162 
PHYSICS: SHELDON AND GEIGER 
Proc. N. a. S. 
The behavior of the galvanometer was also an indication of the dis- 
tinction. In the case of the heater the galvanometer showed a steady 
creep to the maximum value, interpreted as due to the gradual heating 
of the exposed contact. In the case of the light the maximum was reached 
in approximately 13 sec, which was the quarter period of the galvanom- 
eter. A piece of plate glass an inch thick reduced the effect of light 
about 30%. 
Having satisfied ourselves then, that the major portion of the effect 
was in the visible region a series of experiments were carried on with vary- 
ing intensity in an attempt to find an analogy with the photo-electric 
effect. For this purpose a 16 c.p. bulb, such as used in an automobile 
head light, was employed, being chosen because of approximating closely 
to a point source. The intensity was varied by sliding the lamp along 
an optical bench, and not by any change in energy input which might 
vary the wave-lengths in any way. The E.M.F.'s set up by the light 
action were balanced out by a potentiometer method, this null method 
giving very consistent results. Figure 1 shows E.M.F. in volts against 
intensity on an arbitrary scale. 
It would seem from this that it is not similar to what is commonly 
called the photoelectric effect, but while this is probably true, there are 
other things which might alter the shape of the curve. 
First, the shape of the curve might depend on whether we start our read- 
ings from high or from low intensity, since resistance has been found to 
increase with the length of time the current flowed. This was not the 
case here, however, for the same curve was obtained from either direction, 
since a little practice will enable one to succeed in balancing out the effect 
very quickly. 
Secondly, the resistance has been found to change with intensity of 
illumination. Such a decrease in resistance would cause a lower B.M.F. 
across the crystal in proportion to the rest of the circuit, so that the values 
of E.M.F. if corrected, would be raised as we go out along the curve. 
Since, however, the crystal resistance may be from 200,000 to over 1,000,000 
ohms, and the galvanometer resistance was only 400 ohms, the effect 
would be small and would not materially change the shape of the curve. 
It would seem then, from present data, that this is not a true photo- 
electric effect as the term is now used, but probably a light effect on the 
contact potential. 
If we look again at the figure, there is to be seen a point which would 
suggest a bend which might indicate a secondary effect, however the accu- 
racy of our readings do not permit of a certainty in this respect as yet. 
This is being investigated as well as the variation with wave-length and 
it is hoped that these results may be published in the near future. 
The galvanometer used is the Leeds Northrup high sensitivity type 
