Lily's Home Solar Radiometer Crookes Light Mill for Detecting Sunlight and Electromagnetic Radiation, Ideal Gift for The Student or Science Guru, Chain of 3 Clear Spheres Magnificent chain of 3 Radiometers; Large, Medium, Small with drop at the bottom. All made of glass. Large at 3.4 inch dia., Medium at 3.15 inch dia., and Small at 2.36 inch dia. This physical and technical miracle shows you in a clear way how sunlight is converted to energy Hang near a window or under incandescent light and watch as the vanes rotate Lily's Home is a registered trademark of Solar Wholesale, LLC.