232
MODERN GASWORKS PRACTICE
is not desired, however, the portability of the instrument and the faet that retort-house conditions are not liable to cause derangement make it one of the most con-venient for the purpose. One such instrument is that known as the “ Wedge,” which is shown in Fig. 131. The instrument consists of a rectangular brass tube 9 inches long, across which at right angles there is fitted a small telescope of which A is the eye-piece and B the objective, wliile C is a rack and pinion for focusing the telescope on. the heated body of which. the temperature is to be measured. Inside the rectangular brass tube is a carrier D, which can be moved up and down by means of the rack and pinion E. Attached to the carrier, so as to move across the Seid of vision of the telescope, are two glass wedges F, of which one, the essential one, is of dark red glass, while the other is of plain glass, being introduced to correct the
Fig. 131.—The Wedge Optical Pyrometer.
effects of refraction. In use, the telescope is pointed at the heated body or furnace aperture, and the image is viewed as focused on the wedge of dark red glass. The wedge is tlien moved so as to increase the thickness of coloured glass between the hot body and the eye. As it is moved the image gradually fades, and at the point of extinction the scale on the side of the instrument is read for the temperature. The “ Wedge ” Pyrometer is calibrated to cover any range of temperature desired, between the limits of 500 and 2,100° C. The average instrument is made to cover a range of 400°, the scale being divided into intervals of 20°.
The optical pyrometers of the Wanner type employ the principle of comparing the intensity of light emitted from a hot body with that of a standard source of light, and their action may be compared with ordinary forms of photometers. The draw-