MODULATION IN VACUUM TUBES 



455 



As a consequence of the subtraction of quantities of the same order of 

 magnitude, the calculation for the third order coefficient is not capable 

 of any great precision. 



Fig. 5 shows how the fundamental coefficients dix/dEp, dfx/dEc, 

 dRo/dEp, which are involved in the second order term, vary as a 

 function of the grid potential when the plate potential is maintained 

 constant at 120 volts; dRo/dEp is negative in sign. 



Variation of Ci and C-i with Grid and Plate Potentials 



To summarize our analysis up to this point, we have formulated 



an expression for the characteristic surface of a vacuum tube and have 



manipulated it to derive expressions for the fundamental and for 



the second and third order current coefficients. These theoretical 



-18 -Ife -14 -12 



VOLTS GRID POTENTIAL 



Fig. 6 — EL tube No. 109,150. External resistance = 6,000 ohms. 1/ = 1.1 amperes 



relations have been compared with experimental determinations of 



the three quantities involved as a function of the external plate 



resistance, and a sufficiently good agreement has been obtained to 



indicate that the processes which we have treated are sufficient to 



account for experimental observations. We now present calculations 



of the coefficients Ci and C2 as a function of plate potentials and grid 



potentials for several values of the external plate resistance. It is 



seen from Figs. 6, 7, and 8 that these coefficients vary inversely with 



the plate and grid potentials. 

 30 



