Also known as a light mill.
It is a device for demonstrating the conversion of radiation energy into kinetic energy.
Our apparatus consists of an airtight glass bulb containing a partial vacuum. It is 70mm in diameter.
At the center of the bulb is a fine pivot which supports four light weight metal vanes.
One side of each vane is blackened and the reverse side is bright.
This set up is mounted on a round plastic base.
Students can realize that the vanes rotate when exposed to light, the dark sides retreating from the radiation source and the light sides advancing. Cooling the radiometer causes rotation in the opposite direction.
Also faster rotation can be observed for more intense light, providing a quantitative measurement of electromagnetic radiation intensity.