No. 4 – The Radiometer, a.k.a. The Light Mill.

210px-Crookes_radiometer

No aggregation of strange and strangely pleasing stuff would be complete without a radiometer (or, if you’re British, a light-mill).   It was one of the attention-getters in every science museum gift shop in the past, though I notice they seem to be harder to find lately.

Place a radiometer in the sun, and its vanes will spin merrily around inside the vacuum of its sphere.  See it in action here:

http://archive.org/details/RadiometerSolarSpinningSpindleLightMill

though in strong sunlight it will go much faster, to the point of making little fairy-like “tink” sounds as it spins.

I’ve read explanations of the how and why, and it still seems like magic to me.  Since my understanding is tenuous at best, I’ll direct you here http://math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html rather than subject you to my painful paraphrasing.

To have one of these in a window on a sunny day is weirdly satisfying, like having  one of the engines of the world made visible.

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