Radiometer

Radiometer, an instrument consisting of four horizontal arms of very fine glass, carefully poised so as to revolve easily on a point; the tips of the arms having pith discs blackened on one side. The whole is contained in a glass vessel almost but not quite exhausted of air. When exposed to light or heat the arms move round, more or less swiftly according to the strength of the rays. The blackened sides of the vanes are warmer: the molecules of air striking those sides are more heated by the vanes; they rebound after impact with greater velocity: the vanes are driven back by a greater recoil on the blackened sides. The radiometer was invented in 1873–76 by Crookes (q.v.).