2i8 EMISSION OF ELECTRICITY FROM HOT BODIES

methods, however, were compared in independent experiments
and were found to give, identical results when the emission
was homogeneous, and was not changing in character with
lapse of time. With platinum strips the ions given off during
the first twenty hours or so of heating were found to be very
homogeneous and to have a value of M very close to 40.
In the later stages there were indications of ions for which M
was near 23, and sometimes also values of M between 50 and
60 were obtained in the last stages of heating just before
the strips broke. The variation of M with the time of
heating is exhibited in Fig. 24, where the points marked thus

ou 50

Po;
sitive I
ons

........... -"• ........................... '~9R

K +

X

3O
20
<
,40 ---- 2J ---- ^4. X » % »%•
4 Pt Strip * 1 x Pt Strip 4*= 2 o K2 S O4

x
v<

Na+ *

0 A 8 12 to 2O 2-4 ' 2(

Time heated (hours)

FIG. 24.

x represent some of the values given by a particular strip at
times extending over twenty-eight hours. The final high
values are a little uncertain as the strips never last long after
these values have begun to appear. The points thus ^ in
the figure represent values of M given by another strip, and
the circles are values given by the ions from potassium sul-
phate. In all the measurements the strips were kept at the
lowest temperature at which the currents were large enough
for convenient measurement. This involved gradually increas-
ing the temperature during any one experiment, on account of
the decay of the emission with time.perimented with. Most of