_Example._—A tube closed at one end is at 927° C. (1200° abs.), and gives a deflection corresponding to 2 millivolts on the indicator. To find the temperatures which would yield deflections due to 1, 3, 4, and 5 millivolts. Taking the case of 1 millivolt and applying in the formula R_{1} T_{1}^4 2 1200^4 ───── = ───────, ─── = ─────── ; R_{2} T_{2}^4 1 T_{2}^4 1200^4 from which T_{2}^4 = ────── and T_{2} = 1009° abs. 2 = 736° C. Similarly, 3 millivolts represent 1055° C.; 4 millivolts = 1154° C.; and 5 millivolts = 1236° C. These values are readily obtained by the use of four-figure logarithms. Having calculated the temperature corresponding to each whole millivolt, a curve may be plotted to represent millivolts against corresponding temperatures, and intermediate values deduced from it. Evidently, the standard reading must be taken with great accuracy, as the whole scale hinges upon it; and for this purpose an accurate resistance or thermo-electric pyrometer may be used, placed inside the tube of an electric furnace, and the radiation pyrometer sighted on a thin sheet of iron placed just in front of the naked junction. A check at the higher readings of the scale is necessary, as an exact realisation of the fourth-power law is seldom obtained in practice. This may be taken in the same manner, as thermocouples may now be calibrated directly against the gas scale up to 1550° C., thus enabling the gas-scale reading to be transferred to the radiation pyrometer. For delicate readings over a given range, the scale of a mirror galvanometer may be calibrated in this manner, sufficient resistance having first been added in series to ensure that at the highest temperature employed the spot of light will remain on the scale. [Illustration: FIG. 54.—RECORD OBTAINED WITH RADIATION PYROMETER.] =Recorders for Radiation Pyrometers.=—Any of the thermo-electric recorders described in Chapter II may be applied to radiation pyrometers, the chart being suitably divided according to the fourth-power law. When taking a record, the pyrometer is fixed on a stand or bracket and focused on the desired spot. Fig. 54 is an example of a record taken with a Thread recorder and Féry pyrometer, in which the division of the temperature scale according to the fourth-power law will be noticed. It is possible to arrange that the working temperature shall lie on the open part of the scale, by adjusting the sensitiveness of the galvanometer accordingly before calibrating.