The advantages derived from the use of a fixed focus instrument are simplicity and cheapness; but, as many occasions arise in practice in which focusing on an object is a necessity, Foster’s pyrometer must be regarded as a simplified apparatus not capable of the wider applications of Féry’s instruments, but of great service in many cases. Whipple has recently adapted the Féry spiral pyrometer to produce an instrument with a fixed focus, by fastening the instrument to a fireclay tube, on the closed end of which the pyrometer is permanently focused. This form is specially useful for determining the temperature of molten metals, into which the end of the fireclay tube is plunged, thus giving true black-body conditions. [Illustration: FIG. 51.—FOSTER’S PYROMETER, IN USE.] =Paul’s Radiation Pyrometer.=—Thwing, in America, has introduced a radiation pyrometer in which the rays from the furnace enter the wide end of a cone, and by internal reflection are brought to the apex, at which a thermal junction is located. Paul, in this country, has marketed a similar instrument, the action of which is shown in fig. 52, where E is a tube containing a polished cone, C, at the apex of which is fixed a thermal junction, T. Rays from the hot source A A´ enter the tube at D, and pass into the cone, being finally reflected on to T, which is connected to the indicator. So long as the lines joining the outside of the cone with the extremities of the entrance D, crossing at O, fall within the hot source, A A´, the reading will be the same at all distances. Fig. 53 shows the actual pyrometer, mounted on a tripod. [Illustration: 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. [Illustration: FIG. 53.—PAUL’S RADIATION PYROMETER.]