DO J. As POLLOCK AND A. B. B. RANCLAUD. 
below the cool one than when it is vertically above, or than 
when the two carbons are in the same horizontal plane. 
5. Distribution of potential between the carbons.—It is 
well known that from the surface of hot carbon, electrons 
and positive ions are projected. The rate of emission of 
the latter is small and its consideration may be neglected 
in the discussion of the present experiments when the hot 
carbon is negatively electrified. The electrons collide with 
molecules of the surrounding gas, at somewhat various 
distances from the hot carbon surface, the average of 
which may be called the average range of projection. The 
collisions result in the creation of molecular negative ions, 
which, if they are not removed, accumulate near the sur- 
face of the carbon, thus establishing a potential gradient 
through the average range of projection and a consequent 
movement of the ions towards the hot surface. Sucha 
distribution of potential can be realized with the apparatus 
previously described, by applying between the carbons the 
potential difference requisite, at any temperature, to make 
the current in the circuit zero, the hot carbon, in this case 
being positive relatively to the cool one. This distribution 
may be represented, from a merely illustrative point of 
view, by the curve 1 in figure 8, where V is the potential 
and « the distance from the cool carbon, the hot carbon 
rise of potential a b being supposed to take place through a 
distance comparable with the average range of projection 
of the electrons. 
If the hot carbon is made slightly less positive than the 
value required for zero current, a c in curve 1 will slope a 
little upwards; those few molecular negative ions formed 
near the surface of the hot carbon beyond the point a will 
be drawn by the field to the cool rod, though the great 
majority will still go to the hot carbon. A further stage, 
‘with the hot carbon negative with respect to the cool one 
