400 TRANSACTIONS OF SECTION A. 
2. The Photo-electric Properties of Thin Metallic Films. 
By J. Rostinson. 
The dissymmetrical photo-electric effect is observed for metallic films of the order 
of thickness 10°? cm. For very thin films the emergent current is greater than the 
incident. The same statement applies to the velocities of the photo-electric electrons. 
As the thickness of the film increases, a certain thickness is found which makes the 
emergent current equal to the incident. It is found that at the same thickness the 
emergent velocity equals the incident velocity. 
Photo Electric Current 
10 7om 2x10 “cm 
Thickness of Film 
The actual magnitude of the photo-electric current for platinum (either incident or 
emergent) depends on the thickness of the film in a way shown by the accompanying 
curve. At a certain critical thickness (10~-’ cm.) both the incident and the emergent 
effects increase rapidly about thirty times. At this thickness also we have an altera- 
emergent 
tion of the ratio = currents. For films thinner than 10 7 cm. this ratio is 
cident 
constant. At this thickness the ratio begins to decrease as the film increases in thick- 
ness, 
At the same thickness we have also two other phenomena :— 
(a) The photo-electric velocities suffer a sudden change.' As the thickness 
of film increases through 10°7cm., the velocities diminish to about 
one-third of their previous value. 
(6) The specific resistance alters rapidly. 
The photo-electric effects at this thickness can be explained on the view that the 
photo-electric electrons may be produced in one of two ways :— 
(a) Directly by light from molecules of the metal: these we will call primary 
electrons. 
(b) By primary electrons colliding with molecules and producing secondary 
electrons. 
If the thickness 10°’ cm. is of the order of the mean free path of the electrons, then 
for films thinner than 10-? cm. we get only primary electrons which have fairly large 
velocities. | For films thicker than 10°’ om. we get both primary and secondary 
electrons whose velocities are necessarily diminished. 
1 Dike, Phys, Rev., July 1912. 
