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THE QUANTUM [vm. 5 
It should be noticed that the electron gives angular momentum 
to the atom and itself loses only linear momentum along the axis. 
As Sir Alfred Ewing remarks : “ The principle of conservation 
would seem to require the assistance of the aether.” Again, on 
the basis of the model, the ejection of an electron from an atom 
gives the atom an angular impulse. This has a bearing on photo 
electricity, and suggests the importance of considering circularly 
polarized light in our theoretical investigations of fundamental 
phenomena. 
The question may be asked what is the relation between this 
model and that employed by Bohr. This question has been 
discussed by Whittaker, who has shown how it is possible in a 
formal way to assimilate to his own theory the part of Bohr’s 
theory dealing with the transformation into monochromatic 
radiation of the energy set free by the fall of an electron from 
an outer to an inner orbit. In the next chapter we shall show 
that the quantum mechanism affords an opportunity of con 
structing a static model of an atom possessing many of the 
properties of the Bohr atom. But since the advent of the new 
wave-mechanics it is seen that it is no longer merely a choice 
between two alternative models, but a much wider problem 
involving a reconsideration of our ideas as to the nature of atomic 
systems and even of electric charges. 
In concluding this account of the new mechanism suggested 
by Whittaker emphasis must be put on the essential point in his 
work, which is the “ magnetic current.” He says that it is 
simpler “ to assume frankly that magnetic currents can exist 
in an atom, without laying stress on their realization by means 
of bar-magnets.” And again, “ the magnetic current is, in fact, 
a new conception ; and just because it is a new conception there 
must necessarily be some flaw in any attempt to represent it 
by means of old conceptions, such as bar-magnets.”