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.
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.23] ~Additional material in the 3rd edition.~
IT was clearly established by Ampère, as we have seen, that magnetic action is a peculiar form of electromotive actions, and that, in 254 this kind of agency, action and reaction are equal and opposite. It appeared to follow almost irresistibly from these considerations, that magnetism might be made to produce electricity, as electricity could be made to imitate all the effects of magnetism. Yet for a long time the attempts to obtain such a result were fruitless. Faraday, in 1825, endeavored to make the conducting-wire of the voltaic circuit excite electricity in a neighboring wire by induction, as the conductor charged with common electricity would have done, but he obtained no such effect. If this attempt had succeeded, the magnet, which, for all such purposes, is an assemblage of voltaic circuits, might also have been made to excite electricity. About the same time, an experiment was made in France by M. Arago, which really involved the effect thus sought; though this effect was not extricated from the complex phenomenon, till Faraday began his splendid career of discovery on this subject in 1832. Arago’s observation was, that the rapid revolution of a conducting-plate in the neighborhood of a magnet, gave rise to a force acting on the magnet. In England, Messrs. Barlow and Christie, Herschel and Babbage, repeated and tried to analyse this experiment; but referring the forces only to conditions of space and time, and overlooking the real cause, the electrical currents produced by the motion, these philosophers were altogether unsuccessful in their labors. In 1831, Faraday again sought for electro-dynamical induction, and after some futile trials, at last found it in a form different from that in which he had looked for it. It was then seen, that at the precise time of making or breaking the contact which closed the galvanic circuit, a momentary effect was induced in a neighboring wire, but disappeared instantly.24 Once in possession of this fact, Mr. Faraday ran rapidly up the ladder of discovery, to the general point of view.—Instead of suddenly making or breaking the contact of the inducing circuit, a similar effect was produced by removing the inducible wire nearer to or further from the circuit;25—the effects were increased by the proximity of soft iron;26—when the soft iron was affected by an ordinary magnet instead of the voltaic wire, the same effect still recurred;27—and thus it appeared, that by making and breaking magnetic contact, a momentary electric current was produced. It was produced also by moving the magnet;28—or by moving the wire with reference to the magnet.29 Finally, it was found that the earth might supply the place of a magnet 255 in this as in other experiments;30 and the mere motion of a wire, under proper circumstances, produced in it, it appeared, a momentary electric current.31 These facts were curiously confirmed by the results in special cases. They explained Arago’s experiments: for the momentary effect became permanent by the revolution of the plate. And without using the magnet, a revolving plate became an electrical machine;32—a revolving globe exhibited electro-magnetic action,33 the circuit being complete in the globe itself without the addition of any wire;—and a mere motion of the wire of a galvanometer produced an electro-dynamic effect upon its needle.34
But the question occurs, What is the general law which determines the direction of electric currents thus produced by the joint effects of motion and magnetism? Nothing but a peculiar steadiness and clearness in his conceptions of space, could have enabled Mr. Faraday to detect the law of this phenomenon. For the question required that he should determine the mutual relations in space which connect the magnetic poles, the position of the wire, the direction of the wire’s motion, and the electrical current produced in it. This was no easy problem; indeed, the mere relation of the magnetic to the electric forces, the one set being perpendicular to the other, is of itself sufficient to perplex the mind; as we have seen in the history of the electrodynamical discoveries. But Mr. Faraday appears to have seized at once the law of the phenomena. “The relation,” he says,35 “which holds between the magnetic pole, the moving wire or metal, and the direction of the current evolved, is very simple (so it seemed to him) although rather difficult to express.” He represents it by referring position and motion to the “magnetic curves,” which go from a magnetic pole to the opposite pole. The current in the wire sets one way or the other, according to the direction in which the motion of the wire cuts these curves. And thus he was enabled, at the end of his Second Series of Researches (December, 1831), to give, in general terms, the law of nature to which may be referred the extraordinary number of new and curious experiments which he has stated;36—namely, that if a wire move so as to cut a magnetic curve, a power is called into action which tends to urge a magnetic current through the wire; and that if a mass move so that its parts do not move in the same direction across the magnetic curves, 256 and with the same angular velocity, electrical currents are called into play in the mass.
This rule, thus simple from its generality, though inevitably complex in every special case, may be looked upon as supplying the first demand of philosophy, the law of the phenomena; and accordingly Dr. Faraday has, in all his subsequent researches on magneto-electric induction, applied this law to his experiments; and has thereby unravelled an immense amount of apparent inconsistency and confusion, for those who have followed him in his mode of conceiving the subject.
