LONDON. Royal Society, January 20.—Prof. C. S. Sherring-ton, president, in the chair.—Sir Robert Hadfield, S. R. Williams, and I. S. Bowen: The magnetic mechanical analysis of manganese steel. Tests were made on six rods quenched in water, when they are in the non-magnetic condition, and three were afterwards annealed, which rendered them magnetic. The changes in length of the rods when subjected to magnetic fields were determined (Joule effect). In the case of the rods in the magnetic condition the change was an increment for all field strengths. No change in length could be detected for the non-magnetic specimens. The effect on the intensity of magnetisation when subjected to longitudinal stress (Villari effect) was investigated. An absolute method of measuring the intensity of magnetisation when comparatively small was adopted, and for all field strengths the application of tensile stress increased the intensity of magnetisation of the magnetic specimens. The non-magnetic rods showed no change in intensity of magnetisation by being stretched. They showed an intensity of magnetisation about 1/36th of that of the specimens in the magnetic condition, due entirely to oxidation of the skin of the rods.—Dr. W. S. Tucker and?.?. Paris: A selective hot-wire microphone. The instrument consists of an electrically heated grid of fine platinum wire placed in the neck of a Helmholtz resonator. The effect of a sound having the same frequency as that natural to the resonator itself is to produce an oscillatory motion of the air in the neck of the resonator, which in turn causes changes in resistance of the platinum-wire grid. The total resistance change comprises a steady fall in resistance due to an average cooling of the grid, and a periodic change due to the to-and-fro motion of the air. Two methods of using the microphone are described: (1) A bridge method, and (2) an amplifier method. Curves are given showing the sharpness of resonance as measured by the bridge method. Results of experiments on cooling the grid by low-velocity air-currents are described. The principal resistance changes to be expected when the grid is cooled by an oscillatory air-current are: (1) A steady drop due to an average cooling; (2) a periodic resistance change of the same frequency as that of the sound; and (3) a periodic resistance change of frequency twice that of the sound. Further deductions are that the steady change of resistance is proportional to the intensity of the sound, while the periodic resistance change in (2) is proportional to the amplitude.—E. A. Milne and R. H. Fowler: Siren harmonics and a pure-tone siren. The ordinary siren can be regarded as a point source of air of variable flux, the flux being proportional to the area of the orifice exposed by the holes in the disc. The relative intensities of the harmonics for a siren with circular holes and a circular orifice are calculated, and it is concluded that a fairly pure note should be obtained from a siren of this type, in which the distance between the centres of adjacent holes is twice the diameter of the holes. If the original is rectangular in section, the holes can be so shaped that the area of the orifice exposed varies exactly as the sine of the displacement.—L. V. King: The design of diaphragms capable of continuous tuning. Continuous tuning is achieved by the application of air-pressure (or suction). The diaphragm is constructed from a single piece of metal, and consists of a thick, undeformable, central disc connected by a thin, cencentric, annular portion to a heavy, circular rim fitted accurately on a rigid mounting. The application of air-pressure over the interior of the diaphragm alters the tension of the thin annular portion, so that the rigid central portion vibrates about the static equilibrium position with a different pitch. To realise sharp tuning and high sensitivity, diaphragms of this type should be made with almost optical precision in the form of accurate solids of revolution.
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