302 Mr McClelland, On the Action of Incandescent Metals 
Potential of 
Current in arbitrary 
wire in volts 
units 
8 
19 
18 
22 
40 
24 
120 
25 
240 
27 
280 
29 
320 
37 
360 
50 
The curve in Fig. 4 is plotted from these numbers. 
Volts . 
Fig. 4. 
The pressure of the air during the above experiment was 
f mm. and the temperature of the wire the same as in the experi- 
ment illustrated by the curve in Fig. 2. The conductivity when 
the wire is positive is very much less than when the wire is 
negative, as shall be referred to later in the paper. The above 
numbers are not to be compared as regards their absolute values 
with the numbers in the previous experiment when the wire is 
negative. 
The rapid increase of current shown in the curve in Fig. 4 
after the e.m.f. has reached a certain value would seem to indicate 
that the positive ions travelling out from the wire have produced 
secondary ionisation. The e.m.f. for which the secondary ionisa- 
tion begins is not very different in the two cases, which would 
show that the masses in the two cases are not very different. All 
investigations on ionisation produced in air at pressures of 1 mm. 
and less, by Rontgen rays and ultra-violet light as well as in the 
