CRT Paddle Wheel

A beam of cathode rays (electrons) impinging on a paddle wheel cause it to spin and travel down the vacuum tube.

crookes tubes

What it Shows

A paddle wheel is suspended by its axle inside a Crookes tube so that when the paddle vanes spin the entire wheel is free to travel the length of the tube. When 40kV is applied across the tube, the lower vanes directly between the cathode and anode pivot away from the cathode, causing the wheel to move. Flourescent material on the vanes glow green when subjected to the energetic electrons.

How it Works

In 1880 Sir William Crookes argued that the mechanical impulse of the electrons hitting the vanes caused the observed motion—similar to how flowing water turns the arms of a turbine.1 However 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. 2,3

Setting it Up

Please use gloves and safety glasses when handling this fragile vacuum tube. Use the same 6V battery and induction coil used for the Maltese Cross and Cathode Ray Deflection Crookes tubes. Great care must be taken to make sure the tube is level and the frictional forces on the axle of the paddle wheel can be overcome. The tube is only about 12" long and the paddle wheel a few centimeters in diameter, so video projection should be used when presenting to a large audience.

1Sir William Crookes "On Radiant Matter" Popular Science Monthly, London 1880 p.158
2J. J. Thomson "Conduction of Electricity Through Gases" Cambridge University Press 1903 p.501
3H. Starke "Notiz über die mechanische Wirkung der Kathodenstrahlen" Annalen der Physik 3, (1900) pages 101-107. Starke observed the differences in the mechanical reaction for strong and weak electrical currents, changes in vacuum pressure, metallic and non-metallic surface materials, and different angles of incidence of the beta rays on the surface, and concluded that thermal effects were greater than the mechanical impulse imparted by the electrons.