410 Dr. N. Bohr on the Quantum Theory of 



very nearly the same voltages for all the different series in 

 both helium spectra. The absolute values for the voltages 

 •could not be determined very accurately with the experi- 

 mental arrangement, but apparently nearly 30 volts was 

 necessary to produce the lines corresponding to high values 

 •of n. This agrees very closely with the value calculated on 

 the present theory for the energy necessary to remove one 

 electron from the helium atom, viz., 29*3 volts. On the 

 other hand, the later value is considerably larger than the 

 ionization potential in helium (20*5 volts) measured directly 

 by Franck and Hertz *. This apparent disagreement, 

 however, may possibly be explained by the assumption, 

 that the ionization potential measured does not correspond to 

 the removal of the electron from the atom but only to a transi- 

 tion from the normal state of the atom to some other stationary 

 state where the one electron rotates outside the other, and 

 that the ionization observed is produced by the radiation 

 emitted when the electron falls back to its original position. 

 This radiation would be of a sufficiently high frequency to 

 ionize any impurity which may be present in the helium 

 gas, and also to liberate electrons from the metai part of the 

 apparatus. The frequency of the radiation would be 



20*5/3007 = 5*0.10 15 , which is of the same order of magnitude 



as the characteristic frequency calculated from experiments 

 on dispersion in helium, viz., 5*9. 10 15 f- 



Similar considerations may possibly apply also to the 

 recent remarkable experiments of Franck and Hertz on 

 ionization in mercury vapour J. These experiments show 

 strikingly that an electron does not lose energy by collision 

 with a mercury atom if its energy is smaller than a certain 

 value corresponding to 4*9 volts, but as soon as the energy 

 is equal to this value the electron has a great probability 

 of losing all its energy by impact with the atom. It 

 was further shown that the atom, as the result of such 

 an impact, emits a radiation consisting only of the ultraviolet 

 mercury line of wave-length 2536, and it was pointed out 

 that if the frequency of this line is multiplied by Planck's 

 constant, we obtain a value which, within the limit of experi- 

 mental error, is equal to the energy acquired by an electron 

 by a fall through a potential difference of 4'9 volts. Franck 

 and Hertz assume that 4'9 volts corresponds to the energy 

 necessary to remove an electron from the mercury atom, but 

 it seems that their experiments may possibly be consistent 



* Franck & Hertz, Verh. d. D. Phys. Ges. xv. p. 34 (1913). 



t Outhbertson, Proc. Roy. Soc. A. lxxxiv. p. 13 (1910). 



X Franck and Hertz, Verh. d. I). Phys. Ges. xvi. pp. 457, 512 (1914). 





