Crookes' radiometer, 1875.
© Science Museum / Science & Society Picture Library
Sir William Crookes (1832-1919) was an experimental chemist and physicist who devised this radiometer and who also experimented with cathode rays. Millions of air molecules are moving around inside the radiometer. One side of each vane gets hotter than the other side because it is coated with mica, which absorbs electromagnetic radiation such as light. This warms up the surrounding air, which further increases the speed of the molecules. They bump into the vane more often on the warm side, pushing it around.