215 
1921-22.] The Magnetic Character of the Quantum. 
the conception of the magneton, which in some form or other seems to be 
essential for the explanation of magnetic phenomena.* Again, the use of 
inductance in connection with the frequency of the Hertzian oscillator in 
§ 4 implies the existence of tubes of magnetic induction within the atom, 
and we are consequently led once more to postulate the existence of a 
magneton. 
§ 3. No one has made more brilliant use of models for the elucidation 
of physical phenomena than Clerk Maxwell ; at the same time, no one has 
understood more clearly their limitations. Most physicists would agree 
that in the present stage of scientific development it is still necessary for 
them to work with a model, even if the particular model employed should 
require modification or should ultimately be discarded. 
It is as a rule an easy matter to construct a model to represent a limited 
number of physical facts. Difficulties are apt to multiply when we 
endeavour to make our model fit in with other groups of phenomena. One 
of the chief tests of a satisfactory physical theory is its comprehensiveness. 
Perhaps the most important fact which requires explanation in connection 
with atomic structure is the existence of quantum “levels,” now firmly 
established for the outer shell of the atom, and true, if the recent work of 
Ellis on /3 and y rays from radioactive elements he confirmed, for a massive 
nucleus also. If Professor Whittaker’s or any other model can give us a 
clear mental picture of these levels, and with them explain Bohr’s frequency 
condition 
/iv = W2-Wi, 
where W is the energy associated with a particular level, we shall have 
gone a long way towards establishing an enduring theory of atomic 
phenomena. 
It is well known that Rutherford’s theory of the atom, in which there 
is a positive nucleus of extremely small dimensions, arose in connection 
with the scattering of a particles, the electrostatic field of the nucleus being 
responsible for the observed wide-angle scattering. It would be interesting 
to know whether the mechanism imagined by Professor Whittaker would 
account for the facts without necessitating such a concentrated nucleus. 
In a number of papers in the Philosophical Magazine f I have discussed 
the possibility of the core of the atom being of size sufficient to produce 
appreciable magnetic forces, but the objection to such a model has always 
* The difficulty of explaining magnetic effects in terms of the electron theory was clearly 
pointed out by M‘Laren at the British Association meeting of 1913. An abstract of his 
paper appears in the Amiual Report for that year. 
t H. S. Allen, Phil. Mag.., vol. xxix, p. 40, p. 140, p. 714, 1915. 
