Bolometer

Apparatus devised by S. P. Langley, and sometimes called "Langley's thermic balance." It is used as a very delicate measurer of heat. Its action is dependent on the principle that the electrical conductivity of a metal is diminished as it rises in temperature. If a delicate differential galvanometer be acted on by two equal and opposite currents, its needle will remain unaffected; if in one of the circuits is a thin strip of iron or platinum a slight change in the temperature of this will change its conductivity and this will immediately be indicated by a deflection of the galvanometer needle. The balance has been used by Langley for studying the distribution of energy in the ultra-red spectrum, and it is said to be sensitive to differences of temperature of 0.0001° С.