[19] Mass-bestimmungen über die Galvanische Kette. Leipzig, 1831.
[20] Results of the Magnetic Association.
Among ourselves, Mr. Wheatstone has confirmed and applied the views of M. Ohm, in a Memoir[21] On New Instruments and Processes for determining the Constants of a Voltaic Circuit. He there remarks that the clear ideas of electromotive forces and resistances, substituted by Ohm for the vague notions of quantity and intensity which have long been prevalent, give satisfactory explanations of the most important difficulties, and express the laws of a vast number of phenomena [252] in formulæ of remarkable simplicity and generality. In this Memoir, Professor Wheatstone describes an instrument which he terms Rheostat, because it brings to a common standard the voltaic currents which are compared by it. He generalizes the language of the subject by employing the term rheomotor for any apparatus which originates an electric current (whether voltaic or thermoelectric, &c.) and rheometer for any instrument to measure the force of such a current. It appears that the idea of constructing an instrument of the nature of the Rheostat had occurred also to Prof. Jacobi, of St Petersburg.]
[21] Phil. Trans. 1843. Pt. 11.
The galvanometer led to the discovery of another class of cases in which the electrodynamical action was called into play, namely, those in which a circuit, composed of two metals only, became electro-magnetic by heating one part of it. This discovery of thermo-electricity was made by Professor Seebeck of Berlin, in 1822, and prosecuted by various persons; especially by Prof. Cumming[22] of Cambridge, who, early in 1823, extended the examination of this property to most of the metals, and determined their thermo-electric order. But as these investigations exhibited no new mechanical effects of electromotive forces, they do not now further concern us; and we pass on, at present, to a case in which such forces act in a manner different from any of those already described.
[22] Camb. Trans. vol. ii. p. 62. On the Development of Electro-Magnetism by Heat.
Discovery of Diamagnetism.
[2nd Ed.] [By the discoveries just related, a cylindrical spiral of wire through which an electric current is passing is identified with a magnet; and the effect of such a spiral is increased by placing in it a core of soft iron. By the use of such a combination under the influence of a voltaic battery, magnets are constructed far more powerful than those which depend upon the permanent magnetism of iron. The electro-magnet employed by Dr. Faraday in some of his experiments would sustain a hundred-weight at either end.
By the use of such magnets Dr. Faraday discovered that, besides iron, nickel and cobalt, which possess magnetism in a high degree, many bodies are magnetic in a slight degree. And he made the further very important discovery, that of those substances which are not magnetic, many, perhaps all, possess an opposite property, in virtue of which he terms them diamagnetic. The opposition is of this [253] kind;—that magnetic bodies in the form of bars or needles, if free to move, arrange themselves in the axial line joining the poles; diamagnetic bodies under the same circumstances arrange themselves in an equatorial position, perpendicular to the axial line. And this tendency he conceives to be the result of one more general; that whereas magnetic bodies are attracted to the poles of a magnet, diamagnetic bodies are repelled from the poles. The list of diamagnetic bodies includes all kinds of substances; not only metals, as antimony, bismuth, gold, silver, lead, tin, zinc, but many crystals, glass, phosphorus, sulphur, sugar, gum, wood, ivory; and even flesh and fruit.
It appears that M. le Bailli had shown, in 1829, that both bismuth and antimony and bismuth repelled the magnetic needle; and as Dr. Faraday remarks, it is astonishing that such an experiment should have remained so long without further results. M. Becquerel in 1827 observed, and quoted Coulomb as having also observed, that a needle of wood under certain conditions pointed across the magnetic curves; and also stated that he had found a needle of wood place itself parallel to the wires of a galvanometer. This he referred to a magnetism transverse to the length. But he does not refer the phenomena to elementary repulsive action, nor show that they are common to an immense class of bodies, nor distinguish this diamagnetic from the magnetic class, as Faraday has taught us to do.