928 Prof. Horton and Miss Davies on Electron Velocities- 

 Table III. 



The brightest lines of those which suddenly appeared in 

 the spectrum of the glow at about 23 volts. 



X. 



Series. 



5764 



2,jh -4, d 4 ' 



5341 



2,ft -4,dL 



5331 



2, ft o-4, d 3 



5189 



%Po - 4 ^,S5 



5145 



2, p 4 -4-5, s x " 



5116 



2,ft -3-5, Sl ' 



5080 



2, p 8 -5, di 



5038 



2,^o — ^' ^' 



These lines, according to Paschen's classification, all belong 

 to sequences o£ the type (2,p — m, s) or (2, p — m, d) and 

 are subordinate series lines. A few tenths of a volt after 

 this second set of lines became visible, some of them were 

 distinctfy brighter than other lines which were visible at a 

 lower electron velocity and which had gradually brightened 

 as the potential difference was raised. The appearance of 

 the two sets of lines at different electron velocities is therefore 

 not merely due to their production with very different in- 

 tensities aft some definite minimum electron velocity, the same 

 for both, but is due to a genuine difference in the energy 

 required to excite the two sets. Further evidence in support 

 of this view was obtained from a series of measurements of 

 the ionization in the space between the grid and anode taken 

 with the galvanometer while the spectrum was being examined.. 

 In obtaining these measurements, a potential difference was 

 maintained between the grid and anode opposite in direction 

 to that between the filament and the grid, and exceeding the 

 latter potential difference by a constant amount of 3 volts, so 

 that none of the electrons from the filament could reach the 

 anode. During the observations the potential difference 

 accelerating the electron stream from the filament was mea- 

 sured by means of a very high resistance voltmeter connected 

 between the grid and the negative end of the filament. In 

 the curve A of fig. 2, which represents the results obtained 

 with gradually increasing electron velocity, it may be observed 

 that there is a sudden large increase of current at 22"2 volts. 

 The lines \ 5852 and A 5401 and some of the brighter red 

 lines were visible before this increase of current occurred, 

 and the rather feeble glow visible in the tube before 222 volts 

 brightened enormously when the current increase occurred. 



