Societies and Academies

Abstract

LONDON.Royal Society, April 25.—Sir Archibald Geikie, K.C.B., president, in the chair. —J. S. Townsend: The diffusion and mobility of ions in a magnetic field. The mobility and diffusion of ions in a magnetic field is investigated on the same principles as those employed in the ordinary kinetic theory by considering the motion of an ion along its free paths between collisions with molecules. If U and K be the mobility and coefficient of diffusion when the magnetic force is zero, U, and K7, the corresponding quantities in directions at right angles to a magnetic force H, then where w = He/ra and T the mean interval between collisions. The magnetic deflection 0 of a stream of ions moving with a constant velocity in an electric field is also investigated, and a method is indicated of determining the velocity U due to an electric force X. When 9 is small, tan" = HU/X, and when 9 is large, tanflX/ = HUfc. —J. J. Manley: The observed variations in the temperature coefficients of a precision balance. In this paper is given an account of experiments which supplement and extend an earlier research (Phil. Trans., A, ccx., p. 387) dealing with changes which may be observed in the resting points of precision balances. Attention is directed to the following: - (a) the possibility of the change from a positive to a negative value for the temperature coefficient of a balance; (b) the critical temperature range of a balance; (c) the various causes tending _to give rise to a temperature coefficient; (d) the necessity for the " ageing " of a beam either naturally or artificially. In addition to the above, certain minute and temporary lateral displacements of the whole beam are investigated. A method for measuring these movements is given, and their origin disclosed. - Dr. Guy Barlow: The torque produced by a beam of light in oblique refraction through a glass plate. In accordance with the principle that light carries with it a stream of momentum, the passage of a beam of light through a refracting plate should give rise to a torque on the plate, it being supposed that the reaction is on the matter through which the beam is passing. In 1905 Prof. Poynting and the author made experiments which confirmed this result, but as disturbances, due to gas action, were not eliminated, more exact measurements appeared desirable. In the present experiment the original double-prism arrangement was abandoned in favour of a single cube. A glass cube, of i cm. edge, was suspended axially by a fine quartz fibre. A strong beam of light was sent obliquely through the cube, the angle of incidence having been so adjusted that the beam entered through one half of one face, and emerged through the half-face diagonally opposite. The torque was determined from the observed angular deflection of the cube. Observations were made in hydrogen and air with pressures ranging from o-i to 76 cm. Hg. The disturbance due to radiometer action was found to be inversely proportional to the gas pressure, and could be eliminated. After allowing for the reflected beams, the observed torque (of the order 2xio˜6 dyne cm.) was within 2 per cent, of that calculated from the energy of the beam.-Dr. T. C. Porter: Contributions to the study of nicker. Paper III. This paper is a continuation of two former papers: Proc. Roy. Soc., vol. Ixiii., p. 347, and vol. Ixx., p. 313. If n be the number of revolutions per second for a disc with white sector "TO" and the rest black, just to appear flickerless under illumination " I," then n= -27-83 + (8-57 + 2-79 log I) log iv (360-tt'); this holds when I is greater than 3-98. If I be less than 3-98, then n= - 38'6 + (i2-4 + o-77 log I) log w (360 -w).