ADMITTANCES OT PARALLEL-PLANE ELECTRON TUBES 645 



cathode coating." Since the effect was not observed to such a marked degree 

 in the case of the diodes, it seems probable that this is not the correct 

 explanation. 



It is probable that the ol)serve(l variation in bn is a space charge effect. 

 Tt is evident in examining the diode curves that tubes which possessed the 

 higher values for ^o exhibited a greater variation in b. If maximum go can 

 be taken as a measure of the cathode activity, we can then perhaps relate 

 the variation in susceptance with cathode activity and hence with the loca- 

 tion of the potential minimum. A shift in the position of the potential 

 minimum, however, may produce two effects. It varies the transit time of 

 the electrons and changes the degree of space charge in the input space. 

 Either effect might account for the variation of bn- A clue to this effect 

 might be discovered by making measurements on structures with different 

 cathode-grid spacings. 



The following experiments were performed to determine the effect of plate 

 voltage on the input admittance of the triode of Fig. 20. The plate and grid 

 voltages were varied simultaneously in such a way that the sum of the direct 

 currents to the grid and plate remained constant at 30 milliamperes cor- 

 responding to a current density of 184 ma/cm^. The input admittance did 

 not vary from the value shown for this same current density in Fig. 20 even 

 though the plate voltage was varied from 250 volts to 40 volts. In a second 

 experiment the plate potential was maintained at —90 volts with respect to 

 the cathode and the grid potential was varied such that the direct grid cur- 

 rent varied over a range of to 10 milliamperes. Again the admittances were 

 found to be equal to those of Fig. 20 for the corresponding total currents. 

 These two experiments suggest that, for a given geometry, the value of 

 bn is primarily a function of the total current density in the input circuit. 



Acknowledgement 



The author wishes to acknowledge his indebtedness to the late Mr. A. E. 

 Bowen who contributed valuable advice during the course of these experi- 

 ments, to Messrs. J. A. Morton and M. E. Hmes who provided the neces- 

 sary tubes and testers which made this work possible, and to Mr. F. A. Braun 

 who played an indispensable role in the taking and reduction of data. 



Rkferences 



1. "Electron Inertia Effects," F. B. Llewellyn, Camljridge University Press. 

 2 "Equivalent Networks of Negative Grid Vacuum Tubes at Ultra-High Frequencies," 

 F. B. Llewellyn, B. S. T. J., Vol. 15, i)p. 565-586, October 1936. 



3. "Operation of Ultra-High Frec|ucncy Vacuum Tubes," F. B. Llewellyn, B. S. T. J., 



Vol. 14, pp. 632-665, October 1935. 



4. "Theory of the Internal Action of Thermionic Systems at Moderately High Frequen- 



cies," VV. E. Benham, Phil Mag., Vol. 5, pp. 641-662, March 1928; and Vol. 11 

 pp. 457-517, Feb. 1931. 



