page_title: Crookes tube
section_title: Perpendicular emission
hierarchical_section_title: Crookes tube / Experiments with Crookes tubes / Perpendicular emission
caption_attribution_description: English: A Crookes tube to demonstrate the focusing and heating effect of cathode rays made by William Crookes and demonstrated by him at his lecture to the British Association for the Advancement of Science in Sheffield August 22, 1879. It consists of a glass bulb evacuated to a pressure of about 10−6 atm, with a spherical concave negative electrode or cathode (a) at the bottom, a small positive electrode or anode (P) at top, and a metal rod (b) whose tip is at the focus of the cathode. When a high voltage is applied to the tube, cathode rays (electrons) are emitted from the cathode at bottom. They are emitted perpendicularly to the cathode surface, since the electric field is normal to the surface. So all the electrons converge on the tip of the metal rod, and the kinetic energy of the fast moving electrons heats the rod white-hot.
context_page_description: A Crookes tube is an early experimental electrical discharge tube, with partial vacuum, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays, streams of electrons, were discovered.
  Developed from the earlier Geissler tube, the Crookes tube consists of a partially evacuated glass bulb of various shapes, with two metal electrodes, the cathode and the anode, one at either end. When a high voltage is applied between the electrodes, cathode rays are projected in straight lines from the cathode. It was used by Crookes, Johann Hittorf, Julius Plücker, Eugen Goldstein, Heinrich Hertz, Philipp Lenard, Kristian Birkeland and others to discover the properties of cathode rays, culminating in J.J. Thomson's 1897 identification of cathode rays as negatively charged particles, which were later named electrons. Crookes tubes are now used only for demonstrating cathode rays.
  Wilhelm Röntgen discovered X-rays using the Crookes tube in 1895. The term Crookes tube is also used for the first generation, cold cathode X-ray tubes, which evolved from the experimental Crookes tubes and were used until about 1920.
context_section_description: 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.
caption: Crookes tube with concave cathode
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