for Production of Luminosity in Atmospheric Neon. 935- 



In the red part of the glow none but principal series lines are 

 being produced, and, since the velocity of the electrons from 

 the filament gradually decreases as they travel from the grid 

 towards the anode, the experiment clearly shows that these 

 lines can be stimulated when the velocity of the bombarding 

 electrons has been reduced to a value too small to excite the 

 complete neon spectrum. 



An appearance which is the reverse of that just described 

 may be obtained by arranging the electric fields so that they 

 both tend to accelerate the electron stream towards the anode, 

 but with the difference of potential between the filament and 

 the grid considerably less than that necessary for ionization 

 by collisions to take place just below the grid. For instance, 

 in one such experiment, at a pressure of about 1*4 mm., the 

 potential difference between the filament and the grid was 

 maintained at 8 volts and the potential difference between 

 the grid and anode was gradually raised. No luminosity 

 appeared in the gas until the total potential difference between 

 the filament and the anode was about 25 volts, when the central 

 luminous column suddenly appeared, very brilliantly, reaching 

 from near the anode right up to the grid. r l he lower part of 

 this luminosity was of a bright orange colour and the upper 

 end, nearer the grid, was blood-red. On gradually reducing 

 the potential difference across the tube the column became 

 shorter and no longer reached to the grid, and when the total 

 difference of potential between the filament and the anode 

 had been reduced to about 21'5 volts, the glow occupied 

 only about 5 mm. in the centre of the tube and was all of a 

 crimson colour showing only principal series lines of the 

 neon spectrum. On reducing the maximum electron velocity 

 still further, the glow became smaller in size, and finally dis- 

 appeared when the potential difference was between 20 volts 

 and 21 volts. It must be remembered that at the relatively 

 high pressure of this experiment the mean free path of an 

 electron in the gas is small (considerably less than 1 mm.), 

 so that very few of the electrons after acquiring the lowest 

 velocity with which they can collide inelastically with neon 

 atoms, viz. 11*8 volts, will travel to the anode without loss of 

 energy. The appearance of luminosity is thus delayed until 

 the potential gradient between the grid and the anode is such 

 that the electrons can acquire 20 volts velocity in a con- 

 siderably shorter distance. The fact that most of the 

 electrons make inelastic collisions in the gas before they 

 get near to the anode also explains why the glow, when it 

 does appear, does not extend right down to this electrode at 

 this high pressure. 



