DISCONTINUITY OF POTENTIAL AT SURFACE OF GLOWING CARBON. 313 
The heated electrodes were cored carbon rods, 0°5 centi- 
metres in diameter, supplied by Messrs. Siemens Brothers 
for use with their Lilliput arc lamps, with the exception of 
the one employed for the observation at 3040° absolute 
which was a squared rod, 0°5 centimetres on the side, of 
solid Conradty carbon marke C. As the measure with this 
material agrees well with the other results, it may be con- 
cluded that values of the potential difference for zero current 
with hot electrodes of different makes of carbon do not 
seriously differ. 
At 3120° absolute, with 50 volts between the electrodes, 
the ratio of the flow of negative electricity from the hot 
carbon to the flow of positive, when the sign of the potential 
difference was reversed, was as 20 to 1. 
On the assumption thatat high temperatures the potential 
difference for zero current measures the surface discon- 
tinuity of potential, as the number of electrons then pro- 
jected per second far exceeds that of positive ions, the 
curve in figure 1 has been continued to the value, 16°7 volts, 
found by Mr. Duddell, (loc. cit.), for the back electromotive 
force at the anode of an arc between solid Conradty Noris 
carbons. This measure has been plotted for the tempera- 
ture of the crater, 3690° absolute, determined by Messrs. 
Waidner and Burgess* from observations with a Holborn- 
Kurlbaum optical pyrometer, a similar instrument to that 
with which we have estimated the other temperatures. 
Mr. Duddell gives 6°1 volts as the measure of the forward 
electromotive force at the arc cathode; this value corres- 
ponds, on the curve of potential diflerences for zero current, 
to a temperature of 3375° absolute, which, if the assumption 
already stated is legitimate, may be taken as an estimate 
of the temperature of the cathode of the carbon arc. 
1 Phys. Rev., 19, p. 255, 1904. 
