How do radiometers work? Well, the bulb in which the blades spin is a partial vacuum. Partial being the tricky part – another clever scientist by the name of Lebedev noticed that the effect disappeared in a hard vacuum. Curious, yes? The light side of the blades are slightly warmer than the internal air temperature but cooler than the black side. Basically, the principal is the air that hangs out by the cool side of the blade flows slowly to the warm side of the blade. That process is called thermal transpiration. Science is cool
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