328 BELL SYSTEM TECHNICAL JOURNAL 



the initial emission velocities as performing this function. The next func- 

 tion, that of varying the electron velocities, is performed by the grid which 

 varies the potential gradient in the vicinity of the cathode and hence the 

 velocity of the electrons as they approach a potential minimum or virtual 

 cathode which is formed a short distance in front of the cathode by the 

 action of space charge. This virtual cathode performs the third function, 

 that of conversion, by sorting out the electrons and allowing only those elec- 

 trons with emission velocities greater than some specific value to pass. 

 This, then, is one of the conversion mechanisms which we will call virtual- 

 cathode sorting. In this example the virtual cathode occurs very close to 

 the real cathode but this is not always the case. The fourth function, 

 that of utilization, is performed by allowmg the sorted electrons to traverse 

 an electromagnetic field between the virtual cathode and the plate. This 

 operation is completed by the time the electrons have reached the plate. 

 Of course in the triode the plate then performs the final operation, that of 

 collecting the spent electrons and dissipating the remaining energy as heat. 

 It should be clearly reaUzed, however, that this last function need not neces- 

 sarily be performed by the same electrode which provides the output field. 

 Indeed the so-called inductive-output tube proposed by Haeff is a space- 

 charge-control tube in which these two operations are separated. 



Conversion Mechanisms 



But now to get back to a cataloguing of the different kinds of conversion 

 mechanisms. The first general type involves sorting. The first kind which 

 we have mentioned is by virtual-cathode sorting. A second kind of sorting 

 might involve deflecting the electron beam in proportion to its longitudinal 

 velocity instead of reflecting or transmitting it. Various deflection tubes 

 have been proposed from time to time using this mechanism. We shall 

 be forced to neglect this phase of the problem this evening because of time 

 limitations but those of you who are interested wiU find the literature filled 

 with detailed discussions. Still a third type of sorting, sometimes called 

 anode sorting, is used in certain Barkhausen tubes when the plate is oper- 

 ated at or near the cathode potential so that fast electrons are collected while 

 slow electrons are reflected and caused to retra verse a high-frequency field. 

 There are still other types of sorting mechanisms but I will not burden 

 you with these. 



A second general type of conversion mechanism I will call bunching, to 

 distinguish sorting in which electrons are separated according to their 

 velocities from hunching in which electrons of differing velocities are brought 

 together. Now it just happens that many of the older devices used sorting, 

 while many of the newer devices use bunching but this is not universally 

 the case. For example, the magnetron as used at high frequencies and the 



