THEORY OF MULTI-ELECTRODE VACUUM TUBES 51 



the purpose of completing the electrostatic isolation of the plate and its 

 leads from the grid, thus reducing the capacitance between these two 

 elements to the lowest practicable value. This outer screen is of no 

 further concern, since it has no effect on the static characteristics of the 

 tube. The usual characteristics of a typical screen-grid tetrode are 

 shown in Figs. 2, 3, and 4. In this case, as is usual in screen-grid tubes, 

 the shielding is very complete, reducing the direct capacitance between 

 the plate and control grid to a few thousandths of what its value would 

 be in the absence of the screen. 



First, the characteristics of Fig. 2 will be considered. Since the 

 direct capacitance between the plate and the control grid, gi, is ex- 

 tremely small, the electric field in the immediate vicinity of the latter, 

 produced by any potential on the plate, also must be extremely small. 

 The cathode is electrically even more remote from the plate than the 

 control grid, because it not only is shielded from the former by the 

 screen grid, but also has some additional shielding from the control grid. 

 Consequently, the field produced by the plate at points between the 

 cathode and control grid is smaller even than the field produced at the 

 control grid and, hence, is negligibly small. Since, as previously 

 discussed, the total space current is determined almost wholly by the 

 field very near the cathode surface, the plate in this case can have 

 practically no effect on the total space current drawn from the cathode. 

 This is shown by the curves giving the total space current, /(, in Fig. 2. 

 These curves are seen to be so fiat as to be almost entirely independent 

 of variations in the plate voltage. 



The plate, then, in a screen-grid tube plays an essentially passive 

 role which is to collect those electrons that succeed in passing through 

 the screen. The remainder of the space current is collected by the 

 screen, the sum of the plate and screen currents remaining nearly 

 constant with changes in plate voltage. 



There is nothing in the theory of the triode by which to determine the 

 ratio in which space current divides between two or more positive 

 electrodes in a multi-electrode tube. As a rough approximation, one 

 might assume that when their potentials are nearly equal, the currents 

 to the plate and screen would be proportional to the ratio of the area of 

 the openings in the screen to the area subtended by its lateral wires. 

 Also, it might be expected that this ratio would increase slightly with 

 increasing voltage of the plate with respect to the screen, because of a 

 tendency of the plate to pull more electrons through the screen. This 

 effect should be less for very fine mesh screens than for coarse ones. 



From this simple theory, one would expect the plate current-plate 

 voltage characteristics to be very flat for plate potentials higher than 



