ELECTRON BALLISTICS IX HIGH-FREQUENCY FIELDS 327 



will be a volt. Instead of measuring the time required for an electron to 

 traverse any given distance in seconds, it is also convenient to use, as a 

 unit of time, one radian of angle at the operating frequency. We frequently 

 refer to the transit angle of an electron rather than the transit time, although 

 both terms are used. In fact, we may on occasion measure distances in 

 terms of transit angle, and this usage is extended to measure dimensions 

 transverse to the direction of travel of the electron beam. When used in 

 this fashion, we mean that the dimension in question is such that were an 

 electron to be projected in this direction with a velocity equal to that of the 

 electrons in the main beam, the high-frequency field would change through 

 the stated number of radians during the transit time. 



The Five Functions in an Electronic Device 



With this preliminary discussion out of the way we can now answer the 

 question which has probably been troubling quite a few of you. If the only 

 thing we can do to an electron is to change its velocity, then in what basic 

 way does the velocity-modulation tube differ from the conventional negative 

 grid tube or from the magnetron? 



Well, this is an involved story. If we are to make any use at all of an 

 electron beam we must in general perform five distinct operations or func- 

 tions. First we must produce the beam. Then we must impress a signal 

 of some sort onto the beam. From what I have just said this can be done 

 only by varying the velocities of the electrons contained in the beam. The 

 third operation consists in converting this variation into a usable form. 

 It is in this way that the diverse forms of electronic devices differ to the 

 greatest degree. W^e will go into this matter in more detail shortly. The 

 fourth operation consists in abstracting energy from the beam, and the final 

 operation consists in collecting the spent electrons. While these operations 

 are distinct from an analytical point of view, in many actual devices they 

 are performed more or less simultaneously and more than one operation 

 may be performed by certain portions of the tube structure. In fact, in 

 some devices, for example in the space-charge-control tube, the confusion 

 is so great as to make the separation seem rather forced. This very confu- 

 sion may partly explain why vacuum-tube engineers who were steeped in 

 the art were so slow to realize the advantages of this new way of looking 

 at things which I will call the velocity-modulation concept. 



By way of mental exercise in this new way of thinking let us see how 

 we can analyze a simple space-charge-control triode. Well, first of all we 

 have to identify the electron gun which produces the beam. The electrons 

 most certainly come from the cathode, but where is the first accelerating 

 electrode? Actually there isn't any unless we think of the combined d-c 

 field resulting from the d-c potentials on the grid and plate as assisted by 



