Two-way oval radiometer on brass stand

it's not only 8 blades, but the 2 series rotate in opoosite directions! Sir William Crookes’ radiometer (1873) consists of a partial air vacuum inside a glass ball, which contains a set of blades in mica mounted on a spindle, creating the less friction as possible. The four vertical blades equally spaced around the axis, presents one side black and the other bright: the dark face absorb photons of infrared light and become warmer than the white one (where photons are repulsed). With rising temperatures the air expands, molecules close to the black side moves away from each other, and pushes the vane, creating rotating motion as if by magic! A complex object but so interesting...