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SPECIFIC HEAT OF MERCURY BY CONTINUOU...

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Originally appearing in Volume V05, Page 66 of the 1911 Encyclopedia Britannica.
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SPECIFIC HEAT OF MERCURY BY CONTINUOUS ELECTRIC METHOD Flow of Hg. Rise of Temp. Watts. Heat-loss. Specific Heat. gm./sec. do EC MO Per gm. deg. 8.753 11.764 14.862 0.655 .13780 joules 4'594 12.301 7.912 0.685 .03297 cals. It is assumed as a first approximation that the heat-loss is proportional to the rise of temperature do, provided that do is nearly the same in both cases, and that the distribution of temperature in the apparatus is the same for the same rise of temperature whatever the flow of liquid: The result calculated on these assumptions is given in the last column in joules, and also in calories of 2o° C. The heat-loss in this example is large, nearly 4'5 % of the total supply, owing to the small flow and the large rise of temperature, but this correction was greatly reduced in.subsequent observations on the specific heat of water by the same method. In the case of mercury the liquid itself can be utilized to conduct the electric current. In the case of water or other liquids it is necessary to employ a platinum wire stretched along the tube as heating conductor. This introduces additional difficulties of construction, but does not otherwise affect II s= 1 +0.0O0O4t+0.0000009t2 (Regnault), (3) for the specific heat s at any temperature t C. in terms of the specific heat at 0° C. taken as the standard. This formula has since been very generally applied over the whole range 0° to we C., but the experiments could not in reality give any information with regard to the specific heat at temperatures below too° C. The linear formula proposed by J. Bosscha from an independent reduction of Regnault's experiments is probably within the limits of accuracy between too° and 200 C., so far as the mean rate of variation is concerned, but the absolute values require reduction. It may be written- S =Sloo +.00023 (t-100) (Bosscha-Regnault) (4). Me he work of L. Pfaundler and H. Platter, of G. A. Hirn, of J. C. Jamin and Amaury, and of many other experimentalists who succeeded Regnault, appeared to indicate much larger rates of increase than he had found, but there can be little doubt that the discrepancies of their results, which often exceeded 5%, were due to lack of appreciation of the difficulties of calorimetric measurements. The work of Rowland by the mechanical method was the first in which due attention was paid to the thermometry and to the reduction of the results to the absolute scale of temperature. The agreement of his corrected results with those of Griffiths by a very different method, left very little doubt with regard to the rate of diminution of the specific heat of water at 20° C. The work of A. Bartoli and E. Stracciati by the method of mixture between 0° and 3o° C., though their curve is otherwise similar to Rowland's, had appeared to indicate a minimum at 2o° C., followed by a rapid rise. This lowering of the minimum was probably due to some constant errors inherent in their method of experiment. The more recent work of Ludin, 1895, under the direction of Prof. J. Pernet, extended from 0° to too° C., and appears to have attained as high a degree of excellence as it is possible to reach by the employment of mercury thermometers in conjunction with the method of mixture. His results, exhibited in fig. 6, show a minimum at 25° C., and a maximum at 87° C., the values being .9935 and 1.007 respectively in terms of the mean specific heat between o and too C. He paid great attention to the thermometry, and the discrepancies of individual measurements at any one point nowhere exceed 0.3 %, but he did not vary the conditions of the experiments materially, and it does not appear that the well-known constant errors of the method could have been completely eliminated by the devices which he adopted. The rapid rise from 25° to 750 may be due to radiation error from the hot water supply, and the subsequent fall of the curve to the inevitable loss of heat by evaporation of the boiling water on its way to the calorimeter. It must be observed, however, that there is another grave difficulty in the accurate determination of the specific heat of water near too° C. by this method, namely, that the quantity actually observed is not the specific heat at the higher temperature t, but the mean specific heat over the range 18° to t. The specific heat itself can be deduced only by differentiating the curve of observation, which greatly increases the uncertainty. The peculiar advantage of the electric method of Callendar and Barnes, already referred to, is that the specific heat itself is determined over a range of 8° to to° at each point, by adding accurately measured quantities of heat to the water at the desired temperature in an isothermal enclosure, under perfectly steady conditions, without any possibility of evaporation or loss of heat in transference. These experiments, which have been extended by Barnes over the whole range o° to too°, agree very well with Rowland and Griffiths in the rate of variation at 20° C., but show a rather flat minimum of specificheat in the neighbourhood of 38° to 40° C. At higher points the rate of variation is very similar to that of Regnault's curve, but taking the specific heat at 20° as the standard of reference, the actual values are nearly 0.56% less than Regnault's. It appears probable that his values for higher temperatures may be adopted• with this reduction, which is further confirmed by the results of Reynolds and Moorby, and by those of Ludin. According to the electric method, the whole range of variation of the specific heat between to° and 80° is only 0.5 %. Comparatively simple formulae, therefore, suffice for its expression to 1 in 10,000, which is beyond the limits of accuracy of the observations. It is more convenient in practice to use a few simple formulae, than to attempt to represent the whole range by a single complicated expression:- Below 20° C. s=0.9982+0.000,0045 (t-4o)2-o.000,0005 (t-20)3. From 20° to 6o°, s- 0.9982+0•000,0045 (t-40)2 (5). ((s=0 9944 I 000 o4t~ o•000,000y tz (Regnault Above 6o° to 2000 j corrd.) (( s= 1.000+0.000,22 (t-6o), (Bosscha corrd.) The addition of the cubic term below 2o° is intended to represent the somewhat more rapid change near the freezing-point. This effect is probably due, as suggested by Rowland, to the presence of a certain proportion of ice molecules in the liquid, which is also no doubt the cause of the anomalous expansion. Above 6o° C. Regnault's formula is adopted, the absolute values being simply diminished by a constant quantity 0.0056 to allow for the probable errors of his thermometry. Above too° C., and for approximate work generally, the simpler formula of Bosscha, similarly corrected, is probably adequate. The following table of values, calculated from these formulae, is taken from the Brit. Assoc. Report, 1849, with a slight modification
End of Article: SPECIFIC HEAT OF MERCURY BY CONTINUOUS ELECTRIC
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