THEORY OF MULTI-ELECTRODE VACUUM TUBES 53 



of the electrons normal to the surface of the plate and, consequently, in 

 the distance to which they approach the plate in their trajectories 

 before being turned back to the screen. As a result, the plate must 

 become positive by several volts with respect to the cathode before it 

 captures substantially all of the electrons that pass through the screen. 



From this simple theory, the plate-current and screen-current curves 

 would be expected to have the form shown by the ideal curves of Fig. 2. 

 Obviously, the screen-current curves must be complementary to the 

 plate-current curves since the sum of the two currents is substantially 

 constant. 



The difference between these ideal curves and the actual charac- 

 teristics, in the region extending from a few volts to potentials some- 

 what higher than the screen voltage, is attributed to the phenomenon of 

 secondary electron emission. When electrons strike a metal surface 

 with velocities equivalent to more than a few v^olts, other electrons, 

 known as secondary electrons, are liberated from the surface. The 

 number of electrons so liberated varies not only with the velocity of the 

 primary bombarding electrons, but also with the character of the metal 

 surface, the amount of adsorbed gases and other materials on the 

 surface, and other factors. The number of such electrons leaving the 

 surface may even exceed the number of primary electrons striking it, in 

 which case the net current to the metal surface is negative. The 

 velocity of the secondary electrons varies greatly. A very few have 

 velocities approaching that of the primary electrons. The great 

 majority, however, have low velocities equivalent to only a few volts. 



In the screen-grid tube, an appreciable number of secondary electrons 

 is liberated from the plate at potentials between 5 and 10 volts, and 

 they increase in number with plate voltage. For plate potentials lower 

 than the screen potential, in this case 75 volts, the secondary electrons 

 from the plate are drawn to the screen, thus increasing the screen 

 current by the amount the plate current is decreased. When the plate 

 reaches a potential equal to that of the screen, secondary electrons no 

 longer can escape from the plate to the screen, except those emitted 

 with appreciable velocities; consequently, the plate current rises 

 rapidly to its normal value. 



At plate potentials higher than the potential of the screen, secondary 

 electrons emitted from the latter are drawn to the plate. Conse- 

 quently, in this region the plate current is slightly higher than it would 

 be in the absence of secondary electron emission from the screen. The 

 gradual rather than abrupt rise in the plate current curves at 75 volts is 

 attributed primarily to the distribution of velocities with which the 

 secondary electrons are emitted; to a lesser extent, it is dependent also 



