314 J. A. POLLOCK, A. B. B. RANCLAUD AND E. P. NORMAN. 
With a heated iron wire in the place of the hot carbon 
of the previous experiments, the potential difference for 
zero current was 0°85 volts at 1410° absolute and 0°25 volts 
at 1570’, the hot wire being in both cases, negative to the 
cooler electrode. The potential difference witha platinum 
wire, not specially treated, at 1580° absolute was 0°40 volts, 
the hot wire being again negative. Witha Nernst filament, 
designed for 0°25 ampere at 90 volts and used under those 
conditions, the potential difference for zero current was 
0°25 volts. In this case the filament was positive to the 
cooler electrode, and with a potential difference between 
the electrodes of 45 volts, the ratio of the flow of negative 
electricity from the hot filament to the flow of positive, 
when the sign of the potential difference was reversed, was 
as 33 to 1. 
Professor Richardson, (loc.cit.), gives, for the number of 
corpuscles shot off from unit area of a hot conductor per 
second, the expression Aob/6, in which A depends on 
the number of corpuscles per unit volume of the conductor, 
and b on the work done by a corpuscle in passing through 
the surface layer, 4 being the absolute temperature. The 
formula, considering A and b as constants, very well repre- 
sents the observations of its author and others on the 
saturation currents from hot bodies through the range of 
temperature for which it has been employed; from such 
observations the values of b for certain substances have been 
determined. In the theory by which the expression is 
deduced, b is equal to ¢/R, where ¢ is the work done by a 
corpuscle in passing through the surface layer and Ragas 
constant, equal to p/N¢, p being the pressure and N the 
number of molecules per cubic centimetre. The discon- 
tinuity of potential is thus represented by bDR/e, where e 
is the ionic charge. 
The following are the values of the discontinuity of 
potential as calculated by Professor Richardson :— 
