Radiometers: three “light mills” from my growing collection of radiometers (now at 28 so far), including a double with two spinning flys in the same partial vacuum volume. Here the initial light is from a LED, and then a halogen incandescent bulb is turned on which activates the motion. This device goes way back, invented in 1873 by the famous chemist Sir William Crookes when he was investigating partial vacuum environments. Light heats up the “vanes” which then heat up the very thin gas left in