ON THE DEPENDENCE OF THE COEFFICIENT OF FRICTION OF ATMOSPHERIC AIR ON TEMPERATURE. BY ALBERT VON OBER MAYER, CAPTAIN OF ARTILLERY. Of the two hypotheses from which the dynamical theory of gases starts, the older gives the coefficient of friction of gases proportional to the square root, the newer Maxwell's; gives it proportional to the irst power of the absolute temperature. From the retardations of vibrating fisks by the friction of the air. Maxwell found experimentally the power 10. B. Mayer the power; by experments on currents through apillary tubes 0. E. Mayer found the power & J. Paiuj the power of the absolute temperature. For the more certain determination of the ratio, experiments ou currents through four capillaries of glass and one of brass were undertaken: and together with the temperature of the room those of boiling water, congealing paraffin, and a mixture of salt and snow were employed. A irst series of experiments, less accurate, were carried out with the difference of pressure variable; a second, considerably more exact, with the differencs constant. The results of the two series agree very well, and confirm those of Mayer's experiments in a perfectly satisfactory manner. For the coefficient of friction at the temperature t there were found:According to the first series, μ=0-0001706 (1+0002735 †) ; second, μ=0-00013747 (1+0-002723 €). Kaiserliche Academie der Wissenschaften in Wien, Sitzung d. math.naturw. Clusse, Feb. 4, 1875. ON CAMACHO'S NEW ELECTROMAGNET. To the Editors of the Philosophical Magazine and Journal. General Post Office, March 2, 1875. Camacho's concentric-tube electromagnet seems to be identical in principle with one lent me when I lived in Manchester about 1852, by the late Richard Roberts, so well known in connexion with spinning-machinery. His electromagnet was an iron cube sawn nearly through by a number of parallel cuts, crossed at right angles by other cuts, so as to form a series of cores connected at one end. Each of the cores was wrapped with wire, forming electromagnets, connected by a common breech piece in metallic connexion with all the cores. This arrangement was very powerful for its size have no note of its performance et that I ours fait R. Besides, we can shorten the primitive fragment by successiv ings, and thus fix as many points as we will of the right lin sented by equation (2): the quantity of magnetism will be with great exactness. The determination of d is much less precise, since for set value we have only one observation, that which refers to the needle, and moreover d is always very small. Still, by mul observations and taking their means we arrive at satisfactory By means of this method I have studied the manner i the quantity of magnetism and the distance of the poles va needles 0-553 millim. in diameter, tempered very hard, are ized by being passed through one and the same spiral, and tensity of the current is varied. Quantities of magnetism. The average results obtained presented in the following Table, the numbers in both c being expressed in arbitrary units. Several physicists, amongst others Stoletow* and Rowla have recently applied themselves to the determination of wha former calls the magnetizing-function " of various sorts of ir H steel; it is the ratio of the quantity of magnetism H deve F' either temporarily or permanently in unit volume of the subst taken under the form of a cylinder of infinitesimal transvers mensions in proportion to its length, to the force F which prod the magnetism by acting in the direction of the axis of the cyli The clearest way of representing this function consists in structing a curve the ordinates of which are the quantities H, its abscissæ the forces F. At first concave towards the pos ordinates, it afterwards presents a point of inflection and approa asymptotically towards a parallel to the axis of the abscissa. The numbers contained in the preceding Table are proporti Ibid, August 187 * Philosophical Magazine, January 1873. to F and H. The curve given by them presents the same general characters as do Stoletow and Rowland's curves, but with more rigid turns, a concavity towards the positive H strongly pronounced for small values of F, an inflection so elongated that for a considerable portion of its extent the curve is indistinguishable from a straight line; in a word, it has the appearance of a broken line with the obtuse angles much rounded: such are the results given by the construction of the new curve. They confirm the facts discovered by entirely different methods in the case of iron, at the same time that they characterize the peculiar consistence of very stiffly tempered steel, and supply an unexpected verification of the theoretic ideas I have put forth elsewhere on the subject*. Finding it impossible to effect absolute measurements, I could do no more than compare Rowland's results with mine, in the manner indicated by the following Table. C is the abscissa at the origin of the tangent to the point of inflection, C' the abscissa of the point where the tangent meets the asymptote to the curve, L the maximum of magnetization. All the ordinates are expressed as functions of C, all the abscissæ as functions of L. The interval from C to C' might be named interval of rapid magnetization. Within these limits, distant 0.973 C, for iron, 1:608 C, for chilled steel, the quantity of magnetism increases from of its maximum value up to nearly for iron and for steel. The determination, in absolute value, of C, C', and the corresponding ordinates would furnish a good comparison of the magnetic powers of different kinds of steel, iron, &c., and at the same time would fix the limits which it would be absurd not to reach, or not economical to exceed, in the intensity of the currents employed for the magnetizing. In a future communication I shall indicate the results regarding, the distance of the poles, as well as the changes produced in the quantity of magnetism or in the polar distance when the needle is passed repeatedly through the magnetizing spiral.-Comptes Rendus de l'Académie des Sciences, vol. lxxx. pp. 650-653. * "Studies on Magnetism," IV., Phil. Mag. March 1875, p. 199 et seqq. ON THE DEPENDENCE OF THE COEFFICIENT OF FRICTION OF ATMOSPHERIC AIR ON TEMPERATURE. BY ALBERT VON OBER MAYER, JAPTAIN OF ARTILLERY. Of the two hypotheses from which the dynamical theory of gases starts, the ider gives he coefficient of riction of gases propor tional to the square root, the newer Maxwell's; gives it proportional to the irst power of he absolute "emperature. From the retarda tions of vibrating isss by the riction of the air. Maxwell found experimentally the power). E. Mayer the power : by experiments on currents "hrough capillary runes). B. Mayer found the power & Phing he power of ne sosolute temperature. For the more certain letermination of the ratio, experiments on currents through four capillaries of grass and one of brass were undertaken: and together with the temperature of the room those of boiling water, congealing paraffin, and a mixture of salt and snow were employed. Airst series of experiments, less accurate, were carried out with the üfference of pressure variable: a second, considerably more exter, with the difference constant. The results of the two series agree very well, and confirm those of Mayer's experiments in a perfectly satisfactory manner. For the coefficient of friction at the temperature there were found:— second According to the first series, u=00001706 1—040027356) ; μ=000016747 (2-0-902723 4). -Kaiserliche Akademie der Wissenschaften in Wien, Sitzung d. math.naturw. Classe, Feb. 4, 1875. ON CAMACHO'S NEW ELECTROMAGNET. To the Editors of the Philosophical Magazine and Journal. General Post Office, Camacho's concentric-tube electromagnet seems to be identical in principle with one lent me when I lived in Manchester about 1852, by the late Richard Roberts, so well known in connexion with spinning-machinery. His electromagnet was an iron cube sawn nearly through by a number of parallel cuts, crossed at right angles by other cuts, so as to form a series of cores connected at one end. Each of the cores was wrapped with wire, forming a series of electromagnets, connected by a common breech piece in metallic connexion with all the cores. This arrangement was very powerful for its size; I regret that I have no note of its performance. Yours faithfully, R. S. CULLEY, |