EXPERIMENT 8—4
187
begins. Then, lower the voltage a little. If the probe isin
striation, change its position, opening K before handling the tube;
4. Adjust V to a large value and close 82 in such a direction as .
will make the probe negative With respect to the anode. Then
close 81.
The positive ion current will probably be too small to
be observed with the meters specied above. 5. Slowly increase
V and observe the probe currents (i) up
to a point where, after a
slow rise, they start to increase rapidly. Care must be used here.
It is impossible to give general gures for this upper limit since it
..
depends
gas pressure, the tube current and the distance
between the probe and the ano'de.
'
Experiments.—L With constant gas pressure, tube current “and
probe position, plot a volt-ampere characteristic curve, as ingure 8—10.
Test for a
distribution of velocities, as in
'
section 8—9. -Quéer kinds are sometimes found in the logarithmic
'
plot, usually at the higher pressures.
If there are two or more '
straight lines in this plot, there are a corresponding number of
groups of electrons in the discharge, each
Maxwellian dis—
Ÿ
tribution, but differing in their averagevelocity. The percentage
of electrons in eaCh group can be determined as the ratio of the
current ranges of the separate straight lines. When several groups
are found to“ exist, the value of Ve should be taken as the weighted
mean of the values given by the respective_groups. Measure the
slope (:) ofÂa
lme and cohipute
Ve, ?
using .
equations 8—5, 7,‘
‘
,
Ï "
‘
_
‘
‘
NeXt, plot an
in, gure“) 8—12, measure its slope
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In
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.
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