The Discovery of Electron Waves * 



By C. J. DAVISSON 



'T^HAT streams of electrons possess the properties of beams of waves 

 -■- was discovered early in 1927 in a large industrial laboratory in the 

 midst of a great city, and in a small university laboratory overlooking 

 a cold and desolate sea. The coincidence seems the more striking 

 when one remembers that facilities for making this discovery had been 

 in constant use in laboratories throughout the world for more than a 

 quarter of a century. And yet the coincidence was not, in fact, in any 

 way remarkable. Discoveries in physics are made when the time for 

 making them is ripe, and not before; the stage is set, the time is ripe 

 and the event occurs — more often than not at widely separated places 

 at almost the same moment. 



The setting of the stage for the discovery of electron diffraction was 

 begun, one may say, by Galileo. But I do not propose to emulate the 

 gentleman who began a history of his native village with the happenings 

 in the Garden of Eden. I will take, as a convenient starting point, the 

 events which led to the final acceptance by physicists of the idea that 

 light for certain purposes must be regarded as corpuscular. This 

 idea after receiving its quietus at the hands of Thomas Young in 1800 

 return to plague a complacent world of physics in the year 1899. In 

 this year Max Planck put forward his conception that the energy of 

 light is in some way quantized. A conception which, if accepted, 

 supplied, as he showed, a means of explaining completely the distribu- 

 tion of energy in the spectrum of black body radiation. The quantiza- 

 tion was such that transfers of energy between radiation and matter 

 occurred abruptly in amounts proportional to the radiation frequency. 

 The factor of proportionality between these quantities is the ever- 

 recurring Planck constant, h. Thus was reborn the idea that light is 

 in some sense corpuscular. 



How readily this circumstantial evidence for a corpuscular aspect of 

 light would have been accepted as conclusive must remain a matter of 

 conjecture, for already the first bits of direct evidence pointing to the 

 same conclusion were being taken down from the scales and meters of 



* Nobel lecture delivered at Stockholm, Sweden, December 13, 1937 in connection 

 with the award of the 1937 Nobel Prize in Physics by the Swedish Academy of 

 Science to Dr. Davisson and Professor George P. Thompson of London. This lec- 

 ture was originally published in the year book Les Prix Nobel, 1937. 



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