Fig. 52.—Principle of Paul’s Radiation Pyrometer.
Indicators for Radiation Pyrometers.—When the radiations are focused on a thermal junction, the temperature of which is raised in consequence, the E.M.F. developed is in accordance with the laws discussed in Chapter II, and any thermo-electric indicator, if sufficiently sensitive, will serve for the purposes of a radiation pyrometer. The effect on the galvanometer is influenced by: (1) the nature of the junction; (2) the size of the mirror or cone; and (3) the highest temperature attained by the junction. The indicators used in connection with radiation pyrometers are of the pivoted type, which can now be made sufficiently sensitive to give full-scale deflection for a rise of 100° C. in the temperature of the junction. For the junction itself, Heil’s alloy (zinc and antimony in atomic proportions) partnered with constantan has been used, owing to the high E.M.F. developed; but cases of deterioration of this alloy have been noted, causing it to be replaced by some makers by iron. Two iron or copper constantan junctions in series give an E.M.F. for a rise of 100° C., sufficient to work a pivoted indicator, and are preferable to Heil’s couple for a radiation pyrometer.
Fig. 53.—Paul’s Radiation Pyrometer.
Calibration of Indicators for Radiation Pyrometers.—The deflections on the indicators are due to the E.M.F. generated, which is proportional to the difference in temperature between the hot and cold junctions. If both these are at the same temperature—say, 20° C.—the deflection is zero; and on allowing the radiations to fall on the hot junction its temperature is raised by an amount depending upon the intensity of the radiations—say, to 90° C. The deflection produced is then due to a difference of (90 - 20) = 70°, the radiations having raised the temperature of the hot junction 70° above its surroundings. If the surroundings (including the cold junction or junctions) had been at 15° to commence with, the hot junction under the same conditions would have risen to 85°, giving again a difference of 70°, and thus causing the same deflection as before. Provided both hot and cold junctions are located so as to attain the same atmospheric temperature in the absence of radiations, a given quantity of energy impinging on the hot junction will always produce in it the same excess temperature, and will therefore give rise to the same deflection at all ordinary atmospheric temperatures. As the junctions are so arranged in radiation pyrometers as to fulfill this condition, no correction for fluctuations in the cold junctions is necessary. The deflections, therefore, correspond to excess temperatures of the hot junction, which in turn are directly proportional to the energy received by the junction. Readings in millivolts on the indicator thus represent directly the proportions of energy received by the hot junction, 4 millivolts corresponding to twice the energy, which produces 2 millivolts, and so on; and hence the millivolt scale becomes an energy scale.
In order to translate energy into corresponding temperatures, the fourth-power law must be applied. If E1 correspond to an absolute temperature T1 on the part of the black body from which radiations are received, and E2 correspond to another temperature T2, the following relations will hold good:
E1 = K (T14 - x4), and E2 = K(T24 - x4),