IMPEDANCE PROPERTIES OF ELECTRON STREAMS 



479 



second determines the initial speed with which the electrons enter the 

 space adjacent to the control grid. 



In Fig. 9 the measured input capacitance and plate current are 

 shown as functions of the voltage Vgi of the first grid. It is seen that 

 as the plate current increases the capacitance gradually decreases, 

 passes through zero somewhat before the plate current has reached its 

 maximum and then becomes negative; it passes through a minimum 

 and then gradually assumes a positive value equal to about twice the 

 cold capacitance. This behavior of the capacitance is typical of the 

 formation of a virtual cathode, which in the present instance appears 



8 10 12 14 16 18 20 

 VOLTAGE ON FIRST GRID 



22 24 26 



Fig. 9 — Measured input capacitance of experimental vacuum tube no. 55 

 (grid signal — 0.18 volts r.m.s. at 50 kc). 



to be gradual. The negative capacitance is present in a small interval 

 of the voltage Vgi immediately before and perhaps also during a part 

 of the virtual cathode formation. 



Figure 10 shows the measured input capacitance as a function of 

 the control grid bias Vgz for several values of the voltage Vgi. For 

 comparison purposes the corresponding plate currents are also shown. 

 In the region of low plate current where a virtual cathode is present 

 the capacitance is positive and larger than the cold capacitance. 

 Where the plate current curve starts to bend over, that is, where the 



