Abstract
LONDON. Physical Society, April 11.—Prof. S. P. Thompson, president, in the chair.—Dr. R. A. Lehfeldt exhibited an electric heater. The apparatus consisted of a vacuum jacketed glass tube, containing water which was boiled by passing a current through a platinum spiral immersed in the liquid. Tap water is preferable to distilled water, because the small electrolytic action in the former case causes the boiling to proceed quietly. Different temperatures can be obtained by using other liquids.—Mr. Grant exhibited and described an apparatus for vapour pressure measurements. The liquid of which the vapour pressure is required is introduced into the vacuum of a syphon barometer. This is mounted alongside an ordinary syphon barometer, and the upper extremities of both are surrounded by a bath, which can be kept at any desired temperature. The levels of the mercury in the open tubes are then adjusted until the upper mercury surfaces are at the same level. The vapour pressure is then measured by the difference of level in the open tubes. By a simple modification it is easy to investigate the vapour pressure of a liquid in the presence of air. The two chief advantages of the method are (1) the simplification of the temperature correction and (2) the wide range of temperature over which it can be employed with the use of a small bath. Prof. Callendar referred to the advantages of the apparatus, and said that it appeared specially suitable for elementary laboratory measurements.—Mr. J. T. Morris showed an experiment, illustrating the use of kathode rays in alternate current work. The usual form of Braun tube was used, the rays falling upon a luminescent screen and forming a blue spot. A solenoid conveying an alternating current was fixed near the tube. The varying magnetic field caused the spot to oscillate about its mean position. To determine the maximum value of an alternating current, a switch should be arranged to rapidly replace the alternating current by a continuous one. The continuous current is then adjusted until the maximum excursion of the spot is the same as before and the value of the current read off from an ammeter in the circuit. For accurate work, the frequency of the discharge from the induction coil exciting the tube should be adjusted until it is almost exactly in synchronism with the alternating current. The unsteadiness of the spot of light in the zero position limits the accuracy of the measurements. Mr. Morris has tried to reduce this unsteadiness by using an earthed aluminium diaphragm instead of a glass one.—Mr. Morris then showed an experiment on the growth of electric currents in an inductive circuit. An E.M.F. of 0.8 volt was applied to a coil wound on a ring-shaped laminated iron core. When the current had attained its steady value, the E.M.F. was reversed and the variations of the current strength shown by an ammeter. About twenty seconds were required for the current to attain its maximum value in the opposite direction. A secondary coil was also wound upon the same core, and the effect produced upon the growing current by the closing of this secondary circuit was shown. Mr. Morris has determined curves of growth for different currents, and he showed how similar curves could be used to determine experimentally the hysteresis loss in transformers.—Mr. Croft showed some apparatus and devices useful in teaching. The method of determining graphically the focal length of a lens from the distances of conjugate foci from the centre was illustrated. The graphical solution of a quadratic equation was also shown. An apparatus for producing and demonstrating the properties of three-phase currents was exhibited and described. Mr. Croft then showed crystals illustrating the five regular solids, and an electric lamp with the filament in one plane useful for optical work. The flatness of a piece of plate glass can be tested with a scribing block. The point is adjusted to touch the glass in one position. By breathing on the glass and moving the block about it is easily seen if the point leaves the surface.
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Societies and Academies . Nature 65, 579–580 (1902). https://doi.org/10.1038/065579a0
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DOI: https://doi.org/10.1038/065579a0