What is a Radiomoter

The tiny fins inside this radiometer spin when sunlight shines on the device.

Level: Middle School to H.S.

Time involvement: 10-15 minutes.

It looks like a light bulb with some sort of windmill in it. Experts don’t all agree on how exactly it works but it’s fun to watch. Place it in bright sunlight and in seconds the internal “windmill” is merrily spinning. A nearby bright electric light bulb will work too. Move it to a dark corner and it will slow down and stop. No, it has no batteries or hidden mechanism. The four vanes of the “windmill” are painted black on one side and white on the other. It always spins in the same direction relative to the sunlight. A partial vacuum is present inside the radiometer.

Discussion: Although it was invented over a hundred years ago scientists have proposed numerous theories to explain the principle involved. Scientists at major universities have conducted various experiments to clarify what is actually happening. One fact is sure: if normal air pressure is allowed inside, the effect will not work. If a “hard vacuum” is created inside, it will not work! The most agreed to explanation gets into pretty heavy atomic theory. Perhaps the best “take-away” is that even “professional scientists” must submit their theories to experimental tests to validate accuracy. Science moves ahead by discussion and repeated experiments.