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The Freezing Point of Iridium

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The Freezing Point of Iridium RP568 THE FREEZING POINT OF IRIDIUM By F. Henning 1 and H. T. Wensel 2 ABSTRACT The ratio of brightness for red light of a black < body immersed in freezing indium to that of a similar black body immersed in freezing gold was measured both directly and indirectly. The indirect method consisted in determining the ratio of brightness at the iridium and platinum freezing points and calculating the result from the previously determined platinum to gold ratio. The two methods, agreeing with each other within experimental errors, yielded a value for the iridium to gold ratio of 4,380 at wave length 0.652 p. This fixes the freezing point of iridium on the International Temperature Scale as 2,454° C. CONTENTS Pari' I. Introduction 809 II. Previous work 810 III. Present work 813 1. The iridium 813 2. The furnace and crucible Ml 3. Method of observation 81 1 4. Determination of effective wave length Ml IV. Results M7 V. Summary 819 VI. Acknowledgments 819 VII. Appendix. Preparation and purity of iridium 819 I. INTRODUCTION In making an accurate determination of the freezing or melting temperature of a pure metal, one of the objects is usually a more accu- rate establishment of the temperature scale in practical use by pro- viding an additional known fixed point at which temperature measur- ing equipment may be checked. In the present work, however, this item was of comparatively small importance. The extremely high cost of the metal, the difficulty of purifying it to the required degree, and the difficulties encountered in melting the metal under suitable conditions prohibit its use as a secondary thermometric fixed point except in very unusual cases. Indeed these factors would have ren- dered the result obtained in the present work scarcely worth the effort put forth were it not for the fact that data were obtained which are greatly needed in connection with standards in other fields. The color temperature scales used by various laboratories are do! on a very satisfactory basis. Not much is known concerning the the agreement of these scales and in some cases even the basis for 1. « Chief of the Division of Heat and Pressure, Physikalisch-Technisch, for an exchai tion was carried out at the Bureau of Standards under an arrangement these institutions. 2 Chief of the Pyrometry Section, Bureau of Standards. 810 Bureau of Standards Journal of Research [Vol. w scale is not definitely set forth. A project is under way at the Bureau of Standards to establish a color temperature scale based on the colors of black bodies maintained at the freezing points of platinum, rho- dium, and iridium. For this, accurate values for the temperatures of these fixed points are essential. The primary purpose, however, for which this work was undertaken was to secure data bearing upon the basic standards in the field of photometry. If a carbon filament electric lamp operating at the color temperature of the platinum point be photometered against a black body immersed in freezing platinum 3 and a tungsten electric lamp operating at the color temperature of the iridium point be photometered against a black body immersed in freezing iridium, then the relative candlepowers of these two lamps can be calculated from the temperatures of the two black bodies employed. This provides a method for comparing two photometric standard lamps differing widely in color which avoids the difficulties met with in heterochromatic photometry. These two applications of the black body immersed in freezing iridium will be reported in subsequent papers and have been mentioned here merely to explain why extreme accuracy was sought in this determination of the freezing temperature. II. PREVIOUS WORK For the past 25 years the value 2,350° C. has been almost univer- sally accepted as the melting point of iridium. It appears in practically all the tables of melting points, such as those of Landolt and Born- stein, and is listed in the International Critical Tables as 23 50 , indi- cating that this value is uncertain by at least 100°. After the present work had yielded a value some 100° C. higher than this, a critical analysis of the literature led to the conclusion that previous work alone indicated the freezing point of iridium to be 2,435° C. with an uncertainty of perhaps 25° or 30° C. In view of this condition it was thought advisable to discuss the published work in greater detail than would otherwise seem warranted. The high temperature scale