DISCONTINUITY OF POTENTIAL AT SURFACE OF GLOWING CARBON, 31] 
THE DISCONTINUITY or POTENTIAL At THE SURFACE 
orf GLOWING CARBON. 
By Professor J. A. PoLLock, A. B. B. RANCLAUD, and 
H. P. NORMAN. 
The Physical Laboratory of the University of Sydney. 
[Read before the Royal Society of N. S. Wales, December 2, 1908. } 
OwING to the projection of ions by hot substances a dis- 
continuity of potential will occur at the surfaces of the 
electrodes in any circuit in which these latter are formed 
of heated materials. The value of this potential discon- 
tinuity has been calculated by Professor Richardson’ from 
considerations connected with the gas theory of metallic 
conduction, and has been found in the case of carbon by 
Mr. Duddell,? who observed at the surface of the cathode 
of the ordinary carbon arc a forward electromotive force 
of 61 volts, anda back electromotive force of 16°7 volts at 
the surface of the anode. 
In a circuit with one heated electrode in air at ordinary 
pressure, the projection of ions from the hot surface necessi- 
tates the establishment of a potential difference between 
the electrodes if the current in the circuit is to be zero. 
If electrons alone were projected, a layer of gas close to 
the hot surface would become negative to the solid and the 
potential difference between the electrodes for zero current 
might be taken as a measure of the surface discontinuity 
of potential. When positive ions are emitted as well as 
electrons, owing to the difference in the distances from the 
electrode at which the two classes of ions are stopped by 
collision with the gas molecules, the electric force may 
’ Phil. Trans., A 201, p. 497, 1903. * Phil. Trans., A. 208, p. 305, 1904. 
