23o EMISSION OF ELECTRICITY FROM HOT BODIES

I. Oxygen.—The currents with the wire charged positively
were found to saturate very readily at all pressures up to
atmospheric. At high pressures and low temperatures the
emission exhibited a curious instability ; the current under
apparently constant conditions kept suddenly increasing to a
temporary high value and then returning to about the original
level. The cause of this instability has not been discovered,
but on the assumption that it is due to some secondary pheno-
menon, the minimum values of the currents were taken to
represent those due to the direct action of the gas. This diffi-
culty was not encountered to any considerable extent at low

30

20

10

•33

-67

Pressure :
FIG, 25.

I'D

1"33

pressures at any temperature or at high pressures when the
temperature was high.

At low temperatures the saturation current was nearly pro-
portional to the square root of the pressure, when this was under
I mm. At higher temperatures, in the neighbourhood of
1100° C. to 1200° C., the current was almost proportional to
the pressure over this range. At all the temperatures there
was very little variation of current with pressure at high pres-
sures (200 to 800 mm.). The behaviour at 828° C. for pres-
sures below I mm. is shown in Fig. 25.

The variation with temperature of the saturation current at
a constant pressure of 1-47 mm. was also examined. The
superficial area of the hot wire used was 0*223 sq. cm. and itsions