Radiation, and 'Ionization of Helium. 749 



In a second paper by Franck and Knipping *, these authors 

 record results which are particularly interesting on account 

 of their bearing on the newer theories of the structure of the 

 normal helium atom. Bohr originally supposed that the two 

 electrons in the normal atom revolved in the same orbit 

 about the nucleus, and he calculated what energy should be 

 required to remove one of these electrons, i. e. to ionize the 

 normal atom. Bohr's calculated value of the energy required 

 for ionization was found to be higher than that determined 

 experimentally, and it was concluded in consequence that his 

 model of the normal helium atom was incorrect. The expe- 

 rimentally determined values of the energies required for 

 the production of radiation and of ionization in helium, 

 correspond to light vibrations in the extreme ultra-violet 

 region, so that the radiations which, according to Bohr's 

 theory, would correspond to transitions between any other 

 orbit and the normal orbit, all lie in this region. An expla- 

 nation of the known serial systems of the helium spectrum 

 was first obtained by Lnndef, who succeeded in arriving at 

 the general sequence of the lines of these series by taking 

 into consideration the mutual disturbance of the electron 

 orbits. He attributes the principal series to the orbit into 

 which one of the electrons is removed at the radiation 

 potential difference, and explains the existence of two in- 

 dependent serial systems in the helium spectrum by the 

 -existence of two independent sets of outer orbits. One of 

 these sets of outer orbits is coplanar with the orbit of the 

 undisturbed electron, while the plane of the second set of 

 orbits makes an angle with this plane. The two sets are 

 known as the "coplanar" and the "crossed''' systems 

 respectively. Corresponding to the two principal series in 

 helium there are, on this view, two radiation potential differ- 

 ences, at one of which an electron from the normal atom is 

 removed to the first outer orbit of the coplanar system, and 

 at the other of which it is removed to the first outer orbit of 

 the crossed system. The difference between the two radiation 

 voltages should therefore be equal to the difference between 

 the voltages connected by the quantum relation with the 

 limiting frequencies of the two principal series of helium 

 lines. These voltages are 4*78 volts and 3'98 volts re- 

 spectively, so that the interval between the two radiation 

 points should be 0*8 volt. Franck and Knipping in their 

 •earlier paper described experiments to show that two radiation 

 voltages differing by this amount existed. According to 



* J. Franck and P. Knipping', Zeits. f. Phys. i. p. 320 (1920), 



t A. Lando, Phys. Zeits. xx. p. 228 (1919). 



