an instrument used for detecting the radiation of heat. The most common radiometer consists of four aluminum disks, arranged at right angles to each other and attached to a vertical axis, upon which they revolve. This apparatus is enclosed in a bulb-shaped tube, from which the air has been practically all exhausted. One side of the aluminum disk is coated with lampblack, while the other side is bright. The dark side of the disk receives the heat rays and radiates them. The reaction of this radiation causes the disks to revolve. This instrument is very sensitive. Lighting a match near one or even bringing the hand close to it will cause the disks to revolve rapidly.