Solar Radiometers

The Radiometer, also known as light windmill, was invented in 1873 by the English physicist Sir Edward Crooks. It uses light and converts it into energy & motion.

The glass globe is a sealed vacuum to prevent air resistance and the wheel is precisely balanced. The vacuum is important to the radiometer's success. If there is no vacuum (that is, if the bulb is full of air), the vanes do not spin because there is too much drag. If there is a near-perfect vacuum, the vanes do not spin unless they are held in a frictionless way. If the vanes have a frictionless support and the vacuum is complete, then photons bouncing off the silver side of the vanes push the vanes, causing them to rotate. However, this force is exceedingly small. Each paddle on the wheel has a dark side and a shiny metallic side. The dark side of the paddle absorbs more light than the metallic side creating pressure and producing motion. The radiometer works with warm light, that is sunlight or the light from electric bulbs, but not cold neon light.