Crookes tube, device invented by Sir William Crookes (c.1875) consisting essentially of a sealed glass tube from which nearly all the air has been removed and through the walls of which are passed two electrodes. When a high voltage is applied between the two electrodes, electrons are emitted from the cathode and are accelerated toward the anode. Many of these electrons, or cathode rays (as they are usually called), miss the anode and strike instead the glass wall of the tube, causing it to exhibit fluorescence. The behavior of the rays indicates that they travel in straight lines and exert a pressure on any object placed in their path. The Crookes tube was used by Crookes in a number of experiments and was later used in experiments leading to the discovery of X rays by W. C. Roentgen (1895) and of the electron by J. J. Thomson (1897).