448 Prof. F. Horton and Miss Doris Bailey on Measurement 



anode space from travelling to the filament, and at the same 

 time arranging so that no ionization could take place between 

 the first gauze and the filament. In order to secure the 

 sharp rise in the curve it is necessary for the electric fields 

 to be arranged so that the neutralization of the space-charge 

 occurs suddenly. W the neutralization occurs gradually, the 

 increase of electron emission from the filament is also gradual 

 and a sudden rise in the current does not occur. This con- 

 dition is attained when the grids and anode are connected 

 together to form one electrode, as in the experiments the 

 results of which are shown in fig. 2. 



The rise in curve III. of fig. 3 at about 21 volts is due to 

 the ionization of the small trace of mercury vapour which the 

 otherwise pure helium contained, although the ionization 

 chamber opened into a purifying tube containing carbon 

 which was cooled in liquid air. The impurity present in the 

 apparatus during the experiments described earlier was pro- 

 bably gas evolved from the glass in spite of the long treatment 

 to which it had been subjected. The spectrum of the luminous 

 discharge in this case showed no trace of the mercury lines 

 but a very faint indication of a band spectrum, probably due 

 to oxides of carbon. 



The view that the ionization which has been detected in 

 helium when electron velocities of less than 25 volts are 

 used, is due to the indirect ionization of traces of impurities 

 by the helium radiation, is also supported by the results of 

 some experiments in which the spectrum of the luminosity 

 produced in helium by electron bombardment was investi- 

 gated for different velocities of the electron stream. These 

 experiments were performed with a single-grid apparatus, 

 the metal parts of which were not all of platinum and had 

 been contaminated by contact with mercury vapour while 

 they w r ere being used in another research. It was con- 

 sequently found to be impossible to eliminate mercury 

 vapour completely during the investigation, although the 

 amount of this impurity present in the helium must have 

 been very small, for the apparatus was in direct connexion 

 with a charcoal tube cooled in liquid air throughout I he 

 experiments. 



A series of photographs of the spectrum of the luminosity 

 produced in the gas between the anode and the grid was 

 taken by means of a Hilger direct wave-length reading- 

 spectroscope, for various values of the applied potential 

 difference. The lines seen on the photographic plate in a 

 typical case are given in the following table. With the 

 three lower electron velocities, namely 21*4 volts, 23*7 volts, 



