52 BELL SYSTEM TECHNICAL JOURNAL 



the screen potential. Consequently, the plate conductance, given by 



' dEp R, 

 is a very small quantity; and the plate resistance, given by 



dEp 



is a very large quantity compared with its value in triodes, and in- 

 creases with the fineness of mesh of the screen grid. The plate-current 

 curves of Fig. 2 are seen to be in general accord with this simple theory 

 at the higher values of plate voltage, although they are not quite as fiat 

 as might be expected from the theory. This, and the rapid falling off 

 in the vicinity of 100 volts, will be discussed later. 



Taken alone, the extremely high resistance of a screen-grid tube 

 would seem to be a very serious disadvantage. From, equation 14, the 

 tube may be considered as analogous to a generator the electromotive 

 force of which is lueg and the internal resistance of which is Rp, working 

 into an external load resistance R — a generator with extremely high 

 internal resistance. Why this is not fatal to the usefulness of the tube 

 will be pointed out later. 



While the plate current in a screen-grid tube is nearly independent 

 of plate voltage for values of the latter higher than the screen potential, 

 this obviously cannot hold at low values of plate voltage. At zero 

 plate voltage, the plate current must be zero. At this point the screen 

 collects the entire space current and /« = It- As the plate potential 

 increases from zero, the plate current would be expected to rise 

 rapidly, with a corresponding drop in the screen current, as the plate 

 collects more and more of the electrons passing through the screen. 

 However, two factors tend to prevent an abrupt rise in the plate 

 current to its nearly constant value when the plate becomes slightly 

 positive. The first of these is space charge in the region closely 

 adjacent to the plate produced by the electrons that pass through the 

 screen, reach zero velocity in the region adjacent to the plate, and 

 return to the screen. Some of them may perform several oscillations to 

 and fro through the screen before being captured by it. This space- 

 charge effect is largely masked by the more important second factor 

 which is the deflection of the majority of the electrons from their 

 normal paths by the intense electric fields about the lateral wires of the 

 screen. This results in large differences in the components of velocity 



