268 
IOWA ACADEMY OF SCIENCE 
off, but scratching 1 or scarring a rectifying surface usually spoiled more 
or less completely its rectifying properties. In order to destroy the 
rectifying properties, the resistance must have changed in one direction 
or the other.” However, he does not state whether mutilation increased 
the resistance that was least or decreased the resistance in the direction 
of largest value. I assumed that he meant the latter. However, in 
order to be more certain I experimented with a crystal of galenite which 
had slightly greater resistance in one direction than in the other. I 
rubbed one contact surface quite vigorously with rouge on chamois 
cloth. This caused the resistance to decrease about ten times. After 
several months the crystal did not recover from this abrasion, but no 
doubt it would do so in time. 
It is, therefore, obvious that abrasion produces certain very similar 
results in crystals that it does in selenium. 
THE EFFECT OF ALTERNATING CURRENTS. 
Alternating currents produce varying results depending upon the light- 
sensitive selenium and the frequency of the current. Certain frequen- 
cies have been noted to decrease the resistance while other frequencies 
have increased the resistance. Alternating currents produced effects in 
crystal contacts' that might be regarded analogous to those in selenium. 
However, we should investigate the exact conditions of the phenomena 
before we can presume as to the likeness of the effects. 
THE BREAKING DO^VN OF THE RESISTANCE BY HIGH VOLTAGE. 
If a very high voltage is applied across a light-sensitiYe selenium all 
the resistance breaks down and the light sensitiveness is destroyed. Ordi- 
narily the voltage from the secondary of an induction coil is necessary 
for this result. Just how much lower the potential might be I am not 
certain. 
In crystal contacts a potential difference of 30 or 40 volts is almost 
certain to break down the resistance. However, this potential is partly 
determined by the pressure on the crystal surface and the number of 
contacts in series. A number of crystals in series approach more nearly 
to the conditions existing in selenium. 
THE ANTIMONITE CELL. 
If we could establish like behavior between crystal contact resistances 
and the selenium cell, when the two are illuminated, we should have a 
strong presumption in favor of the identity of the two sets of phenomena. 
