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BELL SYSTEM TECHNICAL JOURNAL 



nickel, and that the wave-lenghts X deduced from the diffraction- 

 patterns for beams of electrons of various speeds v agree with those 

 calculated from de Broglie's formula X = hjp = hjinv. A second and 

 independent demonstration was made by G. P. Thomson, who showed 

 later in the same year that beams of high speed electrons are diffracted 

 on transmission through thin films of polycrystalline metal, and that 

 electron wave-lengths computed from patterns so obtained verify the 

 de Broglie relationship. 



It will be well, before considering these earliest experiments in more 

 detail, to present certain others, made more recently, of which the 

 interpretations are more simple. The simplest experimental result 

 which suggests that electrons should be regarded as waves rather 

 than as particles, is perhaps the regular reflection of a beam of electrons 

 from the face of a crystal. The experimental arrangement used in 

 demonstrating this phenomenon is indicated on the left in Fig. 1. 



ELECTRON 

 GUN 



Fig. 1 — Schematic diagram of apparatus for determining angular distribution of 

 electrons scattered without loss of energy by a metalHc crystal, and curves revealing 

 specular reflection of 20-volt electrons. 



Electrons emitted by a hot filament are accelerated and formed into a 

 beam, and this beam is directed against the face of a crystal target at a 

 known angle of incidence. The target in this case is of nickel and its 

 face is parallel to one of the principal sets of atom planes of the crystal 

 (111 planes). The surface is etched and presents to the incident beam 

 a multitude of crystal facets parallel to the plane of the target face. 

 Some of the electrons on striking these facets are scattered without 

 loss of energy. The distribution of these full speed electrons in and 

 near the plane of incidence is then determined by explorations with a 



