—­Hardwicke’s Science-Gossip.

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ELECTRICITY IN TRANSITU—­FROM PLENUM TO VACUUM.[1]

[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.