is defined by the equation xlQg i i -£ _ a) * + 273 tAu + 273 2 in t is which the temperature in degrees centigrade, tAu is the tempera- ture of freezing or melting gold, -j- is the ratio of the intensities of the radiation at wave length X emitted by black bodies at the above two temperatures, and C2 is a constant. A measurement of y-> together with a determination of the corresponding X, obviously determines t, t once Au and C2 are fixed. It is equally obvious that a change in the value assigned to either C2 or the gold point will produce a change in the numerical value of t. During the past 30 years some 10 or 12 different values have at some time or other been assigned » The Waidner-Burgess Standard of Light, B.S.Jour. Research, vol. 6, p. 1103, 1931. wemd] Freezing Point of Iridium S 1 1 to C2 the value on the present 4 , International Temperature Scale l>ein<; 1.432 degrees. cm The gold point on this scale is defined as 1,063 The utility of a temperature determination which has beeD made by measuring X and the ratio of brightness is not affected when ;i new scale is adopted, since the numerical value of the temperature may be equally well calculated on the new scale. If is essential. however, to reduce such values to a common scale lor purposes of comparison. The earliest experiments of any consequence on the temperature of the iridium point are those of Nernst, 5 who measured the bright i of black bodies at the platinum and iridium points in terms of (he Hefner Kerze. The values first reported were, respectively, 91 and 2 1,210 H.K. per cm . Light of the whole spectrum was used instead of monochromatic light, so that equation (1) cannot be applied. Nernst calculated the temperature of the iridium point as 2,203° C. on the assumption that the brightness of a black body is 47 U.K. per cm2 at 1,690° C. and varies as the fourteenth power of the absolute temperature. On the basis of our present knowledge of the relation between the photometric brightness and temperature it is possible to calculate the iridium point from the ratio 1,210 to 91 obtained by Nernst at the iridium and platinum points. Nernst 6 himself, how- ever, showed later that due to contamination of his platinum speci- mens with iridium, the temperature of the furnace in his first work had been about 50° C. above the platinum point when the brightness 2 was measured as 91 H.K. per cm . Taking precautions to avoid this contamination in his later work, he obtained a value of 63.4 H.K. per 2 cm . Nernst made an attempt to express his 1906 results in terms of the Reichsanstalt scale and obtained the value 2,348°, which is equivalent to 2,394° C. on the present scale. However, this value is dependent on the degree of success which Nernst met with in making the transfer. and a more reliable value can be deduced directly from the measured ratio of brightness 1,210/63.4. Calculation based on the LCI. 7 visibility factors shows that two black bodies, one at 2,428° C. and the other at the platinum point, taken as 1,773.5° C. 8 will have a ratio of brightness 1,210/63.4 for a normal observer. The ratio measured by Nernst, therefore, leads directly to a value of 2,428° C. (Int.) for the iridium point, which is probably the best one to be found in the literature, since it is inde- pendent of the photometric unit used and depends only on the purity of the metals and the accuracy with which the ratio determined cor- responds to that for a normal observer. 9 minute Mendenhall and Ingersoll determined the melting point ol temperature bits of iridium placed on top of a Nernst glower. I he Text in annex 4, p. 94; B.SJour Proc. Seventh Gen. Conf. of Weights and Measures, p. 56, 1927; 1928. Research, vol. 1, p. 035, 1928; Zs. f. Phys., vol. 49, p. 742, « Phys. Zeits., vol. 4, p. 733, 1903. 8 Zcits., 1906. Phys. vol. 7, p. 380, o„ „ n a s.-i vol. 16. p. 131. , n 1(Wi n No i 1' aiMR.'! „ 1924. Bee also ».»• • ' ^ Recueil des Travaux, I.C.I., 16th session (Oeneva), p. 67, - 1923-24. 8 B.SJour. Research, vol. 6, p. 1119, 1931. • Phys. Rev., vol. 25, p. 1, 1907. 812 Bureau of Standards Journal of Research [Vol. w of the glower was obtained by extrapolation of the empirical isochro- matic radiation formula log E=K1+ j^ (2) The gold point, taken as 1,065° C. and the platinum point, taken as 1,745° or 1,789° C, were used to determine the necessary constants. The iridium point was reported as 2,292° or 2,388° C, according to the value accepted for the platinum point. The paper does not mention the values obtained for either the E"s or the K's in equation (2).
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