THERMIONIC ELECTRON EMISSION 



459 



Another striking confirmation of the hypothesis that various crystals 

 of a filament have different work functions and thus emit electrons 

 with greatly varying intensities, is given by pictures of such filaments 

 obtained by means of electron optics.* Figure 21 shows an electron 



iL_^fl 





mmlj^ 



.9Rj 



Fig. 21 — Electron-micrograph (above) and photo-micrograph (below) of platinum 



ribbon. 



and photo-micrograph f of a portion of a platinum filament. Cor- 

 responding crystals have been labeled by 1, 2, 3 and 4. The reader 

 can find more cases of correspondence. Of course, a perfect cor- 

 respondence is not to be expected since two neighboring crystals may 

 have the same reflection properties for light while the electron emis- 

 sivities differ and vice versa. 



Fig. 22f — Electron-optical pictures at various stages of heat treatment for a nickel 

 surface coated with oxide. 



Figure 22 shows a series of electron-optical pictures taken by W. 

 Knecht 2« of part of a nickel surface covered with BaO and SrO. The 



* For an interesting and instructive account of the technique and applications, see 

 the book: Geometrische Elektronenoptik by E. Briiche and O. Scherzer, published by 

 J. Springer (Berlin, 1934). 



t I am gratefully indebted to Dr. C. J. Davisson and Mr. C. J. Calbick of these 

 laboratories for this figure. 



X We take this opportunity to thank the publishers of the Annalen der Physik for 

 permission to reproduce this figure from Knecht's article in Annalen der Phjsik 20, 

 180 (1934). 



