Page:Elementary Text-book of Physics (Anthony, 1897).djvu/405

§ 321] of the tube will deflect a discharge as if it were an electrical current.

The explanation of these phenomena was indicated by Crookes, and Spottiswoode and Moulton. The particular form of it here given was developed by J. J. Thomson. It is assumed that they are due to the presence of the gas left in the tube after the exhaustion has been brought to an end. The mean free path of the molecules in the tube is much greater than that at ordinary densities, and they can accordingly move through long distances in the tube before their motion is checked by collisions. It is assumed that the molecules of gas in the tube are dissociated near the negative electrode, and that their negative ions are repelled from it. The phenomena which have been described are then due to the collision of these ions with other bodies or with the wall of the tube, or to their mutual electrical repulsions and to the action between a moving quantity of electricity and a magnet.

The experiments of Spottiswoode and Moulton, who showed that the same phenomena appeared at lower exhaustions, if the intensity of the discharge were increased, are in favor of this explanation. So is also the fact that the Orookes phenomena appear with a maximum intensity at a certain period during the exhaustion of the tube, while if the exhaustion be carried as far as possible, by the help of chemical means, they cease altogether and no current passes in the tube. The connection of these phenomena with the action of the radiometer (§ 223) is also at once apparent.

321. The Röntgen Radiance.—It was discovered by Hertz that the cathode discharge will pass through a thin strip of alumioium-foil placed in its path within the tube. In 1894 Lenard constructed a tube in which a part of the glass wall was replaced by aluminium-foil, and found that when the cathode discharge was directed upon the aluminium-foil a series of phenomena was obtained outside the tube, which he ascribed to the cathode discharge which passed through the aluminium. He found that similar effects could be produced outside a tube in which there was no aluminium window, and so concluded that the cathode discharge could pass through