Normal State of the Helium Atom. 133 



The classical mechanics permits an infinite -variety of orbital 

 systems satisfying the above conditions and a very simple 

 model much like that of Franck and Reiche is obtained if 

 they are so fulfilled that both the resultant angular momentum 

 and the average angular momentum of each electron are 

 equal to h/27r. The electrons revolve about intersecting 

 inclined orbits of the same approximately circular shape, and 

 are always symmetrically placed with respect to the axis of 

 resultant angular momentum. The calculation of the energy 

 promises to be a very tedious task, and has not yet been carried 

 through. If the computed energy of the model proves not to 

 be the same as the experimentally determined energy of the 

 normal atom, a reconsideration of the hypotheses on which 

 the energies of the various models have hitherto been calcu- 

 lated may be necessary. 



Summary. 



The model of the normal helium atom proposed by Franck 

 and Reiche cannot be correct, for it gives no explanation of 

 the chemicnl properties of helium, it gives the wrong ioni- 

 zation potential, and does not harmonize with the observations 

 of Fricke and Lyman on the ultra-violet emission spectrum 

 of helium. 



The experimental facts regarding the excitation of the 

 various lines in the arc spectrum of helium, show that the 

 only stationary states from which direct passage to the normal 

 state is ordinarily possible are those of the seizes (?/?, S); 

 that the direct transfer of atoms from the states (?n, S) to 

 the normal takes place with relative difficulty; and that 

 the presence of some common impurity in helium provides 

 an indirect path for the return of atoms to the normal state 

 which is accompanied by the emission of more than one 

 radiation frequency. 



The application of the Rohr principle of selection to the 

 passage of atoms from the stationary states of higher energy 

 to the normal state, if legitimate, leads to the conclusion 

 that the resultant angular momentum of the normal helium 

 atom is zero. 



Neither of the two possible models having zero angular 

 momentum is very satisfactory, and it seems probable thai 

 the application of the Rohr principle of select : on to atoms re- 

 turning to the normal state will have to be abandoned. .V 

 symmetrical model with one unit of angular momentum, 

 similar in many respects to that of Franck and Reiche, is 

 suggested as a possible solution of the problem. 



Jefferson Physical Laboratory, 

 Harvard University. 

 February 11, 1921* 



