Fig. 56.—Féry’s Optical Pyrometer. External View.
Féry’s Optical Pyrometer.—This instrument (shown in [figs. 55] and [56]) consists of a telescope furnished with a side-branch, in which a standard lamp E is placed. Light from E is focused upon a piece of transparent glass F, inclined at an angle of 45° to the axis of the telescope, from whence it is reflected into the eye-piece. To render the light received from the lamp monochromatic, a piece of red glass is interposed between E and the mirror. The telescope is sighted on the hot substance, rays from which pass through a piece of red glass D, and thence through two wedges of darkened glass, which diminish the intensity to a greater or less degree according to the thickness of absorbent glass interposed, which is reduced by sliding the wedges apart, and increased by the contrary movement. After passing through the wedges, the light proceeds through the inclined mirror to the eye-piece; consequently, the appearance presented to the eye is that of a field illuminated one-half by the standard lamp, and the other by the hot source. The adjustment consists in sliding the wedges, by a screw movement, until both portions of the field are equally illuminated. A temperature scale is provided on the moving piece which actuates the wedges, and is derived by Wien’s equation from the thickness of the wedges interposed when equality is obtained. Calibration is effected by noting the thickness of the wedges corresponding to two known temperatures, from which a straight line connecting thickness with the reciprocal of the absolute temperatures may be drawn, and a table formed giving values of T in terms of the thickness of the wedges. The calibration may be extended indefinitely, the accuracy of the readings depending upon the truth of Wien’s law. Féry’s optical pyrometer is a convenient instrument for occasional readings of high temperatures, combining simplicity with portability.
Le Chatelier’s Optical Pyrometer.—This pyrometer was the original form of instrument in which the temperature of a luminous source was deduced by photometric comparison with a standard light; and Féry’s apparatus, described above, is merely a convenient modification of the original. Instead of the absorbent glass wedges, Le Chatelier employed an iris diaphragm to reduce the quantity of light entering the telescope; the adjustment being carried out by altering the size of the opening in the diaphragm until the brightness of the source agreed with that of the standard. The intensity of the light received in the telescope will vary as the square of the diameter of the opening; and calibration at two known temperatures with a given monochromatic glass enables a temperature scale corresponding to diameter of opening to be computed by Wien’s law. Le Chatelier’s pyrometer is a valuable implement for research work in the laboratory, but is not so convenient for workshop purposes as Féry’s modification.
Fig. 57.—Wanner’s Pyrometer. Section.
Wanner’s Pyrometer.—The principle of this pyrometer is the comparison of the brightness of a red ray from the standard with that of the ray of some wave-length obtained from the source, both rays being produced spectroscopically and therefore being truly monochromatic. The brightness is compared by the aid of a polarising device, resulting in a somewhat complicated optical arrangement, which is shown in [fig. 57]. Light from a standard electric lamp passes through the slit S1, and from the hot source through S2. Both beams are rendered parallel by means of an achromatic lens O1, which is placed at a distance equal to its focal length from the slits. The parallel beams are dispersed by the direct-vision spectroscope P; and then pass through the polarising prism R, which separates each beam into two beams, polarised in planes at right angles. A biprism, B, placed in contact with a second achromatic lens, O2, is made of such an angle that two fields of red light, polarised in planes at right angles, one from the source and the other from the standard, are focused on the slit D. These fields are viewed through an analyser A, and are brought to equal brightness by rotating the analyser, to which a graduated scale is attached, the temperature being deduced from the angle through which the analyser is turned. The calibration is effected by Wien’s law ([equation (3) page 172]), the intensities of standard and source being related to the angle of rotation as indicated by the equation. J2⁄J1= tan2 Θ where J2 and J1 represent the intensities of source and standard respectively, and Θ = angle of rotation. Introducing this value into Wien’s equation ([page 172]), the relation between Θ and T may be shown to take the form log tan Θ = a + b⁄T, where a and b are constants. Hence, if log tan Θ be plotted against 1⁄T a straight line is obtained, and hence by a few observations at known temperatures a calibration curve may be drawn from which intermediate and extraneous readings may be obtained. Messrs Hadfield have introduced a special chart, divided so that actual readings in degrees C. may be taken directly by observing the angle Θ. As sent out for use, the temperature scale is prepared beforehand, so that direct readings may be taken.