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BELL SYSTEM TECHNICAL JOURNAL 



virtual cathode starts to release, the capacitance decreases. It is 

 negative in a small domain and turns ultimately positive. The plate 

 current curve corresponding to Vgi = 30 volts indicates that the 



10 



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Z -5 



-10 



-15 



-20 



-25 



■10 -8 -6 -4 



CONTROL GRID VOLTAGE 



Fig. 10 — Measured input capacitance of experimental vacuum tube no. 55 

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



virtual cathode is present throughout the range of negative bias and 

 the corresponding capacitance curve is positive everywhere. 



Figure 11 refers to capacitance measurements on a tube in which a 



