Societies and Academies

Abstract

LONDON. Physical Society, December 8.—Dr. Alexander Russell, in the chair.—G. Shearer: The relation between molecular and crystal symmetry as shown by X-ray crystal analysis. By X-ray analysis the number of molecules associated with the unit cell is determined. The symmetry number for each of the 32 crystal classes is shown to mean the minimum number of asymmetric molecules necessary in the unit cell to satisfy the symmetry conditions. The symmetry number is the actual number of molecules in the cell when the molecule is asymmetric; if the molecule possesses symmetry, this symmetry appears also in the crystal, and the number of molecules in the unit cell is obtained by dividing the symmetry number of the crystal by the symmetry number of the molecule.—E. A. Owen and G. D. Preston: Modification of the powder method of determining the structure of metal crystals. Plates of aluminium, iron, copper, lead, and magnesium have been examined by means of the Bragg X-ray spectrometer, employing radiation direct from a molybdenum anti-cathode. The maxima in the spectra are sufficiently intense to measure with accuracy, and the crystalline structure of the materials examined are readily determined.—A. B. Wood: The cathode ray oscillograph. The instrument is of the low-voltage type, in which a hot cathode is employed as a source of the electron current. This low-voltage type of oscillograph is much more sensitive than the high-voltage cold-cathode type of M. Dufour. There are various methods of focussing the cathode-ray stream, and it has been proposed to have an external (i.e. outside the vacuum) photographic film. Ordinary gelatin-coated roll films or plates are unsuitable, owing to the marked absorption of the cathode-rays by the gelatin. The best results have been obtained with Schumann plates containing calcium tungstate. This material phosphoresces with a light rich in ultraviolet, and consequently the secondary luminous effect on the Schumann plate is very great. Mechanical, electrostatic, and electromagnetic methods are described for generating a time-axis on the records.—R. Webb: A low-voltage cathode ray oscillograph. The instrument is designed to work at 300 volts. The cathode consists of a hot platinum filament coated with certain oxides, and formed into a circle coaxial with the path of the rays? It is protected from bombardment by positive rays, which would disintegrate it, by a screen in which is cut a circular hole slightly less in diameter than the filament. It has a life of about 200 hours. The anode is a platinum tube through which the rays pass. The deflecting fields are electrostatic, and are provided by two pairs of plates at right angles. The bulb is in the form of a conical flask, the cathode being at the narrow end so that the rays impinge on the flat bottom, which is coated inside with fluorescent matter. The luminous trace of the rays can be seen from outside through the bottom of the flask.