For scientific measurements of temperature by the radiation method, the thermopile, or bolometer, or radiomicrometer, previ ously calibrated by exposure to a black body at a known tempera ture, is directly exposed at a known distance to a known area of the source of radiation. The required result may then be deduced in terms of the area and the distance. The use of extraneous optical appliances is avoided as far as possible on account of selec tive absorption. For practical purposes, in order to avoid trouble some calculations and measurements, an optical arrangement is employed, either lens or mirror, in order to form an image of the source on the receiving surface. In Fery's mirror pyrometer, a mirror which is focused by the pinion, forms an image of the source on a disk, supported by wires of constantan and copper forming a thermocouple, connected by the brass strips to terminals. An observation hole in the wall of the furnace is sighted through an eyepiece, and is made to overlap the disk slightly. The rise of temperature of the junction is assumed to be proportional to the intensity of radiation, and is indicated by the deflexion of a delicate galvanometer connected to the terminals. A lens may be substituted for the mirror at high temperatures, but it is necessary to allow for the selective absorption of the lens, and to a less extent for that of the mirror, by a special calibration of the scale.
Assuming Wien's laws for the distribution of energy in the spectrum (see HEAT), the temperature of a black body may also be measured by observing (r) the wave-length corresponding to maximum intensity in the normal spectrum, which varies inversely as the absolute temperature, or (2) the maximum intensity itself, which varies as the fifth power of the absolute temperature, or (3) the intensity of radiation corresponding to some particular radiation or colour, which varies as an exponential function of T, as given by Wien's formula. Methods (I) and (2) require ela borate apparatus and are impracticable except for purposes of scientific research. The exact application of method (3) is almost equally difficult, and is less certain in its results, but for optical purposes this method may be realized with a fair degree of approx imation by the use of coloured glasses, and forms the basis in theory of the most trustworthy optical pyrometers.
Le Chatelier (Comptes Rendus, 1892, 114, p. 214) was one of the earliest, and has served as a model for subsequent inventors. The standard of comparison is an amyl-acetate lamp, the flame• of which is adjusted in the usual manner and viewed in the same field as the image of the source. The two halves of the field are adjusted to equality of brightness by means of a cat's eye dia phragm and absorption glasses, and are viewed through a red glass, giving nearly monochromatic radiation in order to avoid the difficulty of comparing lights of different colours. Assuming Wien's law, the logarithm of the intensity of monochromatic radiation for a black body is a linear function of the reciprocal of the absolute temperature, and the instrument can be graduated by observing two temperatures ; but it is generally graduated at several points by comparison with temperatures observed by means of a thermocouple.
The Wanner Pyrometer (Phys. Zeits., 1902, p. 112) is a modi fication of Konig's spectrophotometer, in which the two halves of the field, corresponding to the source and the standard of com parison, are illuminated with monochromatic red light polarized in planes at right angles to each other.
The Fery Absorption Pyrometer (lourn. Phys., 1904 p. 32) differs from Le Chatelier's only in minor details, such as the re placement of the cat's eye diaphragm by a pair of absorbing glass wedges. The principles of its action and the method of calibration are the same. The pyrometers of Morse, and of L. F. C. Holborn and F. Kurlbaum depend on the employment of a glow lamp fila ment as standard of comparison, the current through which is adjusted to make the intrinsic brilliancy of the filament equal to that of the source. When this adjustment is made the filament becomes invisible against the image of the source as background, and the temperature of the source may be determined from an observation of the current required. Each lamp requires a separate calibration, but the lamps remain fairly constant provided that they are not overheated. To avoid this, the source is screened by absorption glasses (which also require calibration) in observing high temperatures. Except at low temperatures the comparison is effected by placing a red glass before the eyepiece. At low be called by the International Committee on Weights and Measures, for the revision of this scale as occasion requires.
BIBLIOGRAPHY.-Most of the principles and methods of thermometry are described in text-books on heat, of which Preston's Theory of Heat may be specially mentioned. For recent advances in thermometry the reader should consult the original papers. The greater part of the recent work on the subject will be found in the publications of the Bureau International des Poids et Mesures de Sevres (Paris), of the Reichsanstalt (Berlin), of the Bureau of Standards, U.S.A. (Washing ton), and of the National Physical Laboratory (London) ; Methods of Measuring Temperature by Ezer Griffiths is well up to date and gives very full references to original papers. (H. L. C.)