VIII. 5] WHITTAKER’S QUANTUM MECHANISM 119 
of corresponding frequency. To make the comparison to a 
Hertzian oscillator valid, we must have the wheel execute angular 
oscillations, and this implies that when it is displaced from its 
initial position a restoring moment is called into being, so that 
the angular impulse which it receives as the electron passes 
through the system sets up oscillations like those of the balance 
wheel of a watch. The single line spectra of Franck and Hertz 
show that these oscillations are sensibly isochronous. 
“ In accounting for them it seems preferable not to drop the 
model at this stage. For in the model—as I have described it— 
there are essentially two magnetic systems in any atom : a centre 
one forming what is called the wheel, and another around it, 
which may for distinction be called the ring. (For simplicity of 
description we may confine the consideration to one plane. Thus 
in the figure we have a ring system ABCD as well as a wheel 
system W.) 
“ Each may be taken as rigid, but there is a freedom of relative 
rotation in the plane of the figure, except for the control exerted 
by magnetic forces between the two systems. These forces make 
the system take up a position of equilibrium as sketched, because 
the outer poles of the model have the same name as the inner 
poles of the ring. When an electron passes through and escapes 
it gives an impulse producing relative angular displacement. 
The mutual magnetic forces between the wheel and the ring tend 
to restore the whole to its original configuration. Thus oscilla 
tions are set up which expend their energy on the emitted radia 
tion. In this form, therefore, the model furnishes a mechanism, 
not for extracting a quantum of energy, but for converting that 
into alternating magnetic currents and thereby into radiation.”