Societies and Academies

Abstract

LONDON. Physical Society, November 13.—Dr. C. Chree, F.R.S., president, in the chair.—The photoelectric properties of potassium-sodium alloy: Dr. Fleming. It is well known that, under the action of ordinary and ultra-violet light, the electro-positive metals lose a negative charge of electricity, the effect being most pronounced in the case of rubidium, potassium, and the liquid alloy of potassium-sodium. Potassium and sodium are melted together and then decanted over into a chamber containing a platinum plate, so that a mass of the liquid potassium-sodium alloy having a perfectly clean surface was obtained in a glass tube, and a platinum plate was fixed above it in an inclined position. By means of platinum wires sealed through the glass tube a. contact is made respectively with the platinum plate and the liquid alloy. When the surface of the alloy is illuminated by a powerful beam of light, and the galvanometer connected to the platinum plate and alloy, it is found that a current is produced in this circuit merely by the action of the light, and by balancing this current against a variable electromotive force it was shown that an electromotive force varying from 0.4 to 0.8 of a volt is produced in the photoelectric cell due to the action of the light. This photoelectric effect was greatly dependent on the colour of the light, and especially upon light absorbed by the alloy. It is greatly diminished by making even a feeble magnetic field parallel to the surface of the alloy. Some questions were discussed tending to show that the source of the photoelectromotive force cannot be identical with the volta-contact electromotive force in the circuit, but is due to the absorption of light by the sensitive surface. In all cases of such photoelectric action, the rays which are effective in producing it are those given out by the substance if heated or otherwise made radiant. Thus, zinc is photoelectric under the action of ultra-violet light, and ultra-violet light is largely radiated when an electric spark is taken between the zinc balls, or an electric arc between zinc poles. The photoelectric effects in the case of potassium are probably due to the absorption of the violet ray emitted by potassium when heated.—Electric splashes on photographic plates: A. W. Porter. The author showed, by means of transparencies, the effect of a normal magnetic field upon the patterns which appear in developing a photographic plate which is placed between two terminals between which a sudden discharge passes. This effect is confined to the main line of discharge, and consists in this line being broadened into a band of nearly constant width; a very similar result is obtained if a blast of air sweeps across the plate instead of the magnetic field being employed. This broad band is produced by the glowing gas which surrounds the spark itself. The palm-like figures and the ramifications leading up to them do not seem to be modified by the magnetic field, and the author concludes that these figures are, therefore, determined by events independent of the motion of the changes conveyed along them, or else that the moving charges have a very large mass associated with them.—An anomaly in the lagging of thin wires and narrow pipes: A. W. Porter. When thin wires and narrow pipes are covered with a lagging material, it is not possible at once to say whether the covering will keep heat in or assist it to escape. There is a critical radius equal to the ratio of the conductivity of the material to the emissivity from its surface which the outer covering must exceed before it begins to act as a lagger. Although this seems to have been recognised by Prof. Bottomley and others, the results of experiments which have previously been made would seem to indicate that the effect in practice is very small indeed. The author showed a simple lecture experiment in which a large effect is obtained.—The rate of growth of viscosity in congealing solutions: A. O. Rankine. The author exhibited apparatus and described experiments for the determination of the time variation at constant temperature of the viscosity of gelatin solutions of high concentration.—Note on the re-combihation of ions in air: Dr. Phillips.