﻿Helium Atoms by Low- Voltage Electronic Bombardment, 1145 



they suffice to show that it lies between 4 and 5 volts. The 

 results of these particular experiments do not, therefore, 

 indicate whether the abnormal atoms which are being ionized 

 are those resulting from 20*4 volts electronic-atomic 

 encounters, or those resulting from 21*2 volts encounters. 



It seems possible to account for the beginning of ioniza- 

 tion between 4 and 5 volts without assuming that any one 

 abnormal state of the helium atom is more stable than any 

 other abnormal state. Although helium radiations corre- 

 sponding to various voltages are present in the side tube, 

 very little radiation corresponding to voltages other than 

 204 volts and 21*2 volts is likely to be passed into the main 

 tube for the following reasons : — 



a. Only such radiations as correspond to transitions from 

 an abnormal to the normal state of the atom are capable 

 of being absorbed by normal atoms and subsequently re- 

 emitted. 



/S. Abnormal atoms which have absorbed radiation corre- 

 sponding to voltages higher than 21'2 can revert to the 

 normal state in several ways, the number of which increases 

 as the energy of the abnormal atoms increases. It is there- 

 fore improbable that more than a small proportion of the 

 abnormal atoms corresponding to higher voltages than 21*2 

 revert to the normal condition without passing through 

 intermediate stages, in which case the only portion of the 

 emitted energy which could be absorbed by normal helium 

 atoms would be that corresponding to the final transition. 



Thus the fact that ionization of abnormal atoms did not 

 begin before about 4 volts in our experiments may have been 

 due to the absence of abnormal atoms other than those 

 corresponding to 20 '4 volts and 21*2 volts, and the fact that 

 ionization did begin between 4 and 5 volts, although con- 

 sistent with the view that the 20'4 volts abnormal atom is 

 " metastable " in the sense used by Kannenstine, is not 

 sufficient to prove that this abnormal state is more " meta- 

 stable " than any other abnormal state. 



When arcs are maintained below 20*4 volts in helium, 

 ionization of abnormal atoms only can be occurring. For 

 the maintenance of low-voltage arcs, therefore, it is of 

 importance that none of these abnormal atoms should be 

 lost. When recombination takes place the energy of the 

 radiation emitted during the final transition can be absorbed 

 by normal atoms, and the minimum energy which must be 

 supplied in order that every type oi abnormal atom thus 

 produced may be ionized, is 4*8 volts. If the bombarding 



Phil. Mag. S. 6. Vol. 44. No. 264. Dec. 1922. 4 E 



