﻿178 Short Electric Waives obtained by Valves. 



density of the streams diverted to the plate exceeds or is less 

 than that of those diverted from it. 



If the plate is at the same potential as the centre of the 

 filament, no change should therefore occur. If it is above 

 this the current should drop, and if below the current should 

 rise, which is in agreement with the results in paragraph 4. 



In conclusion, we should like to express our thanks to 

 Professor Townsend, who has assisted us with much valuable 

 advice and criticism. 



Note on the Determination of Ionizing Potentials. 



In the experiments above described, the electric fields in 

 the spaces between the grid, filament, and plate are similar 

 to those used in experiments on the determination of critical 

 potentials when a small quantity of gas is introduced. 



In the latter experiments variations in the plate current 

 are observed as the grid potential is raised, and at certain 

 potentials of the grid abnormal variations in the plate current 

 are observed which are interpreted as indicating certain 

 critical potentials, characteristic of the molecules of the 

 gas. 



The experiments which are here described show that 

 abnormal variations in the plate current are to be expected, 

 due to oscillations which may be maintained where large 

 currents of the order of a milliampere flow from the filament 

 towards the grid when the gas is at a very low pressure. The 

 effect of varying the potential of the grid is clearly shown by 

 the curve of fig. 3. 



In all the ordinary methods of wiring the valve to the 

 cells and galvanometers or electrometers, the system seems 

 to be as often in a state of oscillation as not. 



Even if the plate is so negative that the oscillations are of 

 the weaker type which do not affect the plate current, the 

 difficulty arises that the oscillations superimpose an unknown 

 potential difference between filament and grid, and the 

 electrons are not moving under the fixed field alone. 



It is necessary therefore, in order to obtain reliable values 

 of the critical potential, to take precautions to prevent 

 oscillation, which is best done by using emission currents 

 much less than a milliampere. 



