no 
[VIII. 2 
THE QUANTUM 
described a mechanism within the atom so constituted as to 
compel all exchanges between the kinetic energy of electrons 
and radiant energy to take place in amounts hv, where h is 
Planck’s constant and v is the frequency of the radiation. 
“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 kinetic energy of the electron which 
is absorbed by the atom is given by the equation 
U = hv, 
where h is 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.” 
Whittaker considered the nature of the interaction between 
an atom and an approaching electron. Let us suppose that an 
electron is approaching an atom with a velocity which is not 
great enough to ionize the atom, but which may be great enough 
to evoke a “ single-line spectrum ” (Franck and Hertz, 1914). 
“ The experimental results indicate that the electron, as it 
approaches, experiences a repulsion which is sufficient to turn 
it back altogether if its kinetic energy is less than hv. We have 
first to determine the nature of this repulsion. Now there are 
two kinds of force which are capable of acting on the electron, 
namely, electric force and magnetic force, and we have to decide 
which of these two is operative in the present case. The decision 
is easy, since we know that motion through a field of electric 
force affects the kinetic energy of an atom, while motion through 
a field of magnetic force deflects its direction of motion without 
altering its energy : and in our case the fate of the electron 
depends entirely on whether it possesses enough kinetic energy 
to force its way through the field. The field must therefore be 
electric : that is to say, an electron which is approaching an atom 
experiences, in the vicinity of the atom, a field of electric force. 
“ We have now to consider whether this field of electric force 
is always present in the vicinity of the atom, whether the electron 
is there or not, or whether the field is evoked in some way by the 
approach of the atom. Here again the decision is not in doubt: 
the atom cannot maintain a permanent electric field in this 
vicinity unless it either contains an excess of electricity of one 
sign, or is permanently polarized electrically; both of which 
suppositions are ruled out since the atoms considered are neutral 
atoms which do not respond to an ordinary electric field. We 
thus infer that the electric field about an atom is not permanent, 
but is evoked by the approach of the electron. 
“ We have now to consider what kind of mechanism within