Fig. 13, a, b, c, is
such a tube, and in this model I have endeavored to show the
electrical state of it at a high vacuum by marking a number of + and -
signs. The exhaustion has been carried to 0.0001 millimeter, or 0.13
M, and you see that in the neighborhood of the positive pole, and
extending almost to the negative, the tube is strongly electrified
with positive electricity, the negative atoms shooting out from the
negative pole in a rapidly diminishing cone. If an idle pole is placed
in the position shown at Fig. 13, a, the impacts of positive and
negative molecules are about equal, and no decided current will pass
from it, through the galvanometer, to earth. This is the _neutral_
point. But if we imagine the idle pole to be as at Fig. 13, b, then
the positively electrified molecules greatly preponderate over the
negative molecules, and positive electricity is shown. If the idle
pole is now shifted, as shown at Fig. 13, c, the negative molecules
preponderate, and the pole will give negative electricity.
[Illustration: FIG. 13 A.--PRESSURE = 0.0001 MM. = 0.13 M.]
[Illustration: FIG. 13 B.--PRESSURE = 0.0001 MM. = 0.13 M.]
[Illustration: FIG. 13 C.--PRESSURE = 0.0001 MM. = 0.13 M.]
As the exhaustion proceeds, the positive charge in the tube increases
and the neutral point approaches closer to the negative pole, and at a
point just short of non-conduction so greatly does the positive
electrification preponderate that it is almost impossible to get
negative electricity from the idle pole, unless it actually touches
the negative pole.
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