The Scientific Work of C. J. Davisson 



By KARL K. DARROW 



THE very first piece of work which is published by a physicist who is 

 destined to be great is not often outstanding; but sometimes it has 

 curious affinities, accidental rather than causal, with aspects of the work 

 that was to come thereafter. In the first paper pubHshed by C. J. Davisson, 

 we find him working with electrons, concentrating them into a beam by 

 the agency of a magnetic field, directing them against a metal target, and 

 looking to see whether rays proceed from the target. True, the electrons 

 came from a radioactive substance, and therefore were much faster than 

 those of his later experiments. True also, he did not actually focus the 

 electron-beam. True also, the rays for which he was looking were X-rays, 

 and in these he took no further interest. Yet in nearly all of his subsequent 

 researches he was to use some of the principles of electron-focussing or elec- 

 tron-microscopy; in many, he was to look for things that were emitted by 

 the target on which his electrons fell. This maiden papei was presented be- 

 fore the American Physical Society at its meeting in Washington in April 

 1909; the printed version may be found in the Physical Review, page 469 

 of volume 28 of the year 1909. It was signed from Princeton University, 

 whither Davisson had gone as a graduate student. 



Another characteristic of Davisson's work in his later years was his fre- 

 quent study and use of thermionics. Already in 1911 we find him working 

 in this field — but it was thermionics with a difference. The word "thermi- 

 onics" now signifies, nearly always, the emission of electrons from hot 

 metals; but at first it included also the emission of positive ions from hot 

 metals and hot salts. Though neither useless nor uninteresting, the emission 

 of positive ions is now rated far below the effect to which we now confine 

 the name of thermionics: emission of electrons from hot metals is one of 

 the fundamental phenomena of Nature, and its uses are inimitable. It may 

 be plausibly conjectured that in 1911 the difference in the importance of 

 the two phenomena — emission of positive ions and emission of electrons — 

 was far less evident than it is now. Davisson, working under the British 

 physicist O. W. Richardson who was then professor at Princeton, estab- 

 lished that the positive ions emitted from heated salts of the alkali metals 

 are once-ionized atoms of these metals — that is to say, atoms lacking a 

 single electron. He also showed that if gas is present in the tube, it may 

 enhance the number of the ions but does not change their character. This 



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