﻿Electric Waves obtained by Valves, 177 



negative that the system is unable to oscillate by the method 

 of: driving groups of electrons on to the plate, the oscillation 

 is almost certainly due to a velocity distribution at the 

 grid, as this means that the electrons do not pass the grid in 

 a uniform stream, and allows the integral of expression (2) 

 to be finite and not zero, as it normally is when all the 

 electrons return to the grid. 



The comparative weakness of the oscillations in this case 

 shows that the electrons have all nearly the same velocity 

 when passing the grid. 



The simple theory should therefore be in agreement with 

 the observed facts, as it is when the oscillations are mainly 

 due to the collection by the plate, but as the plate is made 

 more negative with respect to the filament the velocity dis- 

 tribution at the grid becomes more important and the simple 

 theory is less accurate. 



The velocity distribution at the grid will also bo affected 

 by the emission, as this varies the space charge round the 

 filament — this affecting the time the electrons take to pass 

 from, the filament to grid, — and this in turn varies the small 

 effect of the alternating field in this space. 



The effect of the voltage drop of about 4 volts down the 

 filament is that, instead of dealing with one stream in 

 the field due to the grid being charged to V volts, there 

 are a series of streams moving under potentials varying 

 from V to V — 4 (V being the potential difference between 

 the grid and the negative end of the filament). The number 

 of electrons in the various streams varies from a maximum 

 number corresponding to V — 2, the middle of the filament 

 being the hottest. The emission falls off equally on both 

 sides of this middle point. 



In the general case, when the plate is slightly positive with 

 regard to the negative end of the filament when there are no 

 oscillations, some of the streams reach the plate and the 

 remainder approach it closely, but to varying distances. 



If oscillations commence some of these latter streams are 

 periodically diverted to the plate, while in the other half 

 oscillations some of the former are diverted off. 



Thus all the streams concerned maintain the oscillation as 

 in the simple theory, and unless Vis small the wave-lengths 

 they each maintain best are nearly the same, so that the 

 combined effect differs little from that of a single stream 

 moving under potential V. 



The question of whether the mean plate current rises or 

 falls when oscillations begin depends on whether the average 

 Phil. Mag. S. 6. Vol. 44. No. 259. July 1922. N 



