The Diffraction of Electrons by a Crystal of Nickel 



By C. J. DAVISSON 



This article is taken from the manuscript prepared by the author for his 

 address at the joint meeting of Section B of the American Association 

 for the Advancement of Science and the American Physical Society on 

 December 28, 1927, at Nashville, Tennessee. An account of this work 

 giving fuller experimental details is given by Davisson and Germer in the 

 December, 1927, issue of the Physical Review. 



These experiments are fundamental to some of the newer theories in 

 physics. Until they were performed, it could be said that all experimental 

 facts about the electron could be explained by regarding it as a particle of 

 negative electricity. It now appears that in some way a "wave-length" is 

 connected with the electron's behavior. The work thus shows an interesting 

 contrast with the discovery of A. H. Compton that a ray of light (a light 

 pulse) suffers a change of wave-length upon impact with an electron, 

 the change of wave-length corresponding exactly to the momentum gained 

 by the electron. Until Compton's work, all the known facts about light 

 could be explained by thinking of light as a wave motion. The Compton 

 efifect seems to prove the existence of particles of light. 



Physics is thus faced with a double duality. Compton showed that light 

 is in some sense both a wave motion and a stream of particles. Davisson 

 and Germer have now shown that a beam of electrons is in some sense 

 both a stream of particles and a wave motion. 



At the same time, theoretical advances have been made which seem to 

 pave the way for an understanding of this curious situation. A general 

 account of these new developments was given by K. K. Darrow in his 

 series "Contemporary Advances in Physics" in the Bell System Technical 

 Journal for October, 1927. Some remarks on the relation of the Davisson 

 and Germer experiments to the new mechanics were given in this article, 

 p. 692 et seq. — Editor. 



THE experiments which I have been asked to describe are the 

 most recent of an investigation of the scattering of electrons 

 by metals on which we have been engaged in the Bell Telephone 

 Laboratories for the last seven or eight years. 



The investigation had its inception in a simple but significant 

 observation. We observed some time in the year 1919 that when a 

 beam of electrons is directed against a metal target, electrons having 

 the same speed as those in the incident beam stream out in all directions 

 from the bombarded area. It seemed to us at the time that these 

 could be no other than particular electrons from the incident beam 

 that had suffered large deflections in simple elastic encounters with 

 single atoms of the target. The mechanism of scattering, as we 

 pictured it, was similar to that of alpha ray scattering. There was a 

 certain probability that an incident electron would be caught in the 

 field of an atom, turned through a large angle, and sent on its way 

 without loss of energy. If this were the nature of electron scat- 

 tering it would be possible, we thought, to deduce from a statistical 

 study of the deflections some information in regard to the field of the 



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