Subject: Physics Inside the bulb, on a low-friction spindle, is a rotor with several (usually four) vertical metal vanes spaced equally around the axis. The vanes are polished or white on one side and black on the other. When exposed to sunlight, artificial light, or infrared radiation (even the heat of a hand nearby can be sufficient), the vanes rotate with no apparent motive power, the dark sides retreating from the radiation source and the light sides advancing. Simply place near a window or under a direct incandescent light, as the vanes rotate only in light (sunlight or incandescent light). The brighter the light, the faster the vanes rotate. Magnificent glass radiometer, makes a great gift for those who have everything. All made of glass. This physical and technical demonstration clearly shows how light is converted into energy. Place near a window or under incandescent light and watch the vanes rotate. Color: Transparent Material: Glass Size: Base Size: 80mm / 3.1 inches Height: 205mm / 8.1 inches Sphere Diameter: 90mm / 3.5 inches Package Contents: 1 x Crookes Radiometer Note: Light reflection and different displays may cause the color of the item in the picture to appear slightly different from the actual product. Measurement may vary by +/- 1-3cm.