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2026-10-06 Harvard Natural Sciences Lecture Demonstrations, "CRT Paddle Wheel"

URL: https://sciencedemonstrations.fas.harvard.edu/presentations/crt-paddle-wheel Query (Google): Crookes paddle wheel tube "radiant matter" momentum heating explanation Thomson 1903 OR Starling OR replication.

Found [web]

  • A modern working paddle-wheel Crookes tube in Harvard's demonstration collection: at 40 kV "the lower vanes directly between the cathode and anode pivot away from the cathode, causing the wheel to move"; the vanes carry fluorescent material that glows green. Run from a 6 V battery and induction coil; tube about 12 inches long, wheel a few centimetres across. This is a present-day reproduction of Crookes's Fig. 11 effect (the wheel does move).
  • Explanation given by Harvard: Crookes (1880) argued mechanical impulse; "in 1903 J.J. Thomson, citing the experimental work of H. Starke, showed that momentum transfer of the electrons was not enough to account for the observed motion of the paddle wheel, and concluded that the kinetic energy of the electrons only indirectly leads to movement of the paddle wheel via radiometric effect."
  • References: J. J. Thomson, "Conduction of Electricity Through Gases", Cambridge UP 1903, p.501; H. Starke, "Notiz über die mechanische Wirkung der Kathodenstrahlen", Annalen der Physik 3 (1900) 101-107. Harvard summarises Starke: he varied current, pressure, metallic vs non-metallic surfaces and angle of incidence, and "concluded that thermal effects were greater than the mechanical impulse imparted by the electrons."
  • Wikipedia (Crookes tube) adds that Thomson calculated the electron momentum would turn the wheel only about one revolution per minute (no page reference).

Bearing on the claims

  • Claim 5 (radiant matter exerts strong mechanical action): the effect is reproduced today. The mechanism is disputed in the record: Crookes's own reading is direct impact of molecules; Starke (1900, experiments) and Thomson (1903, calculation) attribute most of the force to heating (the radiometer effect). Crookes's own platinum-ring demonstration (vanes spun by a hot wire alone, as fast as by the coil [read: deep read section 3]) is relevant to that dispute. Starke 1900 is a primary experimental test to obtain (German).