BOLOMETER an in strument invented by S. P. Langley, secretary of the Smithsonian Institution, for detecting and measuring small quantities of radiant heat. It consists essentially of a balanced Wheat stone's bridge, one of whose arms is formed by a thin strip of platinum foil, blackened to facilitate the absorption of heat The bridge being in equilibrium, with no current passing through the galvanometer, a ray of radiant heat falling upon the platinum strip warms it slightly, thereby increasing its electric resist ance, destroying the balance Of the bridge and causing an electric current to flow through the galvanometer. By comparing the current so produced with that produced by a source of heat, the intensity of the radiation of which is known, an estimate may be formed of the quantity of heat received from the body under investigation. The instrument is so delicate that it can detect a change of temperature, in the platinum strip, amounting to less than the hundredth-thousandth part of a degree, Fah renheit. Professor Langley considers that it is also capable of measuring small quantities of radiant heat with an error of not more than 1 per cent. Far more accurate results than this, however, have more recently been obtained, par ticularly by C. V. Boys with the (Radio-micro
and by E. F. Nichols. The device of the former was sufficiently sensitive to measure the light received from a standard candle at a distance of more than a mile, while the latter physicist succeeded in measuring such extraor dinarily small amounts of heat as would be received from a standard candle 8 or 10 miles distant. It was by the latter observer that the heat received from certain of the stars was directly measured for the first time.
The bolometer was first devised for the purpose of studying the distribution of heat in the solar spectrum, and it has yielded much valuable information on this subject, especially in the infra-red regions, where Fraunhofer lines exist, although they are invisible to the eye and can only be photographed with difficulty. For more extended descriptions of the instrument, consult of the American Academy of Arts and Sciences (1881, Vol. XVI, p. 342); also (Annals) of the Astrophysical Observa tory of the Smithsonian Institution (1900, Vol. I).