Radiometer according to Crookés. Consists of a glass container on foot with low air pressure and needle-bearing spinner with four vertical mica discs. The discs are black on one side and shiny on the other. The discs thus absorb radiation differently. The higher temperature of the dark side means that the air molecules get greater kinetic energy in collision with the dark side of the discs than with their shiny side. The result is that the spinner starts to rotate.

• Height: 200 mm
• Diam: 70 mm