80 PHYSICS: R.A.MILLIKAN 
plate respectively first become photo-sensitive, and Vo and F'o are the 
respective maximum potentials necessary to stop discharge into the 
cylinder from the plate and from another plate made from the sub- 
stance of the cylinder. 
T'Te<^ue nc Y 
FIG. 4. 
6. Contact EMF's are independent of temperature. This follows 
from Einstein's equation taken in connection with our now well confirmed 
proofs of the independence of photo-potentials upon temperature. 
Where, however, surface films cause variations with temperature of 
photo-potentials, there should be corresponding variations in contact 
EMF. 