But yet other philosophers have regarded these phenomena in other points of view, and have stated the laws of the phenomena in a manner different from Faraday’s, although for the most part equivalent to his. And these attempts to express, in the most simple and general form, the law of the phenomena of magneto-electrical induction, have naturally been combined with the expression of other laws of electrical and magnetical phenomena. Further, these endeavors to connect and generalize the Facts have naturally been clothed in the garb of various Theories:—the laws of phenomena have been expressed in terms of the supposed causes of the phenomena; as fluids, attractions and repulsions, particles with currents running through them or round them, physical lines of force, and the like. Such views, and the conflict of them, are the natural and hopeful prognostics of a theory which shall harmonize their discords and include all that each contains of Truth. The fermentation at present is perhaps too great to allow us to see clearly the truth which lies at the bottom. But a few of the leading points of recent discussions on these subjects will be noticed in the Additions to this volume.
THE preceding train of generalization may justly appear extensive, and of itself well worthy of admiration. Yet we are to consider all that has there been established as only one-half of the science to which it belongs,—one limb of the colossal form of Chemistry. We 257 have ascertained, we will suppose, the laws of Electric Polarity; but we have then to ask, What is the relation of this Polarity to Chemical Composition? This was the great problem which, constantly present to the minds of electro-chemical inquirers, drew them on, with the promise of some deep and comprehensive insight into the mechanism of nature. Long tasks of research, though only subsidiary to this, were cheerfully undertaken. Thus Faraday37 describes himself as compelled to set about satisfying himself of the identity of common, animal, and voltaic electricity, as “the decision of a doubtful point which interfered with the extension of his views, and destroyed the strictness of reasoning.” Having established this identity, he proceeded with his grand undertaking of electro-chemical research.
The connexion of electrical currents with chemical action, though kept out of sight in the account we have hitherto given, was never forgotten by the experimenters; for, in fact, the modes in which electrical currents were excited, were chemical actions;—the action of acids and metals on each other in the voltaic trough, or in some other form. The dependence of the electrical effect on these chemical actions, and still more, the chemical actions produced by the agency of the poles of the circuit, had been carefully studied; and we must now relate with what success.
But in what terms shall we present this narration? We have spoken of chemical actions,—but what kind of actions are these? Decomposition; the resolution of compounds into their ingredients; the separation of acids from bases; the reduction of bodies to simple elements. These names open to us a new drama; they are words which belong to a different set of relations of things, a different train of scientific inductions, a different system of generalizations, from any with which we have hitherto been concerned. We must learn to understand these phrases, before we can advance in our history of human knowledge.
And how are we to learn the meaning of this collection of words? In what other language shall it be explained? In what terms shall we define these new expressions? To this we are compelled to reply, that we cannot translate these terms into any ordinary language;—that we cannot define them in any terms already familiar to us. Here, as in all other branches of knowledge, the meaning of words is to be sought in the progress of thought; the history of science is our 258 dictionary; the steps of scientific induction are our definitions. It is only by going back through the successful researches of men respecting the composition and elements of bodies, that we can learn in what sense such terms must be understood, so as to convey real knowledge. In order that they may have a meaning for us, we must inquire what meaning they had in the minds of the authors of our discoveries.
And thus we cannot advance a step, till we have brought up our history of Chemistry to the level of our history of Electricity;—till we have studied the progress of the analytical, as well as the mechanical sciences. We are compelled to pause and look backwards here; just as happened in the history of astronomy, when we arrived at the brink of the great mechanical inductions of Newton, and found that we must trace the history of Mechanics, before we could proceed to mechanical Astronomy. The terms “force, attraction, inertia, momentum,” sent us back into preceding centuries then, just as the terms “composition” and “element” send us back now.
Nor is it to a small extent that we have thus to double back upon our past advance. Next to Astronomy, Chemistry is one of the most ancient of sciences;—the field of the earliest attempts of man to command and understand nature. It has held men for centuries by a kind of fascination; and innumerable and endless are the various labors, the failures and successes, the speculations and conclusions, the strange pretences and fantastical dreams, of those who have pursued it. To exhibit all these, or give any account of them, would be impossible; and for our design, it would not be pertinent. To extract from the mass that which is to our purpose, is difficult; but the attempt must be made. We must endeavor to analyse the history of Chemistry, so far as it has tended towards the establishment of general principles. We shall thus obtain a sight of generalizations of a new kind, and shall prepare ourselves for others of a higher order.