478 BELL SYSTEM TECHNICAL JOURNAL 



sprung from the atom, so to speak, and it was natural that the first 

 applications should be to the atom. No thought was given at this 

 time, it appears, to electrons in free flight. It was implicit in the 

 theory that beams of electrons like beams of light would exhibit the 

 properties of waves, that scattered by an appropriate grating they 

 would exhibit diffraction, yet none of the chief theorists mentioned this 

 interesting corollary. The first to draw attention to it was Elsasser, 

 who pointed out in 1925 that a demonstration of diffraction would 

 establish the physical existence of electron waves. The setting of the 

 stage for the discovery of electron diffraction was now complete. 



It would be pleasant to tell you that no sooner had Elsasser's sug- 

 gestion appeared than the experiments were begun in New York which 

 resulted in a demonstration of electron diffraction — pleasanter still 

 to say that the work was begun the day after copies of de Broglie's 

 thesis reached America. The true story contains less of perspicacity 

 and more of chance. The work actually began in 1919 with the ac- 

 cidental discovery that the energy spectrum of secondary electron 

 emission has, as its upper limit, the energy of the primary electrons, 

 even for primaries accelerated through hundreds of volts; that there 

 is, in fact, an elastic scattering of electrons by metals. 



Out of this grew an investigation of the distribution-in-angle of these 

 elastically scattered electrons. And then chance again intervened; it 

 was discovered, purely by accident, that the intensity of elastic scatter- 

 ing varies with the orientations of the scattering crystals. Out of this 

 grew, quite naturally, an investigation of elastic scattering by a single 

 crystal of predetermined orientation. The initiation of this phase of 

 the work occurred in 1925, the year following the publication of de 

 Broglie's thesis, the year preceding the first great developments in the 

 wave mechanics. Thus the New York experiment was not at its 

 inception, a test of the wave theory. Only in the summer of 1926, 

 after I had discussed the investigation in England with Richardson, 

 Born, Franck and others, did it take on this character. 



The search for diffraction beams was begun in the autumn of 1926, 

 but not until early in the following year were any found — first one 

 and then twenty others in rapid succession. Nineteen of these could 

 be used to check the relationship between wave-length and momentum 

 and in every case the correctness of the de Broglie formula, X = hip 

 was verified to within the limit of accuracy of the measurements. 



I will recall briefly the scheme of the experiment. A beam of elec- 

 trons of predetermined speed was directed against a (111) face of a 

 crystal of nickel as indicated schematically in Fig. 1. A collector 

 designed to accept only elastically scattered electrons and their near 



