934 Prof. Horton and Miss Da vies on Electron Velocities 



was the first appearance of luminosity was 8*1 arbitrary 

 units, the electron velocity being then slightly greater than 

 20 volts. In another series of observations, in which the 

 gas pressure was higher and in which a more intense 

 electron stream was used, the ionization current at 19"7 volts 

 was as great as 79 of the same units without there being any 

 trace of luminosity, though as soon as the electron velocity 

 exceeded 20*0 volts a glow appeared in the gas. In neither 

 of these series, of observations was a magnetic field used. 

 Since at 20 volts the number of radiating centres is sufficient 

 for luminosity with an ionization current of 8*1 units, the 

 absence of visible light at 19 7 volts with an ionization 

 current of 79 units must be due to the electron velocity 

 being too low, rather than to the glow being produced 

 too faintbv to be visible. 



The experiments have therefore shown that the lines which 

 we have observed in the neon spectrum make their appearance 

 in two stages, as the velocity of the exciting electron stream 

 is raised. The lines of the principal series type depend for 

 their production upon the presence of the 20*0 volts ionization, 

 while the subordinate series lines require the higher ionization 

 velocity of 22'8 volts. No lines in the visible spectrum result 

 from the ionization which occurs at 16*7 volts. With the 

 exception of the green line X 5401, all the lines produced by 

 the 20*0 volts ionization lie in the red and orange parts of 

 the spectrum, while the lines which require 22"8 volts for 

 their excitation are in the green and blue parts of the 

 spectrum. 



The difference in the velocities needed to excite these two 

 sets of lines is beautifully illustrated if the experiments are 

 carried out at a pressure rather higher than those used in 

 most of the foregoing experiments. With a pressure of 

 about 1 mm., and with the magnetic field applied, a brilliant 

 pencil of light can be obtained along the axis of the discharge 

 tube. If the electric fields are arranged so that the electrons 

 are accelerated from the filament to the grid and then retarded 

 between the grid and the anode by an opposing Held suffi- 

 ciently strong to prevent any of them from reaching the 

 latter electrode, luminosity appears when the velocity of the 

 stream reaches 20 volts ; it is then of a blood-red colour, and 

 is confined to a thin layer of gas just under the grid. As 

 the accelerating potential difference is slowly raised, this 

 luminosity gradually extends downwards towards the anode, 

 and, after 23 volts has been passed, the lower end, only, of the 

 column is red, while the rest of it is a brilliant orange colour. 



