ELECTRON BALLISTICS IN HIGH -FREQUENCY FIELDS 323 



fer. I was tempted to say "which results from this transfer" but this implies 

 a cause and an effect, a notion wliich has no place in the present discussion. 

 The dual aspect of any energy-transfer problem must always be kept in 

 mind. Much needless discussion frequently arises between proponents of 

 one point of view and those preferring the other when the only difference 

 is one of language and both groups are really saying the same thing. The 

 electron-ballistic approach yields a simple physical picture; it is capable of 

 being applied to widely differing situations, but it is not well suited for a 

 determination of the reactive contributions of an electron stream. 



Basic Concp:pts 



There are several concepts which we will hnd useful in our analysis. 

 These concepts are extremely simple, so simple in fact that one is tempted to 

 assume that they are well known. However, these concepts are so basic 

 to the subject, and their results so far reaching that we must pause to 

 consider them. 



The first is the concept of total current, as distinguished from its com- 

 ponents. One way of writing Kirchhoff's second law is 



Div. / = (1) 



This simply says that the total current entering or leaving any differential 

 region in space is zero. This expression must of course be generalized by 

 including displacement currents as proposed by Maxwell if applied to 

 alternating currents. The current / is the total current density as here 

 defined. An important consequence of equation (1), actually only an 

 alternate way of stating it, is that the total current always exists in closed 

 paths. Let us take a simple case of a two-element thermionic vacuum tube 

 connected to a batter}-. \'isualize the situation existing if but a single 

 electron leaves the cathode and travels to the plate. The electron takes a 

 finite time to cross from the cathode to the plate. During this time a current 

 exists, the magnitude being given by the relationship 



I = ev 



and according to our premise this current is the same in every part of the 

 circuit. The current begins at the instant that the electron leaves the 

 cathode and it ceases when the electron arrives at the plate. In the appar- 

 ently empty region ahead of the electron there must exist a displacement 

 component, numerically equal to the conduction, or perhaps we should say 

 convection component accounted for by the moving electron. An ammeter, 

 were there one sufficiently sensitive and fast, connected in the external leads 

 would read a current during this same interval of time. 



I have chosen to talk about but a single electron to emphasize the electron- 



