IOWA ACADEMY OF SCIENCE 
263 
Monten as given in his dissertation at the University of Uppsala in 
1909. According to him the conductivity increases more than seventy 
times in going from normal pressure up to 3,000 atmospheres. For 
certain samples of selenium the equation giving the relation between 
pressure and resistance is 
R=2.5 X 10 6 .e- < ? 0083p 
In the lower curve of fig. 1 is given the relation between pressure 
and resistance for galenite as taken from Streintz and Wellik’s paper 
(Phys. Zei'ts. 12, 848, 1911) where 0.6 volts was the difference of po- 
tential across the crystal and its contacts. Curve 2 shows the same 
relation when 0.41 volts was the potential difference. These curves are 
typical of what would obtain with many other crystals. 
Likewise curve 3 gives the variation of resistance of a selenium cell 
when the potential difference was 10 volts and curve 4 shows the same 
relation for 1.4 volts. A glance is sufficient to make obvious the similarity 
between curves (1), (2) and (3), (4). It will be observed that the 
pressure-resistance curve for selenium follows approximately the same 
curve for galenite, where the fall of potential across the latter was 0.41 
volts. Evidently the agreement would be better if the fall of potential 
were yet less in the selenium. This is particularly noticeable at the 
higher pressures. However this might lead to the assumption that the 
selenium in the selenium cells is under an initial pressure of several 
atmospheres due to the packing of the crystals. 
The different values for the scale divisions may obviously be explained 
in one or both of two ways. Either the selenium crystal contact may 
be inherently of higher resistance than those of the frequently studied 
crystals or the pressure of a single crystal may be only a small frac- 
tion of the pressure per unit area. Supposing the resistance of selenium 
to be essentially due to contacts and further that the resistance of each 
contact to vary precisely as it does with galenite, it can easily be de- 
duced that the area of each selenium crystal is approximately .0003 sq. 
cm. But this assumption and conclusion are purely speculative. 
Perhaps the well known change of resistance of carbon granules with 
varying pressure is more nearly akin to the change in selenium than 
those referred to in this paper. I have noted however that the resistance 
of a large number of carborundum crystals when packed under a given 
pressure is unusually large compared with the resistance of a single 
crystal contact. 
