the most sensitive thermometer known to science. Inventedin 1880 by astronomer Samuel P. Langley, the bolometer is used to measurelight from the faintest stars and the sun’s heat rays. It consists of afine wire connected to an electric circuit. When radiation falls on thewire, it becomes very slightly warmer. This increases the electricalresistance of the wire. The difference in conductivity is proportional tothe incident irradiance.