Spectrum Scientifics Blog

In the display windows of our store, about 1/2 dozen of these little guys sit whirling away in the sunlight:

These Radiometers, sometimes known as Crookes Radiometers are fascinating little toys, but most folks have at best a vague idea of how they work. Here’s a quick description: The Vanes in the Radiometer are black on one side and silver (or white) on the other. The black side absorbs the sunlight (or almost any light for that matter) while the other side reflects most of it. Since the black side absorbs so much of it, it needs to emit the energy that it builds. When it does so, it imparts just a tiny, tiny amount of kinetic energy to the vanes. The vanes are attached to an upside-down glass cap that is resting one what essentially a pinpoint. That pinpoint means there is very little friction between the vans and anything else. To add to the design, the bulb of the radiometer is usually vacuum sealed with a low pressure gas much like a regular light bulb. So that little bit of kinetic energy is just enough to get the vanes spinning. As you add more light, such as on a sunny day, the vanes get more energy, emit for energy and impart more kinetic energy to the vanes. On a good day the vanes can be seen whirling at high speed in the sun.

Radiometers usually work well, but every now and then you might get one that ‘jams up’ for some reason. This is usually because the vanes get dislodged from the needle pin they are resting on. This dislodging both imbalances the vanes and ads friction to the system so they don’t work as well. These problems can usually be fixed with a gentle shake of the radiometer to get the vanes back in the proper position. If you lack sunlight, a lightbulb, flashlight, or sometimes even a high powered laser can get the vanes to spin.

Every know and then someone proposes making a version of a radiometer that tries to generate energy by having the vanes spin a small turbine. This can’t work because the radiometer depends on having as little resistance as possible and trying to draw even a trickle of current means more resistance than the Radiometer can overcome. Despite this, Radiometers are an excellent way of showing the energy behind sunlight and how it can be turned into kinetic energy!

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Filed under: Physics, Solar