Fig. 47.—Féry’s Spiral Pyrometer. Section.
Féry’s “Spiral” Radiation Pyrometer.—This instrument differs from the preceding merely in the fact that the rays are focused on a small spiral, formed of a compound strip of two metals, fixed at one end and furnished with a pointer at the free-moving end ([fig. 46)]. The effect of alterations of temperature on this spiral are to cause it to coil up or uncoil, according to whether the temperature rises or falls. This movement is magnified by the pointer, the end of which moves over a dial graduated to read temperatures directly. This arrangement is shown in section in [fig. 47], where C is the mirror, E the eye-piece, S the spiral, P the pointer, and D the dial, viewed through the window W. The appearance of the apparatus when viewed from the front is shown in [fig. 48]. The advantage gained by the use of the spiral is that the instrument is self-contained, no galvanometer being necessary; but, on the other hand, the indications are not so exact, an error of 20° C. being probable at temperatures over 1000° C. In using this pyrometer, it is observed that after focusing the hot substance, the pointer moves rapidly for a time and then pauses, after which it again commences to creep along the scale. The temperature indicated at the moment the pause occurs is generally taken as the reading, but this is not always correct.
Fig. 48.—Féry’s Spiral Pyrometer. Front View.
The creeping movement is probably due to the whole instrument, and the air in the interior, becoming heated by the entering rays, and by proximity to the hot source. In a number of trials made by the author, it was noticed that when the instrument was allowed to stand near the furnace for some time before using, thereby attaining the temperature existing in the vicinity, the “creep” almost entirely vanished. All things considered, the spiral form of Féry’s pyrometer must be regarded as more portable but less accurate than that in which the rays are received on a thermal junction.
Foster’s Fixed-Focus Radiation Pyrometer.—The necessity for focusing, common to all Féry’s radiation pyrometers, is obviated in Foster’s pyrometer, which, however, cannot be used from so great a distance. The principle involved in the fixed-focus pyrometer is that the amount of energy received by a concave mirror and focused on a thermal junction will not vary so long as the area of the surface sending rays to the mirror, through a fixed opening, increases as the square of the distance. This will be understood from [fig. 49], in which C is the mirror, D a thermal junction fixed so as to be in the focus of the opening E F, and A B the heated surface. The lines joining E and F to the edge of the mirror intersect in a point G, and provided the lines G E and G F, if produced, fall within the heated surface A B, the quantity of energy falling on D will always be the same. A cross section of the cone G A B is a circle; and if A B be twice as far away from G as E F, the areas of the circles of which A B and E F are diameters will be in the ratio 4: 1. But as A B is twice as far from G as E F, the intensity of its radiations will be as 1: 4; and hence loss of radiating power is exactly balanced by increase in area.
