This is an IR radiometer comprising 4 vanes mounted on a glass bearing that sits on a pointed metal spindle inside a glass bulb under a partial vacuum. Each vane is black on one side and white on the other. Because the vanes have a high contrast between the two sides (black absorbs and white reflects), a temperature differential is created that in turn creates air currents inside the bulb making the vanes spin when IR (heat) energy is present to create the temperature differential. This heat can be produced by any source such as the sun or any hot object. I have had this radiometer for many years and it still works well - especially when in direct sunlight. The more intense the heat source, the faster the vanes spin.