from Hot Platinum. 
287 
therefore small compared with that in which they move freely, 
so that to determine their equilibrium state we can apply the 
methods of the kinetic theory of gases. In this way we find 
the corpuscles have a distribution of velocity which follows the 
Boltzmann-Maxwell Law, and their average energy is the same 
as that of a molecule of gas at the same temperature. If we 
consider what happens at the surface of the metal we must 
suppose there is here a discontinuity in the potential which 
prevents the negative ions escaping. It is conceivable that as the 
temperature is raised, some of the corpuscles will acquire sufficient 
velocity to enable them to overcome this discontinuity in the 
potential. Since the number of corpuscles with velocity com- 
ponents between u, v, w and u + du , v + dv, w + dw in unit 
volume is 
where n is the total number of corpuscles in unit volume, 3/4 k is 
the energy of a corpuscle and m is its mass ; the number having 
these velocity components which strike unit surface perpendicular 
to u per second is 
If <I> is the work done by a corpuscle in passing through the 
surface layer, the number which escape from unit area of the 
metal surface per second is given by 
since k is connected with 6 the absolute temperature by the 
relation k = ( 2R6 )~ X , R being the gas constant for a single 
corpuscle. If then the negative radiation is due to the corpuscles 
coming out of the metal, the saturation current ( s ) should obey 
b 
the law s= A'0? e~e. This law is fully confirmed by the experi- 
ments to be described. 
In the experiments the current was measured which passed 
from a thin platinum wire to an aluminium cylinder surrounding 
it. To measure the current the cylinder could be put to earth 
through a sensitive Thomson galvanometer but was, otherwise, 
insulated. The wire was heated by a steady current of not more 
