VACUUM TUBE ELECTRONICS 



79 



In general, space-charge conditions existing between grid and plate 

 are quite complicated. An analysis has been made by Tonks * which 

 indicates several distinct classes of space-charge distribution which are 

 possible. In the first place so few electrons may pass the grid mesh 

 that no appreciable space charge is set up between there and the plate. 

 In this instance a positive plate will trap them all, whereas a negative 

 plate will return them all toward the grid. Second, with a fixed posi- 

 tive plate potential an increase in the number of electrons which pass 

 the grid mesh will result in a depression of the potential distribution 

 as illustrated at (a) by the curves in Fig. 9. This depression will con- 



Fig. 9 — Potential distributions in positive grid triodes. 



tinue to increase until a potential minimum is formed. When this 

 potential minimum becomes nearly the same as that of the cathode, 

 either of several things may occur. If the minimum is just above the 

 cathode potential, all electrons will pass that point and eventually 

 reach the plate. However, an extremely small increase in the number 

 of electrons will cause the potential minimum to become equal to the 

 cathode potential. When this happens some of the electrons will be 

 turned back and travel again toward the grid. These will increase the 

 charge density existing and, therefore, cause a further depression in 

 the potential resulting in a mathematical discontinuity so that the 



^ L. Tonks, "Space Charge as a Cause of Negative Resistance in a Triode and 

 Its Bearing on Short-Wave Generation," Pliys. Rev., Vol. 30, p. 501; October (1927). 



