10 
PHYSICS: TATE AND FOOTE 
The results obtained are shown in the accompanying table which also con- 
tains for the sake of completeness the results, already published, for sodium 
vapor. 
METAL 
resonance potential 
(volts) 
WAVE 
LENGTH OF 
RADIATION 
A 
ionization potential 
(volts) 
LIMITING 
WAVE 
LENGTH OP 
SERIES 
A 
Observed 
Calculated 
Observed 
Calculated 
Cadmium 
3.88 
3.79 
3260.17 
8.92 
8.97 
1378.69 
Zinc 
4.10 
4.02 
3075.99 
9.50 
9.37 
1319.95 
Potassium 
1.55 
1.65 
7685.0 
4.10 
4.33 
2856.0 
Sodium 
2.12 
2.10 
5893.0 
5.13 
5.13 
2413.0 
The curves for zinc vapor indicate the possible presence of a secondary 
inelastic encounter, apparently without ionization, at 2.3 volts; but further 
work is in progress to make sure of this point. 
It will be seen that in all cases the results agree within the limits of experi- 
mental error with the values as calculated from the quantum relation hp = 
eV, where v is the frequency of the single radiation in the case of resonance 
potentials or the limiting frequency of the series of radiations in the case of 
ionization potentials. 
It is to be expected that the frequencies corresponding to the ionization po- 
tentials should be the long wave-length limit of the photo-sensibility of the 
vapor in question. Experiments are now in progress to determine whether 
or not this is so. 
Concerning the question of the cause for the appearance of ionization in 
metallic vapors at applied potentials much lower than the ionization potential 
it should be remarked that in no case was there an observable decrease in the 
value of the resonance potential even under conditions which permit of the 
appearance of the complete spectrum at low potentials. It is therefore thought 
that this setting in of ionization at applied potentials much lower than the 
ionization potential is due rather to the presence of electrons having high 
initial velocities than to a decrease in the value of the critical potentials. 
The curves obtained with potassium vapor clearly show the presence of a 
large number of electrons having initial velocities corresponding to 1.6 volts. 
1 Abstract of a paper presented before the American Plrysical Society, December 1, 1917. 
2 Tate, Physic. Rev. Ithaca, N. Y., 10, 1917, (81). 
3 Tate and Foote, Washington, J. Acad. Sci., 7, 1917, (517). 
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