page_title: Crookes tube
section_title: Deflection by magnetic fields
hierarchical_section_title: Crookes tube / Experiments with Crookes tubes / Deflection by magnetic fields
caption_attribution_description: Português: The radiometer, invented in the 1870's by Sir William Crookes, is a device capable.
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: Crookes put a magnet across the neck of the tube, so that the North pole was on one side of the beam and the South pole was on the other, and the beam travelled through the magnetic field between them. The beam was bent down, perpendicular to the magnetic field. To reveal the path of the beam, Crookes invented a tube (see pictures) with a cardboard screen with a phosphor coating down the length of the tube, at a slight angle so the electrons would strike the phosphor along its length, making a glowing line on the screen. The line could be seen to bend up or down in a transverse magnetic field.  This effect (now called the Lorentz force) was similar to the behavior of electric currents in an electric motor and showed that the cathode rays obeyed Faraday's law of induction like currents in wires.  Both electric and magnetic deflection were evidence for the particle theory, because electric and magnetic fields have no effect on a beam of light waves.
caption: Deflection of electron beam with bar magnet