Crookes radiometer consists of a glass bulb from which air has been removed to create a partial vacuum. Inside the bulb is a spindle upon which sits a weathervane style device with (usually) four vertical squares.
These squares are painted black on one side and white on the other. When the device is exposed to direct sunlight or a strong artificial light or heat source, the vanes turn. Cooling causes the vanes to turn in the opposite direction.
Although Crooke believed that the effect was caused by the pressure of light on the vanes, this was dismissed in 1876. There are various explanations as to how the light or heat makes the vanes turn. Many believe that the effect can be explained by using thermodynamic principles. When a heat source is directed at the radiometer it becomes what is known as a heat engine - that is a machine that turns the difference in temperature into movement. In the case of the radiometer the black side of the vanes is heated faster than the white sides through a process known as black body absorption.
The molecules of gas hitting the warmer sides of the vanes (the black parts) will absorb some of this heat and bounce off at a higher speed. Although these molecules are tiny and exert a very small force, because of the partial vacuum the vanes will rotate because of this small 'push'.
Radiometers have been sold worldwide as an interesting novelty ornament and are available through most lab suppliers.