Solar motor in glass bulb (pseudo perpetual), what is it called?

Question

I have a solar motor in a glass bulb with 4 solar panels (a gift). I want to read up on how it functions, but I don't know what it is called. Anyone know?

Answer 1

radiometer

Answer 2

It's called a radiometer and it was first constructed by Sir William Crookes in the 19th century. So google Crookes radiometer. It isn't really a perpetual motion machine of course. It converts the energy of sunlight into mechanical energy. You're not getting anything for nothing. As soon as the sun stops shining, it stops. You'll see that the vanes are shiny on one side and black on the other. Take note of which way round they spin. My guess is that they spin with the shiny side forward. This is because there's still a tiny amount of air in the glass bulb. A black surface absorbs light and heat more than a shiny surface and gets hotter. This heats up the air molecules on the black side, and the higher pressure pushes it round. If you could pump out every last trace of air, the vanes would spin in the opposite direction. If you can't work out why, I'll tell you.

Answer 3

It's called a radiometer. It was invented by William Crookes in 1875, with the idea of proving Maxwell's equations about light. The idea was, if you put a set of panels like that in a perfect vacuum, it would rotate toward the black side because of the (theorized-but-then-unproven) radiation pressure of light; the photons would get a bigger "bounce" off the silver side than the black side. So Crookes built the device, and lo-and-behold, it began rotating -- in the "wrong" direction!

This happened because the bulb was not a perfect vacuum. The residual molecules in the bulb get a bigger "bounce" off the hotter, black side than off the cooler, silver side. And those molecules are far, far heavier than the tiny radiation pressure of light.

Evenutally, people were able to build a radiometer with a perfect vacuum, and it rotated (slowly!) in the "right" direction.