PHYSICS: TATE AND FOOTE 
9 
so related to each other as to satisfy equation (6) increases enormously the 
weight of the conclusion and seems to exclude the possibility that it should 
be the consequence of an obscure and unsuspected systematic error. 
We desire to express our great obligation to Miss Wolfe of the Computing 
Division who has executed most efficiently the numerous least-squares solu- 
tions required for the discussion of the data. 
1 Hale, G. E., Terr. Mag., Baltimore, 17, 1912 (173-178) ; ML Wilson Contr. No. 71, Astroph. 
J., Chicago, 38, 1913, (27-98). 
2 Seares, F. H., Mt. Wilson Contr. No. 72, Astroph. J., Chicago, 38, 1913, (99-125). 
RESONANCE AND IONIZATION POTENTIALS FOR ELECTRONS 
IN CADMIUM, ZINC, AND POTASSIUM VAPORS 1 
By John T. Tate and Paul D. Foote 
University of Minnesota and Bureau of Standards 
Communicated by R. A. Millikan, December 19, 1917 
It has been shown by Franck and Hertz and others that there are, for elec- 
trons accelerated through gases or vapors, certain definite potentials at which 
there is a large transfer of energy from the electron to the atom, as evidenced 
by the emission at these potentials of radiations characteristic of the gas 
atom. This is to be expected on purely mechanical grounds since no consid- 
erable transfer of energy from the light electron to the relatively heavy gas 
atom can take place except when the time of encounter between electron and 
atom bears some simple relation to the characteristic period of one of the 
vibrational degrees of freedom in the atom. It is therefore to be expected 
that there will be a critical potential corresponding to each absorption line 
of the gas and that at this potential the electrons will give up their energy 
to the gas and cause the emission of a radiation of the frequency of the corre- 
sponding absorption line. 
Two types of inelastic encounter between electrons and gas atoms have 
been observed. One of these results in the emission of a radiation of a single 
frequency, without ionization of the gas, while the other ionizes the gas and 
causes it to emit a composite spectrum of radiations. The potential giving 
the first type of encounter may be termed a resonance potential, that giving 
the second type an ionization potential. 
The present paper is an account of an experimental determination of the 
resonance and ionization potentials in cadmium zinc, and potassium vapors. 
The method employed was that described by Tate 2 for the determination 
. of critical potentials in mercury vapor and the apparatus was similar to that 
used by us 3 in our work on sodium vapor. 
