ISO 
IOWA ACADEMY OP SCIENCE 
is filled with sand and the sand falls into the other one that stands in the box. 
Then the full drawer is taken out and replaced by the other one, and the same 
play is repeated as often as required.” In Giltay’s apparatus the rupture is 
produced by the sand falling against the surface of the selenium. 
My method of producing the rupture was by the use of a fine swiss file, so 
ground that it would remove the selenium from between the wires but would 
not act on the wires. It is difficult to say ju^t what would be the difference in 
the rupture produced by the file from that by a sand-blast. Perhaps the file 
would be more coarse in its action. However, as far as the facts go there is 
no indication of any difference in the rupture by whichever method ft is 
produced. If so the filing method has the advantage in that the amount of 
selenium removed can be determined. 
THE RUPTURE EFFECT. 
The discovery of this new effect was somewhat accidental. I was using a 
file to remove the surface layers of the selenium in a Giltay cell, in order to 
find out if all the selenium on the cell were conducting, and also with the hope 
of increasing the light sensitiveness, i.e., the ratio of the conductivity in the 
light to that in the dark. The first test showed the conductivity to increase 
when the selenium was filed away. In the second test the selenium, about 
0.06 gm., between eight parallel wires of the Giltay cell was considered. The 
conductivity in the dark was 87 scale divisions. After filing away a very small 
amount of selenium from all over the surface, the conductivity actually in- 
creased five fold. More than one-half of the selenium was removed before the 
conductivity in the dark fell as low as 170 div. By continuing the process 
until only a thin film of selenium was remaining, the conductivity fell after 
24 hours in the dark to 20 div. The light sensibility was then 66 to 1, by a 
test that gave about 30 to 1 prior to the surface rupturing. The conductivity 
both in the light and in the dark was at first increased by the filing action. 
This indicates a change in the properties of the selenium by the action. . 
Giltay’s discovery was also somewhat accidental. He states: “I had two cells, 
both showing somewhat bluish color as sometimes happens with my cells . 
The dark resistance was 
for 31b. R=190,000 ohms, and in diffuse light R/22.5 
for 32b. R=300,000 ohms, and in diffuse light R/25. 
As the cells when blue do not look well, I treated No. 31b with the sand blast 
so (see Pig. 1) in order to get the usual color of the selenium surface. I expected 
the resistance and also the sensitiveness to get somewhat larger, owing to the 
selenium layer getting somewhat thinner. After having been sand-blasted both 
cells were put into the dark box, and the next day both cells were measured 
again. This showed: 
31b. R= 70,000 ohms, and in diffuse light R/8.2 
32b. R=300,000 ohms, and in diffuse light R/25 
After 15 days both cells were again measured. The temperature of the room, 
was much lower now, . The cells now -showed: 
31b. R=140,000, in diffuse light R25R/25 
32b. R=430,000, in diffuse light R/44 
