Abstract
Annalen der Physik und Chemie, No. 3.—In view of contradictory results got by Sir W. Thomson and M. Le Roux, with regard to the thermo-electric behaviour of stretched wires, Herr Cohn has made a number of experiments, here described in an inaugural dissertation. He finds that the intensity of the thermo-current between stretched and unstretched wire of the same metal, depends (apart from all permanent properties), not only on the present tension of the former, but also, in very different degree according to the material used, on the upper and lower limits between which, since the last complete release from tension, the tensions have varied (the succession alone, not the time, being here determinant). In a thermo element of two similar wires, stretched with equal weights, but of which one (a) has last borne a greater, the other (b) a less weight, the current flows from b to a. In iron and steel the previous tensions even affect the direction in which the thermo-current varies with increasing or decreasing tension. Taking the direction of the current which arises with the first weak stretching, it is opposite for hard and soft wires, and the discordance above referred to is thus explained. Herr Cohn thus formulates his general conclusion: “A stretched wire behaves, ceteris paribus, differently, according as it has before been stretched more weakly or more strongly, and this difference continues till the next alteration of the tension.” —Herr R. Weber contributes a useful paper on the chemical composition of glasses with relation to their resistance to atmospheric influences. He finds that the composition of many well tested lime alkali glasses approximates the proportions 6SiO2, 1CaO, and 1K2O or Na2O; but also, in good glasses, there may be more alkali, if it be compensated with more than 6 equivalents of silicic acid; and less silicic acid may be allowed if the lime be diminished relatively to the alkali.—The transmission of high tones through the telephone is discussed by Herr Hagenbach. From his experiments it appears that the (upper) limit of audibility with the instrument is commonly about two octaves lower than in direct hearing. The cause is found not in the line, nor in the magnet, but in the plate, which, when the variations of magnetism exceed a certain number per second, no longer keeps up with them.—Herr Aron gives a mathematical study of the microphone; inter alia, it is shown that, whereas in the telephone the “clang tint” is exalted, in the microphone it is lowered.—Herr Herwig prosecutes his study of liquid cells as condensers; considering the charge of cells, first by large constant batteries, then by small forces (both acting shortly), and comparing the full charges in cells containing liquids of different resistances.—We further note a new hygrometer by Herr Edelmann, based on the fact that if any space be filled with atmospheric air, and the aqueous vapour then removed, without altering the volume, the pressure decreases by the amount of tension of this vapour. A sinus manometer for measuring small differences of air-pressure (Thomsen), and a simple regulator for the electric light (Stohrer) are also described.—Herr Fröhlich shows the bearing of the principle of conservation of energy on the theory of diffraction.
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Scientific Serials . Nature 20, 45–46 (1879). https://doi.org/10.1038/020045c0
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DOI: https://doi.org/10.1038/020045c0