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The sphere powered by the sun! The vanes in the radiometer are alternately dark and light in colour. When the light strikes these vanes, it transfers heat to them - the light vanes reflect the heat, the dark ones absorb it... the vanes begin to spin as atoms kick off the dark vanes, and as the light gets stronger, the vanes spin faster and faster! Invented by Sir William Crookes in 1875, as a consequence of his studies into Thalium - which he discovered.