A, and the whole of the inside and outside of the tube walls are
coated with metal, D, D, and "earthed" so as to carry away the
positive electricity as rapidly as possible, then it is seen that the
molecules leaving the negative pole and striking upon the idle pole,
C, on their journey along the tube carry a negative charge and
communicate negative electricity to the idle pole.

[Illustration: FIG. 12.--PRESSURE = 0.0001 MM. = 0.13 M.]

This tube is of interest, since it is the one in which I was first
able to perceive how, in my earlier results, I always obtained a
positive charge from an idle pole placed in the direct stream from the
negative pole. Having got so far, it was easy to devise a form of
apparatus that completely verified the theory, and at the same time
threw considerably more light upon the subject. 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.