Vol,. 8, 1922 PHYSICS: SHELDON AND GEIGER 
161 
THE PRODUCTION OF AN E.M.F. ON CLOSED CIRCUIT BY A 
LIGHT EFFECT ON ARGENTITE 
By H. Horton Sheldon and Paul H. Geiger 
Physical Laboratory, University of Michigan 
Communicated, May 12, 1922 
While obtaining data on the change of resistance of Argentite (Ag2S) 
with change of intensity of illumination, it was observed that under proper 
conditions a deflection of the galvanometer was secured on illumination 
when no external K.M.F. was applied to the circuit. The illuminated 
contact was always the negative end of the crystal. 
To study this effect further, the crystal was connected directly across 
the galvanometer and all factors eliminated which might in any way have 
an effect on the galvanometer. It was not difficult to identify the phe- 
nomenon as distinct from pressure effects, etc., but it required more careful 
investigation to make certain that it was not a thermoelectric effect due 
to the heat radiated from the lamp. The fact that the crystal turned 
out to be but little affected by the longer wave-lengths somewhat simplified 
this procedure. A heating coil radiating much more heat than the lamp 
by a thermopile test, was put in place of the lamp, but the result produced 
was extremely slight in comparison, even when the contacts were blackened 
by smoke from naphthalene. 
