The radiometer, also known as a light windmill, is a instructive physical object for demonstrating a theory, which was invented by the English physicist Sir William Crooke in 1873. This small physical and technical miracle shows in a simple way how the “smallest sun power plant in the world” converts light into energy. Depending on the power of the light, a rotation of approximately 2’000 revolutions per minute can be reached by the radiometer.
When warm light, that is sunlight or the light of electric bulbs (but not cold neon light) strikes the wings resting on the needle in the radiometer bowl they start to turn. Depending on the power of the light, the wings turn at varying speeds. Inside the glass bowl there is a partial vacuum produced by the air being thinned in order to keep the balance between air resistance and the turning power of the solar energy. The black surfaces of the wings absorb more light than the white surfaces, therefore the pressure on the black surfaces is much stronger than on the white surfaces. The more the molecules in the bowl heat up, the stronger the pressure on the black surfaces and the wings start to turn steadily.(Molecular theory of Brown.)

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