289 
from Hot Platinum. 
sulphate was determined and no determination was more than 
20 degrees from the true value. This agreement was held to be 
quite good enough for the purpose. 
The following observations were made to see how the current 
from the wire varied with the potential to which it was raised and 
if a saturating current could be obtained. The potentials were 
measured with a Weston voltmeter. A constant current was 
used to heat the wire and it was sought to keep its temperature 
constant by pumping out the gas as fast as it was given off. The 
pressure was thus kept at about '008 mm. ; the resistance of the 
wire is given to show the extent to which the temperature varied. 
The maximum current was roughly 2 x 10“ 7 amperes and is given 
in scale divisions of the galvanometer. 
The slightly smaller current at the higher voltages is doubt- 
less due to the fall in the temperature of the wire as shown by 
Volts on Circuit 
Current in 
Scale Divisions 
Pressure of gas in 
mm. of mercury 
Resistance of wire 
- 40 
205 
•0076 
8-752 ohms 
- 80 
318 
•0083 
8-767 „ 
- 120 
306 
•0072 
8-722 „ 
-160 
315 
•0086 
8-745 „ 
- 256 
275 
•0074 
8-696 „ 
o 
fc- 
CO 
1 
268 
•0088 
8-700 
the resistance column. The table clearly shows that the saturat- 
ing potential is somewhere between 40 and 80 volts since there is 
no increase of the current with higher potentials. No current 
was obtained under these conditions when the wire was positive. 
In measuring the variation of the current from the wire with 
the temperature, a potential of — 120 volts was always put on the 
wire so as to make sure of the current being saturated. In 
general the deflexion of the galvanometer decreased with the time, 
but not more than might be explained by the lowering of tempera- 
ture produced by the gas which seemed always to be given off 
from the hot wire. At temperatures below 1400° very little gas 
was given off and the readings of the galvanometer were quite 
steady, while at higher temperatures this was not the case. The 
curves for lower temperatures are therefore much better than 
