Bolometer

Bolometer (Greek bole, ray; ballein, to throw; and metron, measure; also called actinic or thermic balance), the invention of an American, Professor S. P. Langley, in 1881, for measuring very minute amounts of radiant heat. A strip of platinum forms one arm of an electric balance. Change of temperature in this, even if extremely minute, alters the degree of its electrical resistance, and the alteration is then registered by a delicate galvanometer. The instrument will indicate changes of temperature of less than .0001° Fahr.