prove that no such assemblage of forces acting to and from fixed points, as the forces of magnets do act, could produce a continued motion like that discovered by Faraday. This, indeed, was only the well-known demonstration of the impossibility of a perpetual motion. If, instead of a collection of magnets, the adverse theorists had spoken of a magnetic current, they might probably interpret their expressions so as to explain the facts; that is, if they considered every element of such a current as a magnet, and consequently, every point of it as being a north and a south pole at the same instant. But to introduce such a conception of a magnetic current was to abandon all the laws of magnetic action hitherto established; and consequently to lose all that gave the hypothesis its value. The idea of an electric current, on the other hand, was so far from being a new and hazardous assumption, that it had already been forced upon philosophers from the time of Volta; and in this current, the relation of preceding and succeeding, which necessarily existed between the extremities of any element, introduced that relative polarity on which the success of the explanations of the facts depended. And thus in this controversy, the theory of Ampère has a great and undeniable superiority over the rival hypotheses.

CHAPTER VII.

CONSEQUENCES OF THE ELECTRODYNAMIC THEORY.

T is not necessary to state the various applications

coveries. But we may notice one of the most important, the Galvanometer, an instrument which, by enabling the philosopher to detect and to measure extremely minute electrodynamic actions, gave an impulse to the subject similar to that which it received from the invention of the Leyden Phial, or the Voltaic Pile. The strength of the voltaic current was measured, in this instrument, by the deflection produced in a compass-needle; and its sensibility was multiplied by making the wire pass repeatedly above and below the needle. Schweigger, of Halle, was one of the first devisers of this apparatus.

The substitution of electro-magnets, that is, of spiral tubes composed of voltaic wires, for common magnets, gave rise to a variety of curious apparatus and speculations, some of which I shall hereafter mention.

[2nd Ed.] [When a voltaic apparatus is in action, there may be conceived to be a current of electricity running through its various elements, as stated in the text. The force of this current in various parts of the circuit has been made the subject of mathematical investigation by M. Ohm.1 The problem is in every respect similar to that of the flow of heat through a body, and taken generally, leads to complex calculations of the same kind. But Dr. Ohm, by limiting the problem in the first place by conditions which the usual nature and form of voltaic apparatus suggest, has been able to give great simplicity to his reasonings. These conditions are, the linear form of the conductors (wires) and the steadiness of the electric state. For part of the problem Dr. Ohm's reasonings are as

this

1 Die Galvanische Kette Mathematisch bearbeitet von Dr. G. S, Ohm, Berlin, 1827.

simple and as demonstrative as the elementary propositions of Mechanics. The formulæ for the electrie force of a voltaic current to which he is led have been experimentally verified by others, especially Fechner, Gauss, Lenz, Jacobi, Poggendorf, and Pouillet.

Among ourselves, Mr. Wheatstone has confirmed and applied the views of M. Ohm, in a Memoir1 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 in formulæ of remarkable simplicity and generality. In this Memoir, Professor Wheatstone describes an instrument which he terms the 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.]

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 thermoelectricity was made by Professor Seebeck of Berlin, in 1822, and prosecuted by various persons; especially by Prof. Cumming 5 of Cambridge, who, early in 1823, extended the examination of this property to most of the metals, and determined their thermo-electric order.

2 Mass-bestimmengen über die Galvanische Kette. Leipzig, 1831. 3 Results of the Magnetic Asso

ciation.

4 Phil. Trans. 1843. Pt. 11. 5 Camb. Trans. vol. ii. p. 62. On the Development of Electro-Magnetism by Heat.

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.

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 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, zine, 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.

I do not dwell upon the peculiar phenomena of copper which, in the same series of researches, are traced by Dr. Faraday to the combined effect of its diamagnetic character, and the electric currents excited in it by the electro-magnet; nor to the optical phenomena manifested by certain transparent diamagnetic substances under electric action; as already stated in Book Ix.]

6 See the Twentieth Series of Experimental Researches in Electricity, read to the Royal Society, Dec. 18, 1845.