600 Prof. F. Horton and Miss A. C. Davies on 



at a pressure of 0*35 mm., and shows an increase of positive 

 current at about 55 volts. Tt is therefore possible that this- 

 bend does indicate the production of ionization as a result 

 of encounters between ionized helium atoms and electrons 

 having this velocity, and that it thus affords direct experi- 

 mental evidence of the correctness of Bohr's assumptions as 

 applied to the helium atom which has lost one electron. 



It" collisions occur between ionized atoms and electron?, it 

 follows that the number of collisions between electrons and 

 normal helium atoms will be reduced by the recombination 

 which probably results when the velocity of the impacting 

 electron is lower than some critical value. On this account,. 

 with the intense electron stream which must be used in, 

 order that the electron collisions with ionized atoms may be 

 sufficiently numerous for the removal of the second electrons 

 from these ionized atoms to cause arise in the current curve,. 

 it is possible that there will be such a small proportion of the 

 electrons making second or third inelastic collisions with 

 normal helium atoms that the curve will show no bencisat 

 the electron velocities at which these would normally occur- 

 Thus it was found that when an intense electron stream was 

 used (as in obtaining the curve of fig. 5), the current curve 

 showed a rise at about 55 volts but no bends at 46*0 volts or 

 at 51*2 volts, whereas in curves taken at a similar pressure 

 when a smaller electron current was employed, the former 

 point was not marked but bends at velocities corresponding 

 to multiple collisions were obtained. The absence of the 

 bends at 46*0 volts and at 51*2 volts under the conditions of 

 experiment of fig. 5 may thus be an indication that a large- 

 number of collisions with ionized helium atoms is occurring,. 

 a condition which is essential to the interpretation we have 

 given of the rise at 55 volts. If this interpretation is 

 correct, it might be expected that there would be some' 

 indication of an increase in the measured positive current at 

 an electron velocity, lower than 55 volts, which would 

 correspond to the production of radiation from the ionized 

 helium atom by its collision with an electron. On Bohr's 

 theory such radiation would require for its production an 

 electron velocity of } of 54'16 volts = 40'62 volts. The 

 radiation corresponding to this potential difference would be 

 of such high frequency as to be able to ionize some" of the 

 normal helium atoms present, and it is therefore uncei tain 

 whether its production would show in the current-potential 

 difference curve as an increase of radiation or as an increase 

 of ionization, in a case where the arrangement of the electric 

 fields made it possible to discriminate between these two 



