Optical or Radiation Pyrometers.—Since the intensity of radiation increases very rapidly with the temperature of the source of radiation, instruments for measuring radiation may be applied for measuring temperature, assuming that the laws con necting radiation and temperature are known. The advantage of this method is that the measurement may be made from a distance without exposing any part of the measuring apparatus to the destructive action of high temperatures. Apart from the difficulty of calibrating the measuring apparatus to give temperature in terms of radiation, the chief source of uncertainty in the appli cation of the method is the emissive power of the source of radia tion. The methods principally employed may be divided into two classes :—(1) Radiation methods, depending on the measurement of the radiant energy by means of a radiometer, thermocouple or bolometer; (2) optical or photometric methods, depending on the colour or luminous intensity of the radiation as compared with a suitable standard.
Of the radiation methods the simplest in theory and practice depends on observing the total intensity of radiation, which varies as the fourth power of the absolute temperature according to the Stefan-Boltzmann law (see HEAT) for a perfectly black body or full radiator. In applying this method it is very necessary to allow for the emissive power of the source, in case this does not radiate as a black body. Thus the emissive power of polished platinum at i,000° Abs. is only io per cent., and that of black iron oxide about 4o per cent. of that of a black body; and the percentage varies differently for different bodies with change of temperature, and also for the same body according to the part of the spectrum used for the measurement. Owing to the rapid increase of radi ation with temperature the error due to departure from black body radiation is not so serious as might be imagined at first sight. If the temperature of a polished platinum strip at 1,500° C were estimated by the radiation formula, assuming the constant for a perfectly black body, the error for red light would be about 125°, for green about roo°, and for blue about 75°. Such errors may be corrected when the emissive power of the source at various tem peratures is known from previous experiments, but it is preferable to observe, whenever possible, the radiation from the interior of a uniformly heated enclosure which approximates very closely to that of a black body. (See HEAT.) Radiation pyrometers of this type are generally calibrated by the method of sighting on the interior of an electric furnace con taining a thermocouple or gas-thermometer by which the tempera ture is measured. The gas-thermometer has been employed for verifying the law of radiation up to 1,5oo° C, but the difficulties of obtaining accurate results with the gas-thermometer increase so rapidly above 1,20o° C that it is questionable whether any advan tage is gained by using it beyond this point. The law of radiation
has been so closely verified by observations at lower temperatures that the uncertainty involved in applying it at higher temperatures, in the case of a black body is probably less than the uncertainty of the gas-thermometer measurements, and much less than the uncertainty of extrapolating an empirical formula for a thermo couple. Thus L. F. C. Holborn and W. Wien (Wied. Ann., 1895, 16), by extrapolating their thermoelectric formula, found the value 1,587° C for the melting-point of palladium, whereas Violle found 1,5oo° C by the calorimetric method, and Callendar and Eumorfopoulos (Phil. Mag., 1899, 48) found 1,54o° and 1,55o° C by the methods of the expansion and the change of resistance of platinum respectively. By a later thermoelectric extrapolation Holborn and Henning (Berlin Akad., 1905, 12, p. 311) found 1,535° C for the melting-point of palladium, and 1,710° C for that of platinum, values which were strikingly confirmed by J. A. Harker at the National Physical Laboratory, and by Waidner and Burgess at the Bureau of Standards, U.S.A. Holborn and Valen tiner employing an optical method (Ann. Phys., 1907, 22, p. I) found 1,582° C and 1,789° C for palladium and platinum respec tively. There can be little doubt that the extrapolation of the parabolic formula for the thermocouple at these temperatures is quite untrustworthy and that the values given by the electrical resistance method, or by the laws of radiation, are more likely to be correct. Assuming that the total radiation varies as the fourth power of the absolute temperature, a radiation pyrometer can be calibrated by a single observation at a known temperature, such as the of gold, 1,063° C if a black body is employed as the source; and its indications will probably be accurate at higher temperatures under a similar restriction. If the pyrometer is sighted on the interior of a furnace through a small observation hole it will indicate the temperature of the furnace correctly, pro vided that the temperature is uniform. But it must be rememberci that this condition does not generally exist in large furnaces. Sup pose, for instance, that it is required to find the temperature of the molten metal on the hearth of a furnace viewed through a thick layer of furnace gases, which are probably at a much higher temperature. It is evident that the radiation from the intervening flame may be much greater than that from the metal, and may introduce serious errors. The same objection applies with greater force to optical pyrometers, as the luminous radiation from gases may be of a highly selective character. If, on the other hand, it is required to observe the temperature of metal in a ladle before casting, the surface of the metal must be cleared of scum, and it is necessary to know the emissive power of the metal or oxide exposed.