1921-22.] On the Quantum Mechanism in the Atom. 
129 
X. — On the Quantum Mechanism in the Atom. 
Professor E. Whittaker, F.R.S. 
' (MS. received April 12, 1922. Read May 8, 1922.) 
§ 1. Introduction. 
It is now well established experimentally that when an atom is caused to 
emit radiation of frequency v by collision with an electron, the amount U of 
the kinetic energy of the electron which is absorbed by the atom is given 
by the equation 
V = hv, 
where h denotes Planck’s quantum of Action : an electron whose kinetic 
energy before the encounter is less than hv is incapable of stimulating 
the atom to emit the radiation, and is merely repelled from the atom 
without any loss of energy. It has long been known that the relation 
= hv exists between the frequency of X-rays and the kinetic energy of 
the cathode rays which produce them : and since the discovery of Franck 
and Hertz in 1914 that when mercury vapour is traversed by electrons 
possessing the kinetic energy acquired in passing through a potential fall 
of about 4*9 volts the vapour is stimulated to the emission of the radiation 
of wave-length X = 2536'72 A.U., a great number of experimental in- 
vestigations have been published dealing with “ single-line spectra ” : the 
relation JJ ^hv is always satisfied. 
The aim of the present paper is to investigate the mechanism within 
the atom which compels all exchanges between the kinetic energy of 
electrons and radiant energy to conform to the equation JJ = hv. In § 2, 
by a process of deduction from the known results of experiment, it is 
inferred that the mechanism within the atom must be such that an electron 
approaching an atom induces in the atom what is here called a “ magnetic 
current”: and in § 3 it is shown that this leads directly to the consequence 
that the atom can absorb energy from the electron only in quanta. In § 4 
it is shown that the disturbance in the atom after the collision consists in 
the displacement of a single electron, and it is further shown that the 
radiation emitted by the electron during its oscillatory subsidence to its 
normal state must satisfy the equation U = Aj/. 
VOL. XLII. 
9 
