420 Dr Frank Horton, The ionisation produced by certaim 
case the falling off with time in the case of a freshly heated 
filament would not be expected to be so large as in the case 
of metals, for no doubt a great deal of the absorbed gas is driven 
off while the filament is being fitted into position and during the 
evacuation of the apparatus. When a filament had once been 
heated until the positive emission had been reduced to a steady 
value it was found that the emission quickly became steady on 
re-heating and re-testing, and the value under given conditions 
remained fairly constant. With the filament at about 1450° C, 
and with a potential difference of 205 volts (in addition to that 
due to the heating circuit), a positive thermionic current of 
1:36 x 10-7 ampere was obtained at atmospheric pressure, and on 
gradually pumping down, the current increased to a maximum 
value, generally at about 30 mm. pressure, after which it decreased 
to a minimum at about 2 mm. pressure, and then rapidly increased 
again as the pressure was still further reduced. Sometimes, how- 
ever, this final increase in the current did not occur, and the 
current decreased to a minimum value at the lowest pressure. 
The form of the current pressure curve for the positive emission 
from a Nernst filament is thus exactly similar to that given by 
platinum *. 
The filament was next covered, except for about 1 mm. at 
each end, with a layer of pure sodium phosphate by evaporating 
a water solution of that salt upon it, a few drops ata time. It 
was then fitted into the discharge tube and the positive emission 
was re-tested. In the earlier experiments it was found that the 
phosphate volatilised away from the filament and condensed on 
the walls of the discharge tube. The heating current was there- 
fore regulated so as to keep the filament glowing at as low 
a temperature as possible. Under these conditions it was rather 
difficult to start the filament glowing, and it often “went out” 
before the observations were begun; but ultimately I succeeded 
in obtaining a series of values of the positive emission at 1422° C. 
over a fairly wide range of gas-pressures. Before this series was 
obtained the filament was heated for about two hours in a pressure 
of (002 mm. At the end of this time the emission had been 
reduced to a steady value. The gas-pressure in the apparatus 
was gradually raised to 56 mm., observations of the positive 
emission being taken at several pressures. After each alteration 
of pressure a few minutes were allowed for the current to become 
constant. The pressure was then gradually reduced again, and 
the observations of the current were repeated. Throughout the 
series the temperature of the filament was maintained at 1422° C. 
by adjusting the resistance of the heating circuit. The results 
* F. Horton, Proc. Roy. Soc. A. Uxxxvitt. p. 117, 1913. 
