The Radiometer was originally developed in the mid-nineteenth century by Sir William Crookes and was used to measure the intensity of sunlight.
Inside the Radiometer there are four vanes, each has one blackened side and one silvered side.
It is encased in a clear glass bulb that has a nearly perfect vacuum which allows the air molecules move about more easily. The different colors on either side of the "vane blades" create the convection currents and that causes the blades to spin when exposed to light or heat.
The more intense the light is that shines on it, the faster the vanes inside will turn. With bright light shining on it, the vanes will turn at speeds up to 3,000 RPM!

The Radiometer is a great tool in demonstrating the power of solar energy. A fun demonstration experiment for Teachers uses the "sun block" plastic window film you can buy at hardware or building supply stores. This film is designed to block the suns heat in the summer to keep your house cooler. Build 2 small cardboard "houses" with one having a clear plastic window and the other using the sun blocking window film in the window. When you place the radiometer in the "house", you will see the difference in how fast the radiometer is spinning in the different "houses". You can also add oven and refrigerator thermometers to see the temperature of the "houses".

The Radiometer is 5" high and 3" in diameter.