﻿176 Messrs. E. W. B. Grill and J. H. Morrell on Short 



oscillation o£ maximum amplitude, for a fixed potential 

 between grid and plate, when either the emission is altered 

 or the plate potential is slightly altered with respect to the 

 filament. 



There is also a special case, which is forming the subject 

 of a separate investigation, in which, when the plate is 

 very negative (40 volts or so) with respect to the filament, 

 oscillations can still be produced, but without any current 

 reaching the plate at all. These oscillations are, however, 

 very much weaker and more difficult to produce than those 

 dealt with in our experiments. 



The simple theory which depends on the collection on the 

 plate in each oscillation of a group of electrons will obviously 

 not account for this special case. 



The explanation of the above considerations is to be looked 

 for in the assumptions made in the simple theory. The first 

 assumption that the grid and plate could be regarded as 

 parallel is not important, as the field between cylinders of 

 the size of the grid and plate used is not far from uniform. 

 (It will be noted that the value of T was calculated for the 

 valve used by taking the field between cylinders.) The only 

 difference between cylinders and parallel plates on the simple 

 theory would be to make the ratio T to 1/p for maximum 

 oscillation slightly different. But the second assumption 

 that the electrons pass the grid in a constant stream all 

 having the same velocity requires more careful examination. 

 This velocity is not actually the same for two reasons : 



A. Because there will be alternating potentials between 



the filament and the grid which set up a velocity 

 distribution at the grid. 



B. Because of the voltage drop of the heating current 



down the filament. 



In the usual methods of producing oscillations by means 

 of valves the alternating potential of (A) is most important, 

 as it controls the whole action of the valve, but in our 

 experiments it is only of secondary importance. The 

 alternating potentials induced between filament and grid 

 are smaller than those between grid and plate, and the 

 major part of the work done by the alternating field on 

 the electrons, which is what determines whether the 

 electrons reach the plate or not, is done between grid 

 and plate, and it is therefore nearly correct to say that 

 all the electrons passing the grid at times t , such that 

 expression (1) is positive, reach the plate. 



In the extreme case, however, when the plate is so 



