RADIOMETER (from Lat. radius, ray + Gk. lA(rpov, metron, measure). An instrument invented by Crookes and improved by Pringsheim and E. F. Nichols. It consists essentially of a framework carrying one or more mica vanes supported and free to turn around an axis in an inclosed space from which prac tically all the air has been exhausted; one face of each vane is coated with lampblack, while the other is polished. In the radiometer of Crookes there are four such vanes carried on a little framework, not unlike a wind mill with four arms; and this being pivoted can revolve around an axis. In the form as used by Nichols there are several modifica tions, the most important of which is that the mica vane can no longer revolve freely, but can simply turn through a small angle. In a space so exhausted as that around the mica vanes, the particles of the gas arc few in number comparatively, and their mean free path may be over a centi meter. If, now, ether-waves fall upon the polished surface of the mica, they are reflected and have no action, but if they fall upon the blackened surface they are absorbed, and the temperature of this sur face is raised. As the particles
of the gas now impinge on this heated surface they rebound with an increased velocity, and therefore produce an increased re action against the surface, pushing it back ward. If a Crookes radiometer be exposed to a match, or even the hand if sufficiently close, the mica vanes are set revolving at a high speed. If the gas around the vanes was at ordinary pres sures, the molecules would be so close that any increased velocity of the rebounding particles would be transmitted almost instantly through out the whole gas and so be felt on the polished side as well. Thus there would be no resultant force on the vanes.
In the form of instrument used by Nichols it becomes one of the most sensitive thermometers known, and it can measure most accurately the energy of the ether-waves which its vane absorbs.