ELECTRON BALLISTICS IN HIGH-FREQUENCY FIELDS 347 



high enough to cut off the anode current under static conditions. This field 

 has the vahie computed by Hull: 



6.72 Vf 



H 



R 



Hull's first computation, by the way, was made neglecting space charge, 

 but, strangely enough, the result is not changed by space charge. These 

 electronic oscillations were assumed to be related in frequency to the time 

 of transit of an electron from the cathode to the anode, and at cutoff this is 

 inversely proportional to the field strength, as expressed by the empirical 

 relationship 



\H = 13,100. 



A Ben 



Electronic trajectories for different magnetic fields 

 A — small magnetic field L^b 

 B — moderate magnetic field L ~ 6 

 C — strong magnetic field L<Kb 

 D — critical magnetic field L = 



Fig. 14 — Electronic trajectories for different magnetic fields varying from weak fields to 

 the critical field shown to the right (Brillouin) . 



In general, it was found that best operation occurred when the magnetic 

 field was not quite perpendicular to the electric field. The efficiency and 

 outputs as reported for this type of oscillator were always low, in spite of 

 the large amount of effort devoted to it by an equally large number of work- 

 ers. A second type of oscillation, usually referred to as negative resistance 

 oscillations, has also been the subject of considerable study and some practi- 

 cal use has been made of it at relatively low frequencies. 



Contrasting with this, Posthumus described a third kind of oscillation 

 which he called rotating field oscillations. As in the electronic oscillations 

 the preferred frequency is determined by the magnetic field-strength and the 

 anode potential, the frequency being inversely proportional to the magnetic 

 field-strength. Contrasting with the electronic oscillations, the rotating 



