IOWA ACADEMY OP SCIENCE 
183 
THE EFFECT OF ABRASION ON LIGHT-NEGATIVE SELENIUM.^ 
The fact that light-negative selenium is unstable and increases its resistance 
by merely jarring, would lead one to expect that rupture by abrasion would 
produce a similar increase, rather than the decrease produced in light positive 
selenium. 
The unit of light-negative selenium which I tested had a resistance of about 
19.2 ohms in the dark with .14 volts across its terminals, and 20.7 ohms when 
exposed to the diffuse light of the room. After rubbing a file across one quarter 
of the selenium surface, the resistance increased to 26.2 ohms. And after merely 
drawing the file across the second quarter it further increased to 28 ohms, and 
again similar treatment to the third quarter caused the resistance to rise to 
36.9 ohms. As I did not wish to risk spoiling the sample unnecessarily I did 
not carry the treatment further. After one week in the dark, the resistance 
had recovered to only 34 ohms in the dark. When two months had elapsed the 
conductivity had recovered to 20.7 ohms in the dark and 21.8 ohms in the light 
of the room. It should be noted that the recovery was almost complete and 
also that the process was just as slow as was the recovery with the light-positive 
selenium. It is interesting and important that the abrasion effects are of 
opposite sign in the light-positive and light-negative selenium, and also that the 
effects appear otherwise alike. 
THE NATURE OF THE EFFECT. 
The most elementary explanation of the rupture effect by abrasion may be 
based on the kinetic theory of matter, in which the essential constituents of 
the matter are constrained to act within certain configurations. When the light- 
sensitive selenium is in equilibrium in the dark, the moving parts are con- 
strained within certain configurations, or fixed boundaries, perhaps the bound- 
aries of the crystal surfaces. The same boundaries may prevail more or less 
when the selenium is exposed to light. The filing for example destroys these 
boundaries and so the moving parts of the selenium begin readjusting slowly 
until a new equilibrium is established. The parts of the selenium must be in 
motion all the time. The abrasion merely puts these motions out of their 
usual course. The electrical conductivity fortunately in selenium signifies 
whether or not the equilibrium state is reached with certain parts or how fast 
it is being approached. Probably all matter of crystalline structure undergoes 
similar changes as a result of rupture, but unfortunately most forms of matter 
do not show in a pronounced way this electrical change as a counterpart. 
In two previous papers on the nature of light action in selenium,** I showed 
that the various kinds of behavior of different varieties of selenium, under 
the action of light, could be explained by assuming three components, which 
were in equilibrium according to the reaction Ah±B^C. The rates of change 
maintained one set of fixed values in the dark and another in the light. It was 
necessary to assume that the different varieties of selenium had different initial 
rates of change in the dark, but no explanation was offered as to why this 
should be true, i. e. why all selenium should not tend toward the same equili- 
brium value and the same conductivity in the dark. It was suggested however 
'“See paper by F. C. Brown in Piiys. Zeits. 11, p. 482, 1910, and also paper by Lilali 
E. Crum, on Some Characteristics of Bight Negative Selenium, 33, p. 538, 1911. 
'^*Phys. Rev. 33, p. 1 and p. 403, 1911. 
