[Footnote 1: Presidential address before the Institute of Electrical
Engineers, London; continued from SUPPLEMENT, No. 792, page 12656.]
By Prof. WILLIAM CROOKES, F.R.S.
If an idle pole, C, C, Fig. 12 (P=0.0001 millimeter or 0.13 M),
protected all but the point by a thick coating of glass, is brought
into the center of the molecular stream in front of the negative pole,
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.
[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. This tube is before you, and I will now proceed to
show the change in direction of current by moving the idle pole.
I have not succeeded in getting the "Edison" current incandescent
lamps to change in direction at even the highest degree of exhaustion
which my pump will produce. The subject requires further
investigation, and like other residual phenomena these discrepancies
promise a rich harvest of future discoveries to the experimental
philosopher, just as the waste products of the chemist have often
proved the source of new and valuable bodies.