page_title: Crookes tube
section_title: Paddlewheel
hierarchical_section_title: Crookes tube / Experiments with Crookes tubes / Paddlewheel
caption_attribution_description: English: Drawing of an experimental Crookes tube with a paddlewheel inside, built by William Crookes around 1879. It was called the 'railway' tube, since as the paddlewheel turned it slowly rolled down the rails from one end of the tube to the other. The purpose of the tube was to determine whether cathode rays had mass. The tube was evacuated to a pressure of about 10-6 to 10-7 atmosphere. When the electrodes at each end of the tube were attached to a source of high voltage DC electricity, a beam of cathode rays (electrons) passed down the tube from the cathode to the anode. The upper paddles of the light paddlewheel were in the path of the beam. Crookes found that when the voltage was applied the paddlewheel turned in a direction away from the cathode. He concluded that the momentum of the rays was providing the force which turned the wheel, indicating that cathode rays had mass. However in 1903, J. J. Thompson showed that the motion of the paddlewheel was actually caused by the radiometer effect. When the cathode rays struck the paddle, they heated it. The air next to the hot side of the paddle expanded, pushing the paddle away. All this experiment really showed was that cathode rays could heat objects. Alterations to image: removed figure number and yellowish tint to page, converted to 32 greyscale PNG.
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 tiny vaned turbine or paddlewheel in the path of the cathode rays, and found that it rotated when the rays hit it. The paddlewheel turned in a direction away from the cathode side of the tube, suggesting that the force of the cathode rays striking the paddles was causing the rotation. Crookes concluded at the time that this showed that cathode rays had momentum, so the rays were likely matter particles.  However, later it was concluded that the paddle wheel turned not due to the momentum of the particles (or electrons) hitting the paddle wheel but due to the radiometric effect.  When the rays hit the paddle surface they heated it, and the heat caused the gas next to it to expand, pushing the paddle.  This was proven in 1903 by J. J. Thomson who calculated that the momentum of the electrons hitting the paddle wheel would only be sufficient to turn the wheel one revolution per minute.  All this experiment really showed was that cathode rays were able to heat surfaces.
caption: Crookes' paddlewheel tube, from his 1879 paper On Radiant Matter
exif: {}