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LVII. Critical Velocities for Electrons in Helium. By 

 Frank Horton, Professor of Physics in the University of 

 London, and Ann Catherine Davies, Royal Holloway 

 College, Englejield Green *. 



IN a recent paper f the authors have shown that the first 

 critical electron velocity for the normal helium atom is 

 20*4 volts; and that an electron with this velocity is able to 

 produce radiation from the atom on collision with it. It was 

 also shown that when the velocity of the impacting electron 

 is increased to 25*6 volts, it is able to produce ionization by 

 collision. This value of the minimum ionization velocity is 

 lower than that predicted for the normal helium atom by 

 Bohr's theory, according to which the electron velocity 

 necessary for the removal of one electron from a helium 

 atom is 2*13 times the electron velocity required to ionize 

 -an atom of hydrogen, while the electron A'elocity necessary 

 to remove both electrons from a helium atom at a single 

 collision is 6*13 times the hydrogen ionization velocity. 

 Taking the values 6 = 4*774 x l(j- 10 E.S.U., */ro = l"767 X 10 7 

 E.M.U., and h = 6 547 x 10" 27 gm. cm. 2 /sec, Bohr's theory 

 gives 13*54 volts as the hydrogen ionization velocity. The 

 two critical ionization velocities for helium, calculated from 

 Bohr's theory, are therefore 28*84 volts and 83'00 volts 

 respectively, the difference between these two velocities, viz. 

 54*16 volts, being the electron velocity which, according to 

 the same theory, would be required for the removal of the 

 second electron from a helium atom which had already been 

 ionized. 



After the conclusion of the experiments described in our 

 earlier paper, we investigated the effects of collisions between 

 helium atoms and electrons having considerably greater 

 velocities, with a view to obtaining some further evidence of 

 the validity of Bohr's assumptions, by determining accurately 

 the velocity at which both electrons are removed from the 

 normal helium atom at a single collision. This investigation 

 presented considerable difficulty on account of the fact that, 

 at the higher critical velocities, increases of radiation or of 

 ionization have to be detected in the presence of the effects 

 which occur at lower velocities. Our experiments showed 

 that to obtain a degree of accuracy at all comparable with 

 that attained in the earlier research, it would be necessary 

 to use an electron stream in which all the electrons have the 

 same velocity, so that the breaks in the current curves, 



* Communicated by the Authors. 



f Prop. Roy. Soc. A. vol. xcv. p. 408 (1919). 



