pleased with the prospect of getting away for a few months during the summer of 1926, when he and his wife had planned a vacation trip to relax and visit relatives in England. 'We should have a lovely time,' Davisson wrote to his wife, 'It will be a second honeymoon - and should be sweeter even than the first.'
Something was to happen on this particular trip, however, to turn it into more than the "second honeymoon" Davisson envisaged. Theoretical physics was undergoing fundamental changes at this time. In the early months of 1926 Erwin Schrodinger's remarkable series of papers on wave mechanics appeared, following Louis de Broglie's papers of 1923-24 and Albert Einstein's quantum-gas paper of 1925. These papers, along with the new matrix mechanics of Werner Heisenberg, Max Born and Pascual Jordan, were the subject of lively discussions at the Oxford meeting of the British Association for the Advancement of Science. Davisson, who generally kept abreast of recent developments in his field but appears to have been largely unaware of these recent developments in quantum mechanics, attended this meeting. Imagine his surprise, then, when he heard a lecture by Born in which his own and Kunsman's (platinum-target) curves of 1923 were cited as confirmatory evidence for de Broglie's electron waves! After the meeting Davisson met with some of the participants, including Born and possibly P.M.S. Blackett, James Franck and Douglas Hartree, and showed them some of the recent results that he and Germer had obtained with the single crystal. There was, according to Davisson, "much discussion of them." All this attention might seem strange in light of the relatively feeble peaks Davisson and Germer had obtained, but even these may have been exciting to physicists already convinced of the basic correctness of the new quantum theory. It may also reflect the fact that several European physicists, Walter Elsasser (Göttingen), E.G. Dymond (Cambridge, formerly Göttingen and Princeton), and Blackett, James Chadwick and Charles Ellis of Cambridge had attempted similar experiments and abandoned them because of the difficulties of producing the required high vacuum and detecting the low-intensity electron beams. Apparently they were encouraged by the results which had appeared so unimpressive to Davisson. At any rate, Davisson spent "the whole of the westward transatlantic voyage ... trying to understand Schrodinger's papers, as he then had an inkling . . . that the explanation might reside in them" - no doubt to the detriment of the 'second honeymoon' in progress."
The story is completed (the modern consequences of the experiments you can find in other pages) in Richard K. Gehrenbeck's excellent account in "Physics Today," 31 (1), 34 - 41 (1978), reprinted in the AVS publication "History of Vacuum Science and Technology" (ISBN: 0-88318-437-0)
Davisson shared the 1937 Nobel Prize for Physics with G.P.Thomson (son of J.J.). It is interesting that Davisson's involvement with the wave nature of the electron is better remembered than Thomson's. Perhaps that's the advantage of NOT having a famous father!
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