Fig. 49.—Principle of Foster’s Fixed-Focus Pyrometer.
In the actual instrument the tube in which the mirror is placed is blackened internally, so that no rays reach the mirror by reflection from it. The diameters of the opening E F and the mirror C are such that the perpendicular from G on to A B is ten times the length of A B. Hence, if the heated object be 6 inches in diameter, the limiting distance of G is 10 × 6 = 60 inches. The position of the point G is indicated by a ring on the outside of the tube, and in taking a measurement the tube is brought well within the distance prescribed, which is in all cases ten times the diameter of the heated object. Temperatures are read from a galvanometer connected to the thermal junction, the whole arrangement being portable, as shown in [figs. 50] and [51], which represent the instrument in use.
Fig. 50.—Foster’s Pyrometer, mounted on Stand.
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.
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.