Abstract
IT was shown in a previous article* that solar intensity cannot be accurately ascertained by the thermohelio-meter employed by Père Secchi, owing, among other causes, to the imperfect conductivity of the mercury in the bulb exposed to the sun. Meteorologists, however, do not generally accept the assumption that the conducting power of mercury is so imperfect as to affect materially the correctness of the indication of mercurial thermometers, Deschanel being quoted in support of the opinion that mercury is not an imperfect conductor. We are reminded that Prof. Everett, in a recent translation of the works of the author mentioned, assumed that the conductivity of quicksilver in the bulb of a thermometer is the same as a vessel “with thin metallic sides containing water which is stirred” (see Prof. Everett's translation of “Deschanel's Natural Philosophy,” Part II., pp. 245-387). The subject is so intimately connected with the determination of solar temperature and solar energy, that it has become indispensable to settle the question by some thorough practical test. Accordingly an apparatus, represented by the following illustration (Fig. 1, p. 266) has been constructed by the writer, to ascertain the conductivity of mercury. Before entering on a description, it will be instructive to point out that the heat communicated to the bulb of a thermometer by solar radiation is transmitted to its contents chiefly by convection, hence that the altitude of the sun during the observation influences the accuracy of the indication. This will be readily comprehended. Fig. 2 (p. 267) represents the bulb of a thermometer exposed to the rays when the sun's zenith distance is 65°; Fig. 3 representing the bulb when the zenith distance is 18° 23´, the latter being the minimum at the Observatory of the Roman College, where the thermo-heliometer has been long employed for the purpose of ascertaining the intensity of solar radiation. Referring to Fig. 2, it will seen that the blank crescent c, whose varying thickness indicates very nearly the amount of heat imparted at each point of the spherical surface presented towards the sun, occupies a nearly vertical position. The mercury contained within the space indicated by the said crescent, having its specific gravity reduced by the radiant heat, will ascend; while the mercury on the opposite side, which retains its specific gravity, will descend; thus a circulation will be established by means creasing the size of the bulk the transmission of heat from a to b will be retarded unless the conductivity of mercury be perfect. Hence the size of the bulb is an element affecting the accuracy of the indication—a circumstance atal to the employment of a spherical bulb in the thermo-heliometer.
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ERICSSON, J. Conductivity of Mercury . Nature 6, 265–268 (1872). https://doi.org/10.1038/006265a0
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DOI: https://doi.org/10.1038/006265a0