IOWA ACADEMY OF SCIENCE 
265 
THE APPARENT INVALIDITY OF OHM’S LAW. 
It is well known that the resistance of light-sensitive selenium is a 
function of the electro-motive force across the circuit. Practically every 
investigation of crystal contacts has recognized a similar phenomenon 
with crystals, although the relation has usually been expressed with the 
resistance as a function of the current. There is no obvious reason why 
the variations from Ohm’s law in crystal contacts as well as in selenium 
can not arise from a common action. But there is no evidence that shows 
whether it is the action of the electrical current per se or the electrical 
stress as indicated by the fall of potential in the circuit, that produces 
the variation from Ohm’s law. I prefer to attribute the change of con- 
ductivity in crystals and in selenium to the electric intensity. This notion 
makes it somewhat easier for me to perceive the relation of the phe- 
nomena to the dynamic equilibrium of selenium. In the lower curve 
of fig. 2 is shown the manner of variation of the resistance of galenite in 
contact with gold plates* when the potential is varied. The upper curve 
shows the corresponding variation in light-positive selenium. With the 
scale chosen the rate of changes of resistance is nearly the same in both 
instances. Where the potential difference in the selenium changes from 10 
to 30 volts, the resistance changes from 50,000 to 38,000 ohms, while with 
the galenite when the potential difference changes from .2 to .6 volts the 
corresponding drop of resistance is from 53 ohms to 30 ohms, i. e., with 
the same ratio of increase of voltage there is not only a decrease of resist- 
ance in both, but the percentage decrease is of approximately the same 
order of magnitude. Perhaps if the data for galenite covered the range 
from 0.1 to .3 volts we should find the percentage change of resistance 
more nearly equal to that of selenium. It is noteworthy that the potentials 
are of the order of 100 times greater in the selenium when the same slope 
of the curve is approximated. This compares favorably with the increased 
scale of pressure as shown in fig. 1, necessary for the comparison of the 
pressure-resistance curves. 
THE CHANGE OF RESISTANCE WITH TIME AFTER THE POTENTIAL DIFFERENCE 
IS APPLIED. 
Selenium and crystal-contact resistances present essentially the same 
behavior by continued application of potential differences across the re- 
sistance. In general the resistance of selenium decreases after closing the 
circuit as it does in crystal contacts. According to Pierce if (Phys. Rev. 
borundum and then in the same manner the voltage is decreased, it is 
♦•Streintz and Wellik Phys. Zeits loc cit. 
