Radiometer

—An apparatus invented by Sir William Crookes which demonstrates ~ Radio-micrometer. the power of light. A set of metal vanes, blackened on one side and bright on the other, is fixed on a vertical axis, capable of free revolution, within a gla bulb from which the air is exhausted. Radiant heat received by the vanes causes them to rotate ata peed depending upon the intensity of the radiaion.