radiometer 3
the radiometer needs to be in a partial vacuum in order to spin. if the gas is too dense then the tiny forces at play here cannot overcome air's viscosity. if it's in a perfect vacuum then again it stops spinning. apparently the rarefied gas plays an important role. the black side of the vane heats the gas which expands and pushes the vane. sounds good eh? this is the encyclopedia britannica explanation. it's wrong. the gas quickly reaches an equilibrium state with a temperature gradient and uniform pressure. no pressure difference - no spin.