914 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



planes of W have different work functions, it is to be expected that different 

 regions of the point will emit various current densities. Quantitative cal- 

 culations show that the ratio of highest to lowest current densities should be 

 at least 300. The current density might also be expected to vary if i^F 

 varies due to small local elevations or depressions from the paraboloid. 

 Such hills and valleys or ridges might result in 10-fold variations in current 

 density. Both types of variations are illustrated in photographs which are to 

 follow. 



NORMAL CLEAN W Ba ON ENLARGED PLANES 



T = 2400" K V = 30 SECONDS T=1210°K V = 1 MINUTE 



T = 300*'K l-40/lA V=9000 T=300°K L=40//A V=7500 



Vs SECOND '/5 SECOND 



Fig. 2 — Field emission patterns from normal clean tungsten and from Ba on W with 

 enlarged planes. Note elliptical structure around central dark region in (b). The treat- 

 ment which conditioned the W point is given above the black line; the constants used 

 in taking the photograph are given below the lines. 



The various physical constants pertaining to the experimental tube are 

 summarized in the left part of Fig. lb. The error in the value of r is probably 

 less than 20% and the error in the magnification is probably less than 30%. 



Field Emission from a Paraboloidal Surface of a W 

 Single Crystal 



Reproductions of photographs of the screen are shown in Figs. 2, (a) 

 and (b), and 3 (a). Figure 3 (b) shows the indices of the principal regions and 

 zones appearing in these and subsequent reproductions. For the experi- 

 mental tube and for a point with a radius of curvature of 3 X 10~^ cm, the 

 distance between two nearest 211 planes is about 2000 A. Other dimensions 

 can be scaled off on the reproductions. In these and other reproductions, 

 the treatment given the surface is described in the upper part of the print- 



