204 J. A. POLLOCK AND A. B. B. RANCLAUD. 
To get an estimate of the temperature of the cool carbon 
when currents were measured, the junction of a thermo- 
electric thermometer of platinum and platinum-iridium 
wires, 0°5 millimetres in diameter, was placed in the position 
of the end of the carbon rod, the rod being temporarily 
removed. The temperature given by the thermometer, 
under any circumstances as to distance of the junction 
from the heated cylinder and length of time in position 
close to it, is considered to be that of the end of the carbon 
under similar conditions. 
3. Flow of negative electricity from the hot carbon.— 
The general form of the relation, without reference to 
exact scale, between potential difference and current for 
temperatures of the hot rod in the neighbourhood of 1800° C. 
is shown in figure 3, where ordinates represent currents, 
and abscissee potentials of the hot relatively to that of the 
cool carbon. : 
General form of the relation between potential differ- 
ence and current. 
