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Societies and Academies

Nature volume 92, pages 467469 (18 December 1913) | Download Citation



LONDON. Physical Society, November 28.—Prof. C. H. Lees, F.R.S., vice-president, in the chair.—Prof. H. L. Callendar: The expansion of silica. In attempting to deduce the expansion of mercury by the weight thermometer method with silica bulbs it was necessary to determine the expansion of specimens of silica from the same source as the bulbs, and to extend the observations of expansion over the range 0° C. to 300° C. Specimens which had been exposed to high temperatures appeared to give lower results over the range 0° C. to 0° C. than specimens which had not been heated above 300° C. during the measurements. Specimens of the same material, (1) in the form of rods were obtained and were heated and tested by the Newton ring method over the range 0° C. to 300° C.; and (2) in the form of tubes, which were tested by the Fizeau method over the range —20° C. to 150° C. The difference between the axial and radial coefficients of the tube specimens had also been tested. The expansion of the silica rod gave results agreeing with the extrapolation of the curve representing the original observations between 300° C. and 1000° C. The silica rods showed at first some peculiarities due to intrinsic strain, but settled down into a cyclic state which could be represen ted over the range o° C. to 300° C. by the formula 108 × mean coefficient o° to t = 78.0ndash;8650/(t + 175), but the variation of the coefficient with temperature was rapid and peculiar over this range and could not be represented by a formula of the usual type. The axial expansion of four different specimens had been measured, and could be represented between —20° C. and 1500 C, with a little divergence by the formula, 108 × mean coefficient 0° to t = 29.0 + 0.25t — 0.00070t2, which agreed over this range with the formula found for the rods, but was inadmissible for extrapolation to 300° C. The difference between the radial and axial coefficients was tested. Differences of the order of 5 or 10 per cent, in the expansion in different directions appeared to be persistent, and were not removed by heating the specimens to 1000° C. or cooling in liquid air. It was concluded that the differences in the radial coefficient might be due to distortion of the ring. It was considered that the most probable result for the cubical coefficient would be obtained by assuming it to be three times the linear. Owing to the smallness of the expansion of silica, and its comparative freedom from hysteresis, the possible uncertainty with the silica bulbs was probably less than 1 in 1000, in spite of the imperfect annealing.—F. J. Harlow: The thermal expansions of mercury and fused silica. A more complete set of observations of the relative coefficients of expansion of mercury in silica than those previously published are obtained by the use of an electrically heated oil bath. The observations comprise readings at frequent intervals up to 300° C, and are in good agreement with the earlier observations. Tables are included giving representative observations and the final results. From the values of the coefficients of expansion of silica determined by Prof. Callendar, the coefficients of absolute expansion of mercury are calculated.—Prof J. A. Fleming: An experimental method for the production of vibrations on strings. An apparatus for the production of vibrations of strings loaded or unloaded was shown. The vibrations are produced on a string by attaching one end to the shaft of a small continuous-current motor of about 1/8 h.p. The other end of the string is attached to a fixed point which can be moved by means of a screw, in some cases a spring balance being interposed to measure the tension. When the motor is started the string has a circular motion given to its end which is equivalent to two simple harmonic motions at right angles to each other. If the tension is adjusted rightly the string then vibrates in sections, and the number of sections can be adjusted. The distance from node to node can then be measured easily, and the frequency determined from the speed of the motor. In this way the velocity of the wave is measured, and can be compared with the velocity determined by taking the square root of the quotient of the tension by the linear density of the string. This method is useful in studying the properties of loaded strings. When the wave-length on the string extends over a distance of more than eight or ten loads, the string vibrates as if the loading matter were distributed uniformly, but the string cannot propagate vibrations when the half wave-length approaches equality to the distance between two loads. It is possible to show the reflection of a wave at a load placed at any point on the string, and also that this reflection is reduced by tapering off the loading. With this loaded vibrating string all the phenomena of inductive loading in telephone cables on the Pupin system carr be imitated.

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