v(ijU/: coxrnMPOR.th'y .ini-,i\crs i\ I'livsics- ix asi 



was wlu'ii \vi- siipposi'd till- fU-clroii lo la- sliiixliiiK still, anil llie 

 kiiR'tic t'iH'rn\- (if llii- fli'c'trmi, wiiirli is l ;wt'-. In this last expression, 

 V stands for the speed of the electron in its orbit; now, wv-/r is the 

 "centrifugal force" acting upon the electron, which is equal (and 

 opposite) to the attraction exercised hy the nucleus upon the electron., 

 which is r, /•-; so that \ mv" is equal to -{-e-/2r, and the total energy 

 of the atom has the salue —e-/'2r. As before, this is the energy- 

 value referretl to the slate of the ionized atom. 



This c|uantity —e-'2r must be permitted to assume the successive 

 energy-values of the succcssi\e Stationary States, and no others; 

 we must have 



-r 2r= -Rh w- (M = 1, 2, 3, 4 ) (6) 



Kach of these equations defines a value of r, as follows: 



r = n-e- 2Rli (w = 1 , 2. :5, 4 ) (7) 



If we say that the electron may revolve in and only in such circles 

 as have the radii given by the equations (7), we thus define an atom- 

 model interpreting the Stationary States. Is this atom-model 

 superior to the tentative one which was described just before it? 

 Not in any way which has yet been brought to notice. Xo reason 

 is given why the electron should revolve in a circle instead of spiralling 

 into the nucleus, nor why it should revohe in these and only in these 

 circles, nor why it should cause radiation of a peculiar frequenc\' to be 

 emitted when it passes from one of these circles into another. .Ail 

 of the objections which I suggested, a few paragraphs above, thai llie 

 reader might raise against the then-mentioned atom-model with 

 the stationary electron, may equally well be raised against this atom- 

 model with the revolving electron. Why then should we attach 

 greater importance to this one than to that? Partly, as I said, be- 

 cause this atom possesses intrinsic magnetic properties, while to the 

 other one magnetic qualities would have to be ascribed by an addi- 

 tional assumption; but chiefly because Bohr discovered certain dis- 

 tinctive features of the circular orbits defined by (7), which set them 

 apart from all others. These we now e.xamine. 



To understand the first of these features, it is nece.ssar>- lo consider 

 the angular momentum of the atom. Sooner or later we shall have 

 to make a slight alteration in the reasoning indicated in the last para- 

 graphs; it may as well be made now even though it is not yet necessarj-. 

 Heretofore I have tacitly assumed that the nucleus stands still while 

 the electron revolves around it. As a matter of fact, if the atom may 

 be repnscnted as a solar system in miniature, the nucleus and the 



