Bolometer

Bolometer, an instrument invented (1881) by Professor Langley for the measurement of the intensity of radiant heat—essentially a Wheatstone's Bridge (see ELECTRICITY, Vol. IV. p. 268), arranged so that no current passes through the galvanometer. The arms of the bridge are made of a substance the resistance of which varies greatly with the temperature. Hence, if one arm be exposed to radiation from which the other is screened, the galvanometer needle will be at once deflected. The instrument may be made much more sensitive than a thermopile. A good one can measure variations of temperature of $\backslash frac\{1\}\{10000\}$th of a degree centigrade, and can detect a variation of $\backslash frac\{1\}\{1000000\}$th.