The Radiometer provides an introduction to the study of thermodynamics that sits on your desk or table top. Set it in sunlight or incandescent light and watch the mill inside spin!

The radiometer consists of four vanes, each has one blackened side and one silvered side. It is encased in a clear glass bulb that has a near perfect vacuum. So what happens to make the radiometer "work"? 

Crookes originally believed that sunlight falling on the light-mill turned the vanes so that the black surfaces were being pushed away by the light. He thought that this demonstrated that light radiation pressure on the black vanes was turning it round just like water in a water mill.

The problem with Crookes' explanation is that light falling on the black side should be absorbed, while light falling on the silver side of the vanes should be reflected. This means that there would be twice as much radiation pressure on the metal side as on the black and the mill would actually turn in the opposite way.

What really happens is that the faster molecules from the warmer side of each vane strike the edges obliquely and impart a higher force than the colder molecules. The movement of the vane as a result of tangential forces around the edges, is away from the warmer gas and towards the cooler gas with the gas passing round the edge in the opposite direction. Therefore, the movement is actually due to what happens near the edges of the vanes rather than at the faces. (This correct answer, thermal transpiration, was first provided by Osborne Reynolds in 1879.)