Crookes Radiometer

Push the lever of a normal motor and you get rotation by pushing on something solid. Push light at a radiometer’s vanes and it spins anyway — even though light has no mass to speak of and the room is in a partial vacuum, with almost nothing to push against.

That’s the trick. Sir William Crookes thought he’d found light pressure when his invention started spinning in 1873. He was almost right. The real cause turns out to be even stranger: the dark side of each vane heats up slightly, kicks any stray air molecule away a tiny bit faster, and the resulting recoil makes the whole thing turn. A few molecules of air, behaving like a frustrated crowd at a turnstile, push the vanes around at roughly two revolutions per second in direct sunlight.

Sealed glass bulb. Four mica vanes inside, each black on one side and silver on the other, mounted on a needle bearing. A little less air than nothing — about a hundredth of the atmosphere outside the bulb. Aim it at any heat source — a lamp, an open window, a hand — and watch the room spin a piece of glass.

Sir William filed for patents under his middle name, Mooney, because everyone at the Royal Society was already so tired of his ghost-investigation hobby. The radiometer is named after him anyway.