Here in the basement, we have all sorts of curiosities. (Hence this strange blog.) Among them, a Crookes radiometer happily spinning in the sunshine. It’s a delight on these crisp, clear, wintry mornings.
The short version: inside a partial vacuum, little black-and-white vanes sit atop a low-friction pivot. When heated, typically by sunlight (but a flashlight will do), it begins to spin, as if propelled by an invisible thrust from the black sides of the vanes. We’ve been told it’s a process of thermal transpiration that makes this happen, but no matter what, it’s oddly hypnotic.