IOWA ACADEMY OF SCIENCE 
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using amalgamated surfaces, according to Koenigsberger, Reicenheim 
and Schilling.* * But we can not be certain that mercury acts in the same 
way in selenium that it does with other crystals. If a small globule of 
mercury is put on a crystal surface* the contact resistance will not be 
destroyed. It is probable that the surface tension of the mercury keeps 
the globule from making the intimate contact that is made by the amal- 
gam under pressure. A gold electrode in contact with a crystal of 
galenite had a resistance of 50 ohms with 150 gms pressure. After the 
gold was amalgamated the resistance was .087 ohms with 150 gms pres- 
sure and .057 ohms with 1224 gms. The change of resistance with pres- 
sure and with the current is comparatively very small after amalga- 
mation. This together with the fact that the change of resistance by 
pressure is greater than the resistance of the crystal itself, furnishes 
almost a conclusive proof that the pressure effect and the current effect 
is of the nature of a change of resistance at the point of contact with 
the crystal. 
In selenium the mercury vapor forms mercuric selenide. Whether 
it is this selenide or the free mercury molecules or semi-free molecules 
in the selenide, that makes the light conductivity can not be definitely 
settled yet. However, if we take amorphous selenium and let the mer- 
cury vapor act on it, we obtain almost as high a conductivity if only 
about one per cent of the selenium is acted upon as we do if the entire 
quantity of selenium is transformed into selenide. This is very much in 
favor of the presumption that it is the free or free acting mercury 
molecules that is responsible for the very high conductivity. The specu- 
lation that we may find it convenient to make is that mercury in a 
finely divided state may bring about the same action in crystal contacts 
that it does in selenium. The presumption is based on the apparent 
similarity of the effects. 
THE EFFECT OF ABRASION. 
In two earlier papers* I have described the effect of rupturing the 
surface of light-sensitive selenium by abrasion. The immediate effect 
of the abrasion is to increase the conductivity. This effect is; likewise 
duplicated in crystal contact resistance. I may quote from Flowers,* 
'“A crystal having a rectifying surface was often found to have other 
rectifying surfaces underneath and parallel when the layers were split 
*Phys. Zeits, 12 p. 11, 39, 1911. 
*See paper by Alan E. Flowers on crystal and contact rectifiers, Phys. Rev. 29, p. 
451, 1909. 
*F. C. Brown, Phys. Rev: 34, 201, 1912, Phys. Zeits. 1912. 
*Phys. Rev. 29, p. 453, 1909. 
