Crookes Solar Radiometer

The Crookes Solar Radiometer is a light-bulb shaped apparatus, clear glass with a weather-vane shaped apparatus inside. Close inspection of the apparatus reveals that the vanes, four in number, are white on one side and black on the other. The vanes are mounted perpendicularly on a needle-like device and are perfectly balanced. They are mounted on the pin so it can spin freely, like a compass needle.

The inside of the solar radiometer is nearly a perfect vacuum, which means almost all the air has been removed. This allows the remaining air molecoules to move about freely.

The radiometer works in light. Light particles strike the vane, which are reflected by the light side. The dark side absorbs the light. A difference in temperature results on the opposing sides of the vane because of this. The air molecoules in the bulb " bounce" off the dark side harder than they do the light side. Convection energy currents formed from this activity cause the radiometer to spin. The stronger the light, the faster the spin.

It was thought originally by the inventor, Sir William Crookes, and other scientists at the time that light pressure striking the vanes is what caused it to spin. Further experimentation revealed the true nature of it's motion - the molecular motion of the air left in the bulb, as it will not function in a perfect vaccum. And if there is too much air in the bulb it will not function because of the "drag" produced by the excess air molecules.

The radiometer measures the intensity of radiant energy. Sun or artificial light makes it work. The device was invented in the late 19th century by Sir William Crookes. It is not used much as a scientific instrument because of the difficulty in callibrating it and because it is not sensitive enough to be of real scientific value.

The Crookes Solar Radiometer is a fun thing to have around, though, because the spinning vanes are eye-catching. Placed in a window, it makes a great ornament.