416 Dr Frank Horton, The ionisation produced by certain t 
readings of the optical pyrometer were taken at several tame 
peratures between .900° C. and 1600° C. From these observa- | 
tions the temperature of a Nernst filament corresponding to any’ 
reading of the pyrometer between these limits can be ascertained | 
with fair accuracy. In most of the experiments described in this | 
paper the actual temperature of the filament is of little consequence, | 
for the pyrometer was usually employed simply to adjust the| 
temperature to a constant value. > | 
| 
' 
The negative enussion from Lime. i 
a 
A Nernst filament was heated in a good vacuum (the residual! 
gas having a pressure of 0001 mm.) until the negative current) 
obtained from it under a potential difference of 214 volts had) 
become fairly constant, a condition which is obtained in a much 
shorter time than when platinum is being experimented on. || 
A series of observations of the thermionic emission from the | 
filament at different temperatures was then taken, and the results 
are shown in the curve of fig. 1. In this ordinates represent 
3°78 x 10-8 ampere 
Current: 1 
1400 1500 1600 1700 
Temperature, Degrees centigrade 
Fig. 1. 
current, one division = 3°78 x 10-8 ampere, and abscissae repre- 
sent temperatures on the centigrade scale. The curve given 
shows that at temperatures above about 1500° C. the thermionie 
current increases directly with the temperature and not in the 
