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Crookes radiometer, a device invented by Sir William Crookes in 1873. This is an arrangement of four vanes mounted on a low-friction spindle and set inside a sealed glass bulb with a partial vacuum. One side of each vane is polished, the other painted black. When a light is shone on the apparatus, the exposed dark side of one vane warms up and heats the air next to it. At the edges of the vane, cooler air tends to flow towards the warmer side by the process of thermal transpiration. Energising the air to flow makes the vane move in the opposite direction (i.e. away from the darkened side) and the whole assembly starts to turn, exposing the dark side of each vane in turn.
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