The radiometer has four vanes, silvered on one side and blackened on the other. The vanes are mounted in a glass bulb which is evacuated to a pressure of approximately 0.01 mm of Hg. At this pressure, the mean free path is about the dimensions of the system so that a maximum number of molecules can carry energy between the walls of the vessel and the vanes. The momentum change is greater from the hot black surfaces where the molecules gain energy , so that the rotation is toward the silvered surfaces.

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