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Societies and Academies

Nature volume 101, pages 98100 (04 April 1918) | Download Citation

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Abstract

LONDON. Royal Society, March 21.-Sir J. J. Thomson, president, in the chair.-Dr. C. Chree: The magnetic storm of December 16-17, 1917, as recorded at Kew and. Esk-dalemuir Observatories. The magnetic storm of December- ^6-17, 1917, was of very considerable though not outstanding magnitude. It commenced between 8h. and 9I1, on December 16, and had not wholly subsided before the afternoon of the following day. Attention is directed in the paper to the curves for the twenty-four hours commencing at 8h. on December 16. The most active period of disturbance wTas between 15I1. (3 p.m.) on December 16 and 4b; on December 17. A prominent feature in the curves was a succession of oscillations of periods averaging about twenty minutes. There were also, especially at Esk-dalemuir, some very large short-period oscillations. The paper compares the- oscillations recorded at the two observatories, and gives estimates of the rate of.change of the magnetic elements during the most rapid movements. The amplitude and rapidity of the changes proved to be mu'ch greater at the more northern station. -E. A. Owen: The absorption of X-rays. (1) The absorption coefficients of a number of substances for a radiation of wave-length 0-586 x io˜8 cm. (the a-Hne of palladium) have been determined, and the values obtained confirm those of Bragg and Pierce in.the case of elements used in common. (2) The atomic fluorescent absorption coefficient is {proportional approximately to the fourth power of the atomic number of the absorber. (3) The following relation exists between the atomic fluorescent absorption coefficient, atomic number of the absorber, and the wave-length of the radiation absorbed, /a=CN4A3, where C is a constant over certain ranges, but changes abruptly at critical points. This relation is independent of the scattering coefficient; it refers only to the loss of energy of X-radia-tion by the production of corpuscular radiations and the fluorescent X-radiations that accompany them. (4) Calculations based on the above general relation show that the molecular total absorption coefficients of different substances observed by Auren with radiation of wave-length 0-35 x io˜8 cm. may be deduced very approximately from the atomic total absorption coefficients obtained for different elements with radiation of wave-length 0-586 x io˜8 cm. if the coefficient of scattering be assumed to have a constant value of 0-2 for all elements from hydrogen to bromide for both these radiations.

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https://doi.org/10.1038/101098b0

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