56 SECTIONAL ADDRESSES. 
is well known that Larmor has shown that one result of the establish- 
ment of such a field is to endow an electronic orbit with a uniform 
rotation about the direction of the magnetic field, the angular velocity 
: BS oH Langevin has also pointed out that the 
2m ¢ 
being given by @ = 
size and form of the electronic crbit remain unaffected by the magnetic 
field. Ehrenfest’s hypothesis asserts that if the magnetic field be 
established slowly the energy of the electron in its orbital motion and 
the frequency of its revolution.in the orbit may be changed, but the 
number of quanta defining its energy undergoes no modification. With 
the adoption of these principles it is an easy matter to show that when 
we quantise the angular momentum about the direction of the magnetic 
field the normal Zeeman components are exactly the same as those 
provided by the older classical theory of Lorentz. The singular beauty 
and simplicity of this method of explaining the normal Zeeman effect 
constitute one of the finest achievements placed to the credit of the 
quantum theory. 
Efforts to explain the abnormal Zeeman effect have not as yet met 
with the same siccess. Among the contributions made to this subject 
perhaps that of Heisenberg ** is the most stimulating and suggestive. 
In addition to offering an explanation of the abnormal Zeeman effect 
it constitutes an attempt to account for the doublet and triplet structure 
of series spectra. 
Taking for example the case of an alkali element, Heisenberg 
postulates that through magnetic coupling a movement of rotation 
‘within an atom of these elements involves simultaneously the valency 
electron and the core of the atom as well. According to the theory it 
is supposed that in the various stationary states there is a partition 
of the angular momentum between the two, one-half an azimuthal 
quantum being assigned to the core and k-4 azimuthal quanta to the 
electron. The author supposes further that through space quantisation 
the two axes of rctation are in the same direction, and that the rotation 
of the core and that of the electron may take place either in the same 
sense or in opposite senses. As far as the radial quanta for the 
electronic orbits are concerned, it is assumed that they are given by 
n’+4 where n’ has integral values. This device leads to the result 
that the total quantum number characterising the orbit of the electron 
is an integer n that is equal to the sum k+n'. In this way the author 
is enabled, at the same time, to characterise the spectral terms in the 
Rydberg series formule by integra] quantum numbers. 
This scheme, it will be noted, provides for the binding of the valency 
electron in one or other of two energy levels and reduces the frequency 
difference characterising the members of the doublet series of the spectra 
of the alkali elements to a manifestation of what is practically a Zeeman 
effect produced by an internal atomic magnetic field. To account for 
the triplet structure of series spectra such as we obtain with the 
alkaline earth elements, Heisenberg supposes the magnetic coupling 
48 Heisenberg, Zeit. fiir Phys., No. 8, p. 257 and p. 278, 1922. 
