The Bell System Technical Journal 



Vol. XXI F July-October, I g45 Nos.3-4 



Physical Limitations in Electron Ballistics* 



By J. R. PIERCE 



Introduction 



THE subject of this talk is "Physical Limitations in Electron Bal- 

 Hstics". It is pleasant to have a chance to talk about such physical 

 limitations, because there is so little we can do about them. And, 

 although these limitations are apt to be discouraging, a knowledge of them 

 is very valuable, for it keeps us from spending time trying, like the in- 

 ventors of perpetual motion machines, to do the impossible. 



As electron ballistics is particularly subject to physical limitations, there 

 are so many that it is impossible to discuss all of them thoroughly at this 

 time. Also, many of the limitations are of a rather complicated nature, 

 and to deduce them from basic principles in a quantitative way requires 

 much thought and patience. I think the best I can do is to try to mention 

 most of the chief limitations, as a warning to the uninitiated that rocks lie 

 ahead in certain directions, but to concentrate attention on only a few of 

 them. I have chosen this evening to devote particular attention to lim- 

 itations that bear on the production and use of electron beams in which 

 considerable current is required, such as those used in cathode ray tubes 

 and high-frequency oscillators, and to mention only briefly as a sort of 

 introduction problems pertaining more closely to low-current devices such as 

 electron microscopes. 



The Wave Nature of the Electron 



One of the most important limitations in electron microscopy is the dual 

 nature, wave and corpuscular, of the electron. Without making any 

 attempt to justify or explain the combination of wave and particle con- 

 cepts which is characteristic of modern physics, we may describe its con- 

 sequence at once; very small objects don't cast distinct shadows. This 

 cannot be explained merely in terms of the physical size of the electron and 

 the object. When an electron beam is reflected from a surface of regularly 



* A lecture given under the auspices of the Basic Science Group of the New York Sec- 

 tion of the A.I.E.E., as a part of an Electron BaUistics Sj'mposium, Columbia University, 

 March 21, 1945. 



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