type X-ray tubes were used until about 1920, when they were superseded by the hot cathode Coolidge X-ray tube. Experiments During the last quarter of the 19th century Crookes tubes were used in dozens of historic experiments to try to find out what cathode rays were. There were two theories: British scientists Crookes and Cromwell Varley believed they were particles of 'radiant matter', that is, electrically charged atoms. German researchers E. Wiedemann, Heinrich Hertz, and Eugen Goldstein believed they were 'aether vibrations', some new form of electromagnetic waves, and were separate from what carried the current through the tube. The debate continued until J.J. Thomson measured their mass, proving they were a previously unknown negatively charged particle, the first subatomic particle, which he called a 'corpuscle' but was later renamed the 'electron'. Maltese cross Julius Plücker in 1869 built a tube with an anode shaped like a Maltese Cross facing the cathode. It was hinged, so it could fold down against the floor of the tube. When the tube was turned on, the cathode rays cast a sharp cross-shaped shadow on the fluorescence on the back face of the tube, showing that the rays moved in straight lines. This fluorescence was used as an argument that cathode rays were electromagnetic waves, since the only thing known to cause fluorescence at the time was ultraviolet light. After a while the fluorescence would get 'tired' and the glow would decrease. If the cross was folded down out of the path of the rays, it no longer cast a shadow, and the previously shadowed area would fluoresce more strongly than the area around it. Perpendicular emission Eugen Goldstein in 1876 found that cathode rays were always emitted perpendicular to the cathode's surface. If the cathode was a flat plate, the rays were shot out in straight lines perpendicular to the plane of the plate. This was evidence that they were particles, because a luminous object, like a red hot metal plate, emits light in all directions, while a charged particle will be repelled by the cathode in a perpendicular direction. If the electrode was made in the form of a concave spherical dish, the cathode rays would be focused to a spot in front of the dish. This could be used to heat samples to a high temperature. Deflection by electric fields Heinrich Hertz built a tube with a second pair of metal