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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Crookes tube | 4/4 | https://en.wikipedia.org/wiki/Crookes_tube | reference | science, encyclopedia | 2026-05-05T09:36:41.769217+00:00 | kb-cron |
=== Spectral shift === Eugen Goldstein thought he had figured out a method of measuring the speed of cathode rays. If the glow discharge seen in the gas of Crookes tubes was produced by the moving cathode rays, the light radiated from them in the direction they were moving, down the tube, would be shifted in frequency due to the Doppler effect. This could be detected with a spectroscope because the emission line spectrum would be shifted. He built a tube shaped like an "L", with a spectroscope pointed through the glass of the elbow down one of the arms. He measured the spectrum of the glow when the spectroscope was pointed toward the cathode end, then switched the power supply connections so the cathode became the anode and the electrons were moving in the other direction, and again observed the spectrum looking for a shift. He did not find one, which he calculated meant that the rays were traveling very slowly. It was later recognized that the glow in Crookes tubes is emitted from gas atoms hit by the electrons, not the electrons themselves. Since the atoms are thousands of times more massive than the electrons, they move much slower, accounting for the lack of Doppler shift.
=== Lenard window ===
Philipp Lenard wanted to see if cathode rays could pass out of the Crookes tube into the air. See diagram. He built a tube with a "window" (W) in the glass envelope made of aluminum foil just thick enough to hold the atmospheric pressure out (later called a "Lenard window") facing the cathode (C) so the cathode rays would hit it. He found that something did come through. Holding a fluorescent screen up to the window caused it to fluoresce, even though no light reached it. A photographic plate held up to it would be darkened, even though it was not exposed to light. The effect had a very short range of about 2.5 centimetres (0.98 in). He measured the ability of cathode rays to penetrate sheets of material, and found they could penetrate much farther than moving atoms could. Since atoms were the smallest particles known at the time, this was first taken as evidence that cathode rays were waves. Later it was realized that electrons were much smaller than atoms, accounting for their greater penetration ability.
== See also == Crookes radiometer – 1873 device that rotates when exposed to light
== References ==
== External links ==
An illustration of a "maltese cross" Crookes tube. The Cathode Ray Tube site Crookes and Geissler tubes shown working Java animation of a Crookes tube "The Cathode Rays". Library. Oracle Thinkquest Education Foundation. Archived from the original on 2008-05-06. Retrieved 2008-04-28. History of d Jenkins, John. "Crookes and Geissler tubes". Spark Museum. Retrieved 2008-04-29.