PHYSICS: MILLIKAN AND SOUDER 
19 
EXPERIMENTAL EVIDENCE FOR THE ESSENTIAL IDENTITY 
OF THE SELECTIVE AND NORMAL PHOTO- 
ELECTRIC EFFECTS 
By R. A. Millikan and W. H. Souder 
RYERSON PHYSICAL LABORATORY. UNIVERSITY OF CHICAGO 
Received by the Academy, December 3, 1915 
In the fall of 1913, while studying the photo-electric properties of 
freshly cut surfaces of the alkali metals in extreme vacua, we observed 
that immediately after first cutting, the fresh surface of sodium showed 
very large photo-sensitiveness when tested with monochromatic light of 
wave length 5461 A., even when the vacuum was of the order 10"-^ mm. 
as measured by a McLeod gauge. But after several weeks of experi- 
menting and many cuttings a condition was reached in which a freshly 
cut surface was completely insensitive when illuminated with this wave 
length. The lost sensitiveness reappeared, however, in the course of 
not more than two minutes after cutting, and grew rapidly to a very large 
value in fifteen or twenty minutes. When the gas pressure was of the 
order of 0.01 mm. the same phenomenon occurred but the rise to a maxi- 
mum value was less rapid. From these results we began to surmise that 
photo-electric currents must be due to the influence of some active gas, 
which diffused from the walls to the metal and whose action upon the 
surface was retarded by the presence of an inert gas. 
While we were in the process of testing this hypothesis by repeating 
the experiment under as diverse conditions as possible the papers of K. 
Fredenhagen,^ H. Kuster,^ G. Wiedmann and W. Hallwachs^ appeared 
nearly simultaneously, all taking the point of view which had been 
suggested to us by our own experiments, and the last one of them, 
namely, that of Wiedmann and Hallwachs. asserting with consid- 
erable positiveness that the presence of a gas is a necessary condition 
for the appearance of photo-electric currents in potassium, and that the 
"photo-electric effect in general takes place in the mixture formed by 
the body and the absorbed gas.'' 
By this time, however, we had obtained data which convinced us 
that the point of view taken by these authors was untenable and in 
April, 1914, at the meeting of the American Physical Society, we pre- 
sented a paper entitled 'Effect of Residual Gases on Contact E. M. F.'s 
and Photo-currents,'^ in which we described fully and exhibited to the 
society the curves shown in figure 1. A glance at these curves shows 
at once that the freshly cut surface is initially strongly photo-sensitive- 
to lines 2535 and 2804 and that its sensitiveness for these Hues diminishes 
