Page:Elementary Text-book of Physics (Anthony, 1897).djvu/404

390 vacuum-tube m which the exhaustion is such that the phenomena are those here described is often called a Geissler tube. As the exhaustion is raised still higher, the rosy light in the tube fades out, the non-luminous space around the negative electrode becomes very much greater, and the phenomena in the tube become exceedingly interesting. They were discovered and have been carefully studied by Crookes, and the vacuum-tubes in which they appear are hence called Crookes' tubes. They may be most conveniently described by assuming that the molecules of gas in the tube break into their constituent ions in the region near the negative electrode, and that the negative ions are repelled from that electrode. The stream of negative ions may be called the cathode discharge. This view receives some support from the fact that the relations of current and resistance in the tube are such as to indicate a counter electromotive force at the negative electrode.

The region occupied by the discharge from the negative electrode may be recognized by a faint blue light, which was not visible in the former condition of the tube. At every point on the wall of the tube to which this discharge extends occurs a brilliant phosphorescent glow, the color of which depends on the nature of the glass. The discharge seems to be independent of the position of the positive electrode, and to take place in nearly straight lines, which start normally from the negative electrode. If two negative electrodes be fixed in the tube, the discharge from one seems to be deflected by the other, and two discharges which meet at right angles seem to deflect one another.

If the discharge from a flat electrode be made to fall upon a body which can be moved, such as a glass film, or the vane of a light wheel, mechanical motions will be set up.

If the negative electrode be made in the form of a spherical cup, and a strip of platinum-foil be placed at its centre, the foil will become heated to redness when the discharge is set up.

There is no evidence that two discharges in the same direction act directly on each other, but a magnet brought near the outside