Proceedings of the American Academy of Arts and Sciences. CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL LABORATORY, HARVARD COLLEGE. ON THE THOMSON EFFECT AND THE TEMPERATURE COEFFICIENT OF THERMAL CONDUCTIVITY IN SOFT IRON BETWEEN 115° AND 204° C. BY EDWIN H. HALL, L. L. CAMPBELL, S. B. SERVISS, INVESTIGATIONS ON LIGHT AND HEAT MADE and publishED, WHOLLY OR IN PART, WITH APPROPRIATION FROM THE RUMFORD FUND. CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL LABORATORY, HARVARD COLLEGE. ON THE THOMSON EFFECT AND THE TEMPERATURE COEFFICIENT OF THERMAL CONDUCTIVITY IN SOFT IRON BETWEEN 115° AND 204° C. BY EDWIN H. HALL, L. L. CAMPBELL, S. B. SERVISS, AND Presented May 9, 1906. Received December 7, 1906. INTRODUCTION AND SUMMARY OF RESULTS. A PAPER printed in these Proceedings for May, 1905, by the authors of the present paper gave the following results as obtained from the study of a certain specimen 2 of soft iron: Composition iron, 99.93%; carbon, 0.059%. Thermal conductivity, k: 0.1528 at 28.2° C., with a temperature coefficient 0.0003 (?) from 28° to 58°, 66 66 66 0.0007 (?) 13° 87°, the latter value being the more reliable. Electric resistance, absolute: 11365 at 0° C., with a mean temperature coefficient 0.00519 from 0° to 100°. "Thermo-electric height" with copper: 1 Mr. Churchill was not engaged in the earlier part of the work described in this paper, but he worked with me during the greater part of July, 1906, in a supplementary investigation.-E. H. H. 2 It is proverbial that the various properties of any given metal, as set down in physical tables, are usually obtained from different specimens. It has seemed to us worth while to study one particular kind of iron in as many of its different aspects as practicable. The study is still in progress. We have now to add for the same specimen of iron: Temperature coefficient of thermal conductivity between 115° and 204° C., referred to the value of the conductivity at 115° — — - 0.00068, approximately. Electric resistance, absolute: 17260 at 100° and 26140 at 218.2°, with a mean temperature coefficient 0.00661 (on the basis of the 0° value of the resistance) between 100° and 218°. Thomson effect coefficient, v (=σ÷T, where σ is the fraction of a calorie absorbed per second in the Thomson effect by a current of 10 amperes flowing from a cross-section at temperature (T-) absolute to a cross-section at temperature (T+) absolute), from a combination of our new observations with our old ones, approximately (see pp. 614, 615). from 32° C. to 182° C. (The rate of change of v here indicated would make the tangent of the angle of inclination of the iron line on the ordinary thermo-electric diagram about 40 per cent greater at 182° C. than at 32° C.) Battelli, who worked about twenty years ago with a specimen of iron which he does not describe, except as to its shape and size, got values from which we can deduce the approximate law Lecher, who also in his recent paper fails to describe the quality of the iron with which he worked, gets σ as 278 × 10, that is, v = — 855 × 10-10, at 52° C., and gives a formula which, when allowance is made according to the data of Bède, for the change of specific heat of iron with change of temperature, indicates a considerable increase, about 12 per cent, in the numerical value of v between 50° and 130°, a nearly constant value from 130° to 160°, a slight numerical fall between 160° and 180°, and an increasing rate of fall with further rise of temperature. It appears, then, that there is general agreement among those who have worked directly on the Thomson effect in iron that the line for this metal on the thermo-electric diagram should be one of increasing steepness from 50° C. to the neighborhood of 130° C.; though Lecher is in disagreement with others as to the general change of inclination of the line above this temperature. It is noted that lead, in which the Thomson effect is undoubtedly very small at certain temperatures, has apparently been carefully examined only in the neighborhood of 50° C. (by Le Roux, by Haga, and by Battelli) and 110° C. (by Battelli), and the question is raised whether it is not desirable to examine copper, which is mechanically a much better material to work with than lead, very carefully through the widest practicable range of temperature, with a view to establishing a reference line for the thermo-electric diagram. Incidentally, it is observed that the thermal conductivity of calcined oxide of magnesium, packed to a density of about 0.17 gram per cu. cm., is considerably less, perhaps 30 per cent less, at 218° C. than at 100° C. GENERAL ARRANGEMENT OF APPARATUS. The general method followed in our recent work was the same as that which was described in the paper already referred to; but, as many changes in detail were necessitated by working at a higher temperature, it seems desirable to show by means of diagrams the later form of the apparatus. Figure 1 shows a horizontal section through the middle at the level of the two main iron bars, a and ẞ, each 1 cm. in diameter, which were the subjects of the research. The pots are of cast iron, about 16.5 cm. long, in the direction of the bars, 27 cm. wide, and 25 cm. deep. Around the main bars are two "guard-rings" (see Figure 2, a vertical cross-section of the apparatus), each consisting of twenty iron bars about 0.6 cm. in diameter, the primary object of which is to lessen lateral outflow of heat from the main bars. The outer guard-ring is a new feature of the apparatus. Behind the brass nuts n on the main bars are, first, washers of brass; next, washers of leather prepared for plumbers' use, to bear a rather high temperature.3 Mica and a plumber's cement, consisting of silicate 3 A trial which we now know to have been too short had persuaded us that this prepared leather would withstand the action of hot naphthalin. When, after a long course of experiments, the main apparatus here described was closely examined, it appeared that the leather washers had been reduced to dust. But the cement had apparently held, so that, so far as we know, there was no leakage at the flanges of the main bars during the experiments on the Thomson effect, though there was such leakage in experiments made later, in July, 1906. See p. 619. |