207 
DISCHARGE OF ELECTRICITY FROM GLOWING CARBON, 
the hot carbon is negative with respect to the cool one, the 
relations between them, for distances separating the car- 
bons of 1, 2, and 3 millimetres, are shown in figures 4, 5, 
and 6. The final portions of the curves have been drawn 
vertical, the abscissz of the vertical parts being the values 
of the voltages at which arcing just occurred. The numbers 
attached to the curves are the values of the temperatures 
of the hot and cool carbons respectively. 
4. Critical values.—The phenomenon of an abrupt change 
from a non-luminous to a luminous discharge is described 
by Professor Thomson’ for the case of a gas at low pressure. 
A similar sudden change takes place in air at normal pres- 
sure, and under the circumstances of our experiments the 
alteration, from the one régime of current flow to the other, 
can be brought about by a change in the potential differ- 
ence between the carbons of about a volt. 
The currents just before arcing occurs can be deduced 
by graphical extrapolation, as shown in the above diagrams; 
the approximate values so found are collected in table I. 
TABLE I. 
Distance Temperature. Critical voltage | Current just 
between for arcing. before arcing. 
carbons. Hot carbon. Cool carbon. Volts. Milliamperes. 
1 mm. 18002-C. 1360° C. 43°5 2°37 
i 1680 1240 55-0 2-50 
. 1550 1100 68:5 2 66 
‘i 1415 920 83-0 2-82 
|} 2mm. | 1800 1120 53:0 1°85 
Bes: 1680 980 66-0 1:95 
. 1550 860 81:0 2:07 
a 1415 650 SAD 2°20 
3 mm 2360 1070 39°5 0:97 
A 1800 880 70-0 112 
1680 750 85-0 1:20 
’ Loc. cit. 
