Abstract
LONDON. Physical Society, June 22.—Prof. Reinold, F.R.S., President, in the chair.—The following communications were made:—Note on some photographs of lightning, and of “black” electric sparks, by Mr. A. W. Clayden. The lightning photographs, three in number, were obtained during the storm on June 6. Two flashes, seen on one plate, show complicated and beautiful structure: one of them is a multiple flash, and flame-like appendages point upwards from every angle; the other is a broad ribbon, and, although the plate shows signs of movement, the displacement is not in a direction such as would produce a ribbon-like effect from a linear flash. The second plate shows four flashes, none of which are ribbon-like, though the camera had moved considerably. The third plate was exposed to six flashes, one of which was believed to pass down the middle of the plate; but, on development, only a triple flash in one corner of the plate was seen. Careful search, however, revealed the central flash as a dark one with a white core, and other dark flashes were subsequently found. The plate was very much over-exposed, and this suggested that black flashes might be due to a sort of cumulative action caused by the superposition of the glare from a white cloud upon the normal image of the flash. To test this, sparks from a Wimshurst machine were photographed, and, before development, the plates were exposed to diffused gas-light for a short time. The bright sparks yielded normal images with reversed margins, and the faint ones were completely reversed. Other experiments showed the reversal to spread inwards as the time of exposure to gas-light increased. Finally, reversal was effected by placing a white screen behind the spark, to represent a white cloud, the only illumination being that of the spark itself. In the discussion which followed, Mr. W. N. Shaw exhibited a photograph taken during the same storm, which is particularly rich in dark flashes branching outwards from an intensely bright one. In some places the bright line has dark edges, and in one part a thin bright line runs along the middle of an otherwise dark portion of the flash. In answer to Mr. Inwards, Mr. Shaw said the plate was exposed about half a minute, and the former thought that, under those conditions, the appearance of the plate did not contradict Mr. Clayden's hypothesis. Speaking of the same photograph, Prof. Perry considered that Mr. Clayden's observations would explain the result, for a bright flash required more exposure to diffused light to reverse it than a faint one did. Prof. Ramsay reminded the meeting that Prof. Stokes's “oxides of nitrogen” explanation was still a possible one; and Mr. C. V. Burton asked whether they may be due to faint sparks cutting off light from brightly illuminated clouds, just as a gas-flame absorbs light from a brighter source. In reply, Mr. Clayden thought the “oxides of nitrogen” hypothesis improbable, and said his experiments did not enable him to answer Mr. Burton's question. As regards Mr. Shaw's plate, he believed the diffused light from the clouds would be sufficient to reverse the fainter tributary flashes, although it was insufficient to reverse the primary one. From data obtained when the ribbon-flash was taken, he had made some calculations which gave the height of the clouds about 1000 yards, and the ribbon-flash 1300 yards long and 100 yards wide.—Researches on the electrical resistance of bismuth, by Dr. Ed. von Aubel. The paper, which is in French, was taken as read. A translation will appear in the Proceedings of the Society.—Expansion with rise of temperature of wires under pulling stress, by J. T. Bottomley, F.R.S. The investigation was to determine whether the coefficient of expansion of wires depends on the stress to which they are subjected, and was undertaken in connection with the secular experiments on the elasticity and ductility of wires, now being conducted at Glasgow University. Two wires, about 17 feet long, of the same material, were suspended side by side within a tube, through which steam could be passed to change the temperature. One wire was loaded to half, and the other to one-tenth its breaking weight, and, in the preliminary experiments the elongations were read by a Quincke's microscope cathetometer. About 150 heatings and coolings, extending over three months, were necessary to bring the heavily loaded wire to its permanent state, so that consecutive expansions and contractions were equal. When this stage was reached, hooks of peculiar shape were attached to the lower ends of the wire. These hooks form a relative geometrical guide, and their horizontal parts mutually support a small table which carries a plane mirror. If the wires expand or contract unequally, the mirror becomes tilted, and the relative displacement is observed by means of a telescope and scale fixed nearly vertically over the mirror. From experiments on copper wires, the coefficient of relative expansion was found to be 0˙32 X 10-6 per degree Centigrade, or about 1/55 of the ordinary linear expansion of the material. The heavily loaded wire expanded most. The results for platinoid give 0˙27 X 10-6 as the relative coefficient under the conditions named above; this is about 1/57 of the ordinary linear expansion, which, from separate experiments, was found to be 15˙4 X 10-6. Mr. H. Tomlinson thought the probationary period for copper might be considerably shortened by repeatedly putting on and taking off the load, and by subjecting the wire to torsional oscillation. With iron wires this would not be the case, for they behave in a most peculiar manner, and require long periods of rest after each oscillation. From experiments he had conducted during the last two years, he found that the permeability of iron could be enormously reduced by repeated heatings and coolings whilst undergoing magnetic cycles of small range. Mr. Gregory said the paper threw considerable light on some experiments on the sag of stretched wires upon which he was engaged. He also suggested heating the wires by electric currents. In reply, Mr. Bottomley said he had considered it important to leave the wires untouched after being suspended, and as regards heating by electricity he thought that convection-currents would make the temperature non-uniform.—Owing to the absence of Prof. S. P. Thompson, his “Notes on Geometrical Optics” were postponed.
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Societies and Academies. Nature 40, 262–264 (1889). https://doi.org/10.1038/040262b0
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DOI: https://doi.org/10.1038/040262b0