BOLOMETER, an instrument for measuring radiation by means of the rise in temperature of a blackened metal strip in one of the arms of a resistance bridge. In the first bolometer, invented by Langley, a Wheatstone bridge (q.v.) was used in conjunction with a galvanometer which showed a deflection proportional to the intensity of radiation (when the deflection was not great). Lummer and Kurlbaum's bolometer consists of four platinum gratings (each of which is made of a series of strips) inserted in the arms of a Callendar-Griffiths bridge (q.v.) ; two of these gratings, in opposite arms of the bridge, are placed one behind another, so that the openings of one are opposite the strips of the other, and are exposed to the radiation, the other opposite pair being shielded ; this arrangement doubles the effect on the galva nometer, and also compensates any extraneous temperature changes. The spectrum bolometer consists of a single strip set on edge, in an arm of a bridge ; it is used for exploring the distribu tion of intensity of radiation in a spectrum. (See RADIATION,