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BELL SYSTEM TECHNICAL JOURNAL 



must be in the neighborhood of Stt before we may expect to secure 

 oscillations. 



This would correspond to a frequency somewhat higher than is 

 often associated with this type of oscillation. It must be remembered 

 however, that the particular case considered was that of a tube with 

 its plate at a slightly positive potential, whereas the majority of the 

 experimental frequency observations were made with the plate either 

 slightly negative, or, if positive, adjusted so that a virtual cathode was 

 formed inside the tube, and many of the electrons w^ere turned back 



Fig. 13 — Modification of Fig. 12 produced by added inductance. 



before they reached the plate. The curves of Fig. 1 1 do not apply to 

 these cases. 



Therefore, let us see what happens when the plate is operated at a 

 negative potential so that all of the elections are turned back before 

 they reach it. At the outset, it should be remarked that this condition 

 does not prohibit the presence of direct-current plate current after 

 the oscillations have built up to a finite amplitude. The analysis applies 

 to the requirements for the starting of ihe oscillations, only, so that 



