5.5 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Crookes tube | 3/4 | https://en.wikipedia.org/wiki/Crookes_tube | reference | science, encyclopedia | 2026-05-05T09:36:41.769217+00:00 | kb-cron |
=== Cathode rays move in parallel lines === 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. Cathode rays heat matter which they strike. 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.
=== Electrostatic deflection ===
Heinrich Hertz built a tube with a second pair of metal plates to either side of the cathode ray beam, a crude CRT. If the cathode rays were charged particles, their path should be bent by the electric field created when a voltage was applied to the plates, causing the spot of light where the rays hit to move sideways. He did not find any bending, but it was later determined that his tube was insufficiently evacuated, causing accumulations of surface charge which masked the electric field. Later Arthur Schuster repeated the experiment with a higher vacuum. He found that the rays were attracted toward a positively charged plate and repelled by a negative one, bending the beam. This was evidence they were negatively charged, and therefore not electromagnetic waves.
=== Magnetic deflection ===
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 static electric and magnetic fields have no effect on a beam of light waves in vacuum.
=== Paddlewheel ===
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.
=== Electric charge === Jean-Baptiste Perrin wanted to determine whether cathode rays actually carried negative charge, or whether they just accompanied the charge carriers, as the Germans thought. In 1895 he constructed a tube with a 'catcher', a closed aluminum cylinder with a small hole in the end facing the cathode, to collect the cathode rays. The catcher was attached to an electroscope to measure its charge. The electroscope showed a negative charge, proving that cathode rays really carry negative electricity.
=== Anode rays ===
Goldstein found in 1886 that if the cathode is made with small holes in it, streams of a faint luminous glow will be seen issuing from the holes on the back side of the cathode, facing away from the anode. It was found that in an electric field these anode rays bend in the opposite direction from cathode rays, toward a negatively charged plate, indicating that they carry a positive charge. These were the positive ions which were attracted to the cathode, and created the cathode rays. They were named canal rays (Kanalstrahlen) by Goldstein.