Radiometer

An instrument consisting of four vanes poised on an axis so as to be free to rotate, and contained in a sealed gla ve el almost perfectly exhausted. The vanes of mica are blackened on one side. On exposure to light or a source of heat (ether waves) the vanes rotate. The rotation is due to the beating back and forth of air molecules from the surface of the vanes to the inner surface of the gla globe.