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BELL SYSTEM TECHNICAL JOURNAL 



distance from the cathode. We can in fact calculate the potential at any 

 distance along the beam by the well known Child's law equation 



/ = 2.33 X 10~'^F''V^' 

 a . 



CATHODE 



k l I I I I I I 



• .-AAAi^VNAi'V%A/^\\^^.\AiA/V\iv^/X^i'Vv/vi^A/V^iAA/4/\VvKA^Xv\A/^ 



-11 ' ' 



' + 



Fig. 9 — Avoiding beam divergence by means of a longitudinal electric field. 



Here V is the anode voltage, x the cathode -anode spacing, / the current 

 in amperes and A the cathode area. 

 Suppose we take as an example 



.1 = 1 cm- 



/ = .01 amp. 



X = 10 cm 



