Abstracts of Technical Articles from Bell System Sources 



Electricity in Gases} Karl K. D arrow. The material in this 

 paper was presented as the Joseph W. Richards Memorial Lecture 

 delivered before the Electrochemical Society at Cincinnati, April 23, 

 1936. This lecture presents in a vividly descriptive manner some of 

 the material published in past issues of the Bell System Technical 

 Journal. 



Electron Diffraction Experiments Upon Crystals of Galena} L. H. 

 Germer. Cleaved surfaces of galena crystals yield electron diffraction 

 patterns made up of Kikuchi lines, and spots which are drawn out into 

 streaks by refraction. After etching, the spot pattern predominates 

 and the individual spots are sharp. The lines are then rather diffuse 

 and ill-defined. Rocking curves upon various Bragg reflections from 

 the surface plane prove that the imperfection of a certain crystal does 

 not exceed about 15 minutes, and that the projections through which 

 the electrons pass are relatively thick. Estimates of imperfection and 

 thickness made from rocking curves are in approximate agreement with 

 those obtained from widths of Kikuchi lines. 



A galena crystal which has been filed or ground parallel to a cube 

 face exhibits two dilTerent sorts of surfaces. There are smooth 

 "mirror" surfaces from which large blocks of the crystal have been 

 mechanically torn, and there are very deeply scratched portions of the 

 surface. The "mirror" surfaces give diffraction patterns which are 

 qualitatively similar to patterns from cleaved surfaces, although there 

 are notable differences. From mirror surfaces produced by filing, 

 Kikuchi lines are very diffuse or are entirely missing, and diffraction 

 spots form an extended array. The diffuseness of the lines and the 

 extent of the array of spots correspond to great crystal imperfection, or 

 to exceedingly thin projections. Reasons are advanced for believing 

 in imperfection rather than extreme thinness. 



The deeply scratched portions of the surface of a galena crystal give 

 diffraction patterns which are entirely unlike patterns from cleaved 

 surfaces. Before etching, Debye-Scherrer rings are produced. After 

 a light or moderate etch a complex pattern appears, the nature of which 

 is related to the angle between primary beam and direction of filing. 



1 Transactions Electrochemical Society, Vol. LXIX, 1936. 

 ^Phys. Rev., October 1, 1936. 



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