PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 7 DECEMBER 15, 1921 Number 12 
THE GROWTH AND DEC A Y OF PHOTO-THERMIONIC CURRENTS 
FROM OXIDE COATED FILAMENTS 
By H. D. Arnold and Herbert E. Ives 
Research Laboratories ov the American Telephone & Telegraph Company 
and Western Electric Company, Inc. 
Communicated by F. B. Jewett, October 18, 1921 
T. W. Case 1 and E. Merritt 2 have recently described experiments in 
which the oxide coated filament of an audion is subjected to illumination, 
with the result of increasing the current between filament and plate. 
In the experiments of Prof. Merritt this added space current was shown 
to be principally due to short wave-length radiation, and was explained 
as a true photo-electric effect, that is, the direct emission of electrons 
due to illumination. He found the added current to be dependent upon 
the filament temperature. This might be taken to indicate that the 
emission of photo-electrons is a function of the temperature of the surface, 
which disagrees with conclusions drawn from experiments upon other 
photo-electrically active materials. 
In some recent experiments we have found this effect to have character- 
istics sufficiently different from those of the true photo-electric effect to 
raise doubts as to the identity of the two phenomena. If they are not 
identical then the new effect is not necessarily evidence for a dependence 
of the photo-electric effect on temperature. 
The experiments here reported have to do with the rate of growth and 
decay of the light induced additions to the space currents. These were 
recorded on a string galvanometer through a distortionless amplifier 
system. In every case voltages were high enough to carry across all 
electrons liberated. A carbon arc lamp with glass condensing lens was 
used as a light source, and between the condenser and the oxide coated 
filament was interposed either a deep red or a deep blue glass (transmitting 
