﻿[ 1140 ] 



CV. TJie Ionization of Abnormal Helium Atoms by Low- 

 Voltage Electronic Bombardment. By FltANK Horton, 

 Sc.D., and Ann Catherine Davies, D.Sc. * 



IN a recent paper in the Astro-physical Journal t F. M. 

 Kannenstine describes experiments which show that 

 with alternating electromotive forces, an arc in helium can 

 be maintained, and even made to strike, at about 5 volts, 

 provided the frequency of the alternations exceeds a certain 

 limiting value. Evidence of the maintenance of arcs in 

 helium at voltages below the first critical electron energy 

 for this gas (20*4 volts) has been given by other observers J, 

 and it might be expected that the limiting voltage for the 

 maintenance of the arc in helium would be the difference 

 between the normal ionizing voltage (25*2 volts) and one 

 of the two critical electron energies for the production of 

 radiation (20*4 volts and 21*2 volts), i. e., the limiting voltage 

 might be expected to be either 4*8 volts or 4*0 volts. Kan- 

 nenstine's- experiments seem to be the first in which a limit 

 approximating to either of these values has been reached, 

 and also the first in which the arc has been made to strike 

 below 20*4 volts in helium. This striking of the arc at 

 voltages below the resonance value in experiments with 

 alternating electromotive forces is not, however, so much at 

 variance with theory and with the results obtained by other 

 experimenters as it appears to be at first sight, for what 

 Kannenstine found was that after the cycle of voltages had 

 once been completed, the arc struck at about 5 volts during 

 subsequent cycles if the frequency of the alternations 

 exceeded 220 per second. Hence the effect obtained was 

 not the striking of the arc in normal helium, but the 

 striking of the arc in helium containing- some abnormal 

 atoms for the production of which a higher voltage had been 

 employed. 



Kannenstine has pointed out that his results can be inter- 

 preted on the view that there is one of the states of the 

 helium atom, intermediate to the normal state and the singly 

 ionized state, in which the atom can exist for a perceptible 

 interval of time, which he gives as about *0024 sec. This 



* Communicated by the Authors. 



f F. JM. Kannenstine, Astro. Phys. Journ. vol. Iv. p. 345 (1922). 



t K. T. Compton, E. G. Lilly, and P. S. Olmstead, Phys. Rev. vol. xvi. 

 p. 282 (1920). A. C. Davies, Proc. Boy. Soc, A., vol, 100. p. 599 

 (1922). 



