A sensitive instrument used to measure radiant heat. The original form consists of two elements, each comprising blackened platinum strips (about 10−3 mm thick) arranged in series on an insulated frame to form a zigzag. The two elements are connected into the adjacent arms of a Wheatstone bridge; one element is exposed to the radiation, the other is shielded from it. The change in the resistance of the exposed element, as detected by the bridge galvanometer, enables the heat reaching it to be calculated.
Modern semiconductor bolometers are now common, in which the platinum is replaced by a strip of semiconductor: this has a much greater (though usually negative) temperature coefficient of resistance, and makes the system more sensitive.