The cathode rays from one of the two electrodes at the lower right-hand corner of the tube impinge on a four-bladed vane that is balanced on a sharp point so that it will spin freely when struck by the electrons. To reassure the students that this is not a radiometer effect, the cathode rays from the other cathode strike the opposite side of the vanes, and it spins in the opposite direction. The students can thus see that the cathode rays carry kinetic energy.

   To add to the effect, the electric turbine I use for demonstrations at Kenyon has dots on the vanes that fluoresce when the beam strikes them.

   This apparatus is at Hampden-Sydney College in Virginia.
 

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