VACUUM TUBE ELECTRONICS 



77 



velocities with which electrons are emitted from the cathode. Prac- 

 tically, the difference is negligible except at extremely high frequencies. 

 Since the impedance between the cathode and potential minimum is 

 small compared to the plate impedance, its effect is merely to add a 

 loss to the system which increases with frequency since the plate im- 

 pedance approaches a capacity as the frequency approaches infinity. 

 The grid cathode path presents less difficulty, although a somewhat 

 less rigorous treatment is given here. As pointed out, the force from 

 the grid acts on the high charge density region existing near the 

 potential minimum. The impedance between cathode and grid, there- 

 fore, consists of two parts in series; namely, capacity between grid and 

 potential minimum and impedance between potential minimum and 

 cathode, the latter part of this impedance being common both to plate- 

 and grid-current paths. 



Zd = To + iXr 



aeg = (ix+iv)eg 



O 



(M,+-iv)i 



4 

 C=-yC, 



l-=-[^C,ro2 

 M'=M'0 



■o 



Fig. 8 — Equivalent network of plate-cathode path of negative grid 

 triodes for transit angles less than 0.3 radian. 



If we were to connect the grid and cathode terminals of such a 

 triode to a capacity bridge and measure the capacity existing there 

 when the tube was cold and when the cathode was heated, we should 

 find that the capacity would exhibit a slight increase in the latter case. 

 The reason for this increase may best be explained by noting that in 

 the cold condition the electrostatic force from the grid is exerted on the 

 cathode itself, whereas in the heated state, the force acts on the elec- 

 trons near the potential minimum, thus resulting in an increased capac- 

 ity in series with a resistive component. 



In some measurements of the losses in coils which were made at a 

 frequency of 18 megacycles, J. G. Chaffee of the Bell Telephone Lab- 

 oratories has found that a loss existed between grid and cathode of 

 vacuum tubes which was much greater than can be accounted for by 

 any of the dielectrics used and which was present only when the tube 



