« PreviousContinue »
This augmentation varies, too, a great deal, from one metal to another. We remark that tin, thallium, cadmium, zinc, lead, are found together towards the upper part: at about 200° and 230° their resistance has doubled. Still above them are found steel and iron: the resistance of the latter has doubled at 180°, quadrupled at 430°, at 860° is about nine times as great as at zero. Palladium and platinum, on the contrary, approach the axis of the temperatures; it is only in the vicinity of 400° and 450° that the augmentation has acquired a value equal to that of the primitive resistance. Gold, copper, silver, form an intermediate group. It may therefore be said generally that, the less elevated the fusing-point of a metal, the more rapidly does its conductivity diminish: iron and steel form an exception to this law. In alloys the variation is always less than in the metals which constitute them. In certain of them (German silver for example) it is very slight; and this renders them valuable for the construction of standards and resistance-coils. Approximately, it is in the metals in which the resistance is greatest that its increase, under the influence of heat, is relatively the most rapid. The slight differences of composition which alter so profoundly the absolute resistance, have but a feeble influence on the relative value of its augmentation by rise of temperature.-Bibliothèque Universelle, Archives des Sciences Phys. et Nat., vol. li. pp. 284-287.
ON THE CONSERVATION AND THE PROPERTIES OF A PLATE OF PAL-
The Author has found that it is only after being heated to redness that a plate of palladium, charged with hydrogen by electrolysis, loses the hydrogen which it held by occlusion. This is readily ascertained by immersing the plate in a solution of ferrideyanide of potassium. In fact, as long as hydrogen is still present at the surface of the palladium, reduction of the ferrideyanide into ferrocyanide is observed, which is easily recognized by means of the properties of the salts of protoxide of iron.
There are also other metals which thus absorb electrolytic hydrogen-as nickel, zinc, and cobalt.
When a plate of palladium is coated with palladium-black, it becomes saturated with hydrogen much more rapidly. If when thus saturated it is wrapped in gun-cotton, the latter explodes at the end of a few seconds, and the plate burns during five or six minutes with a flame of feeble brightness.
A plate of palladium charged with hydrogen and left in the air, loses in time the gas occluded. Placed under water deprived of air, under absolute alcohol, or ether, it loses at first a part of its hydrogen with effervescence, but retains the rest without apparent change. Bibliothèque Universelle, Archives des Sciences, vol. li. p. 185.
M. L. de la Rive, by a sudden and very great diminution of the conductivity; nevertheless bismuth and antimony form an exception, and become, on the contrary, better conductors on melting.
* Pogg. Ann. Jubelband, p. 150.
XI. Studies on Magnetism. By E. BoUTY, Professor of
UP P to the present time there does not exist a complete theory of the magnet. Notwithstanding the relative simplicity of the phenomena presented by soft iron, one could not expect to explain these apart; and the study of steel magnets is still too little advanced to supply the elements of a satisfactory physical theory.
Such being the situation, I thought that an experimental and close investigation of the phenomena presented by steel magnets (e.g. those accompanying their production, union, or separation) would not be devoid of interest. The present is a first attempt in this direction. The questions which form the subject of it, though hitherto very little studied, would yet offer numerous numerical verifications for any accurate theory of magnetism; and this would suffice to render highly important researches of the sort we have undertaken.
Most of the investigations the subject of which has been magnetization by currents refer to soft iron. Lenz and Jacobit, Joulet, Müller, Wiedemann || especially, and more recently Quintus Icilius, Stoletow **, and Rowland ++ preoccupied themselves with determining the magnetic moments, temporary or perma
* Translated from a copy, communicated by the Author, of a Thesis presented to the Faculty of Sciences, Paris, 1874. † Pogg. Ann. vol. xlvii. (1839).
$ Pogg. Ann. vols. lxxix. & lxxxii. (1850, 1851). Phil. Mag. S. 3. vol. ii. (1839). Ibid. vols. c., cvi. & cxvii. (1857-1862). **Phil. Mag. January 1873. Phil. Mag. S. 4. Vol. 49. No. 323. Feb. 1875.
¶ Ibid. vol. cxxi. (1864). ++ Ibid. August 1873.
nent, developed by a current of given intensity in a bar placed in the axis of a spiral excited by the current. Several of these physicists treat also, subsidiarily, the same question for steel. As regards the accessory phenomena accompanying magnetization, they are so numerous and varied that they constitute an inexhaustible mine which still, notwithstanding numerous labours, has scarcely been touched. We will cite only those memoirs which have the closest connexion with the subject of the present investigation.
Quetelet studied the magnetism produced in a bar of steel by friction with a magnet. He established that the magnetism increases, up to a certain limit, with the number of the frictions, according to precise laws, to which we will return by-and-by. Hermann and Scholz, under the direction of Frankenheims, proved an analogous augmentation when a bar of steel is brought near the pole (free or covered with paper) of an electromagnet, or when a steel bar is several times introduced into a spiral traversed by a current.
Coulomb, and afterwards Lamont, in their numerous studies on all branches of magnetism, have enriched the science with observations on the influence of the temper of steel upon its moment of saturation, and on the phenomena which accompany the union or separation of superposed magnetized plates. Villari**, and long previously Abria††, made some experiments on the brief duration of the phenomenon of magnetization.
The temporary magnetization of steel, observed for the first time by Musschenbroek and Epinus, has been the subject of interesting memoirs by Poggendorff‡‡ and Wiedemann §§.
But the most complete investigation we possess on steel magnets is found in the recent labours of M. Jamin. These researches, which it is not our duty to estimate here, open to physicists a path in some sort quite new, and in which we should be happy to have made one step.
I. METHODS OF MEASUREMENT.
The determination of the magnetic moment of a magnet is most frequently effected by one of the two methods indicated
* Ann. de Chim. et de Phys. Ser. 4. vol. liii.
† De naturali magnetismo in Chalybem inducendo quanto momento sit tempus. Vratisl., 1865.
Quanti sit momenti tempus in magnetismo inducendo, certa quadam fluidi galvanici intensitate adhibita. Vratisl., 1863. § Pogg. Ann. vol. cxxiii.
Mémoires de l'Académie, passim. ** Pogg. Ann. 1873.
tt Annales de Chimie et de Physique, Ser. 3. vol. i.
Comptes Rendus de l'Acad. des Sciences, 1873-74.
§§ Galvanismus, vol. ii.