32 SECTIONAL ADDRESSES. 
so move until an electric field is set up which equilibrates this difference 
of potential energy. There will thus be an intrinsic or contact difference 
of potential between metals which is equivalent to the difference in 
the values of w and is equal to the difference in w divided by the 
electronic charge.°® 
Photoelectric Action. 
We have seen that there is a connection on broad lines between 
thermionic emission and both contact potentials on the one hand and 
photoelectric emission on the other. ‘lhe three groups of phenomena 
are also related in detail and to an extent which up to the present 
has not been completely explored. In order to understand the present 
position, let us review briefly some of the laws of photoelectric action 
as they have revealed themselves by experiments on the electrons 
emitted from metals when illuminated by visible and ultra-violet light. 
Perhaps the most striking feature of photoelectric action is the 
existence of what has been called the threshold frequency. For each 
metal whose surface is in a definite state there is a definite frequency 
%» Which may be said to determine the entire photoelectric behaviour 
of the metal. The basic property of the threshold frequency n, is 
this: When the metal is illuminated by light of frequency less than 
m no electrons are emitted, no matter how intense the light may be. 
On the other hand, illumination by the most feeble light of frequency 
greater than , causes some emission. The frequency », signalises a 
sharp and absolute discontinuity in the phenomena. 
Now let us inquire as to the kinetic energy of the electrons which 
are emitted by a metal when illuminated by monochromatic light of 
frequency, let us say, n. Owing to the fact that the emitted electrons 
may originate from different depths in the metal, and may undergo 
collisions at irregular intervals, it is only the maximum kinetic energy 
of those which escape which we should expect to exhibit simple 
properties. As a matter of fact, it is found that the maximum kinetic 
energy is equal to the difference between the actual frequency n and 
the threshold frequency », multiplied by Planck’s constant h. In 
mathematical symbols, if v is the velocity of the fastest emitted electron, 
m its mass, e its charge, and V the opposing potential required to 
bring it to rest, 
eV=1m wW=h (n—N,). 
From this equation we see that the threshold frequency has another 
property. It is evidently that frequency for which kinetic energy and 
stopping potential fall to zero. This suggests strongly, I think, that 
the reason the electron emission ceases at », is that the electrons 
are not able to get enough energy from the light to escape from the 
metal, and not that they are unable to get any energy from the light. 
The threshold frequencies have another simple property. Tf we 
measure the threshold frequencies for any pair of metals, and at the 
5 This statement is only approximately true. In order to condense the 
argument certain small effects connected with the Peltier effect at the junction 
between the metals have been left out of consideration. 
