1921-22.] Note on Professor Whittaker’s Paper. 
143 
XI.— Note on Professor Whittaker’s Paper “ On the Quantum 
Mechanism in the Atom.” By Sir J. Alfred Ewing, K.C.B., F.R.S. 
(MS. received May 6, 1922. Read May 8, 1922.) 
In Professor Whittaker’s highly interesting and suggestive application of 
my model the postulated structure is a system of magnets rigidly connected 
like the spokes of a wheel, having poles of the same name at the circumfer- 
ence, the corresponding poles of contrary name being all at the centre. 
He shows that when a bombarding electron, moving in the direction of the 
axis, approaches such a wheel it sets the wheel turning, the rotation of the 
poles constituting what he calls a “ magnetic current.” If the initial velocity 
of the bombarding electron falls short of a certain limiting value, the 
encounter is perfectly elastic, and the electron is reflected back without 
loss of energy ; the point at which its direction of motion becomes reversed 
may be on either side of the plane of the wheel, depending on the electron’s 
initial velocity of approach. If the initial velocity exceeds the stated limit, 
the electron passes clear through the system and escapes, but with a 
deflnite reduction of velocity , corresponding to a quantum of energy which 
it has given up to the wheel. When the encounter is elastic, the angular 
velocity which the wheel acquires during the approach of the electron is 
lost during the return of the electron. When the encounter is not elastic, 
and the electron has passed clear through, the wheel is left with an angular 
velocity co] and the energy which that represents is the source of the 
consequent radiation. In Professor Whittaker’s notation, the energy com- 
municated by the electron to the atom in that case is 
U 
JAco^ 
2ehM2 
where M is the sum of the magnetic poles at the circumference, and A is 
the moment of inertia of the wheel, e being the charge of an electron. 
The angular momentum communicated to the wheel is 
Ao)=2eM. 
The electron, whose mass is m, has its speed reduced from u to v, which is 
such that 
This is the quantum of energy, which is related to the frequency of the 
consequent radiations by the Planck equation 
V = ?lv. 
