himself at first adopted this view;6 but he now conceives that the bismuth is not made polar, but is simply repelled by the magnet; and that the transverse position which it assumes, arises merely from its elongated form, each end trying to recede as far as possible from the repulsive pole of the magnet. Several philosophers of great eminence, however, who have examined the subject with great care, adhere to Dr. Faraday's first view of the nature of Diamagnetism —as W. Weber,7 Plücker, and Mr. Tyndall among ourselves. If we translate this view into the language of Ampère's theory, it comes to this:-that as currents are induced in iron and magnetics parallel to those existing in the inducing magnet or battery wire; so in bismuth, heavy glass, and other diamagnetic bodies, the currents induced are in the contrary directions:these hypothetical currents being in non-conducting diamagnetic, as in magnetic bodies, not in the mass, but round the particles of the matter. Magneto-optic Effects and Magnecrystallic Polarity. Not even yet have we terminated the enumeration of the co-existent polarities which in this province of nature have been brought into view. Light has polar properties; the very term polarization is the record of the discovery of these. The forces which determine the crystalline forms of bodies are of a polar nature: crystalline forms, when complete, may be defined as those forms which have a certain degree of symmetry in reference to opposite poles. Now has this optical and crystalline polarity any relation to the electrical polarity of which we have been speaking? However much we might be disposed beforehand to conjecture that there is some relation between these two groups of polar properties, yet in this as in the other parts of this history of discoveries respecting polarities, no conjecture hits the nature of the relation, such as experiment showed it to be. In November, 6 Faraday's Researches, Art. 2429, 2430, 7 Poggendorf's Ann. Jou. 1848. 1846, Faraday announced the discovery of what he then called 'the action of magnets on light.' But this action was manifested, not on light directly, but on light passing through certain kinds of glass. When this glass, subjected to the action of the powerful magnets which he used, transmitted a ray of light parallel to the line of magnetic force, an effect was produced upon the light. But of what nature was this effect? When light was ordinary light, no change in its condition was discoverable. But if the light were light polarized in any plane, the plane of polarization was turned round through a certain angle while the ray passed through the glass:-a greater angle, in proportion as the magnetic force was greater, and the thickness of the glass greater. A power in some respects of this kind, namely, a power to rotate the plane of polarization of a ray passing through them, is possessed by some bodies in their natural state; for instance, quartz crystals, and oil of turpentine. But yet, as Dr. Faraday remarks, there is a great difference in the two cases. When polarized rays pass through oil of turpentine, in whatever direction they pass, they all of them have their plane of polarization rotated in the same direction; that is, all to the right or all to the left: but when a ray passes through the heavy glass, the power of rotation exists only in a plane perpendicular to the magnetic line, and its direction as right or left-handed is reversed by reversing the magnetic polarity. In this case, we have optical properties, which do not depend on crystalline form, affected by the magnetic force. But it has also been found that crystalline form, which is so fertile a source of optical properties, affords indications of magnetic forces. In 1847, M. Plücker,10 of the University of Bonn, using a powerful magnetic apparatus, similar to Faraday's, found that crystals in general are magnetic, in this 8 Silicated borate of lead. See Researches, § 2151, &c. Also flint glass, rock salt, water (2215). * Researches, Art. 2231 10 Taylor's Scientific Memoirs, vol. v. : sense, that the axes of crystalline form tend to assume a certain position with reference to the magnetic lines of force. The possession of one optic axis or of two is one of the broad distinctions of the different crystalline forms and using this distinction, M. Plücker found that a crystal having a single optic axis tends to place itself with this axis transverse to the magnetic line of force, as if its optic axis were repelled by each magnetic pole; and crystals with two axes act as if each of these axes were repelled by the magnetic poles. This force is independent of the magnetic or diamagnetic character of the crystal; and is a directive, more properly than an attractive or repulsive force. Soon afterwards (in 1848) Faraday also discovered11 an effect of magnetism depending on crystalline form, which at first sight appeared to be different from the effects observed by M. Plücker. He found that a crystal of bismuth, of which the form is nearly a cube, but more truly a rhombohedron with one diagonal a little longer than the others, tends to place itself with this diagonal in the direction of the lines of magnetic force. At first he conceived 12 the properties thus detected to be different from those observed by M. Plücker; since in this case the force of a crystalline axis is axial, whereas in those, it was equatorial. But a further consideration of the subject, led him13 to a conviction that these forces must be fundamentally identical: for it was easy to conceive a combination of bismuth crystals which would behave in the magnetic field as a crystal of calespar does; or a combination of calcspar crystals which would behave as a crystal of bismuth does. And thus we have fresh examples to show that the Connexion of co-existent Polarities is a thought deeply seated in the minds of the profoundest and most sagacious philosophers, and perpetually verified and illustrated, by unforeseen discoveries in unguessed forms, through the labours of the most skilful experimenters. 11 Researches, Art. 2454, &c. 12 Art. 2469. 13 Art. 2593, 2601. Magneto-electric Machines. The discovery that a voltaic wire moved in presence of a magnet, has a current generated in it, was em ployed as the ground of the construction of machines to produce electrical effects. In Saxton's machine two coils of wire including a core of soft iron revolved opposite to the ends of a horseshoe magnet, and thus, as the two coils came opposite to the N. and S., and to the S. and N. poles of the magnet, currents were generated alternately in the wires in opposite directions. But by arranging the connexions of the ends of the wires, the successive currents might be made to pass in corresponding directions. The alternations or successions of currents in such machines are governed by a contrivance which alternately interrupts and permits the action; this contrivance has been called a rheotome. Clarke gave a new form to a machine of the same nature as Saxton's. But the like effect may be produced by using an electro-magnet instead of a common magnet. When this is done, a current is produced which by induction produces a current in another wire, and the action is alternately excited and interrupted. When the inducing current is interrupted, a momentary current in an opposite direction is produced in the induced wire; and when this current stops, it produces in the inducing wire a current in the original direction, which may be adjusted so as to reinforce the resumed action of the original This was pointed out by M. De la Rive in Machines have been constructed on such principles by him and others. Of such machines the most powerful hitherto known is that constructed by M. Ruhmkorff. The effects of this instrument are exceedingly energetic. current. 1843.14 Applications of Electrodynamic Discoveries. The great series of discoveries of which I have had to speak have been applied in many important ways 14 Traité de l'Elect. i. 391. to the uses of life. The Electric Telegraph is one of the most remarkable of these. By wires extended to the most distant places, the electric current is transmitted thither in an imperceptible time; and by means of well-devised systems of operation, is made to convey from man to man words, which are now most emphatically 'winged words.' In the most civilized states such wires now form a net-work across the land, which is familiar to our thoughts as the highway is to our feet; and wide seas have such pathways of human thought buried deep in their waves from shore to shore. Again, by using the chemical effects of electrodynamic action, of which we shall have to speak in the next Book, a new means has been obtained of copying, with an exactness unattainable before, any forms which art or nature has produced, and of covering them with a surface of metal. The Electrotype Process is now one of the great powers which manufacturing art employs. But these discoveries have also been employed in explaining natural phenomena, the causes of which had before been altogether inscrutable. This is the case with regard to the diurnal variation of the magnetic needle; a fact which as to its existence is universal in all places, and which yet is so curiously diverse in its course at different places. Dr. Faraday has shown that some of the most remarkable of these diversities, and probably all, seem to be accounted for by the different magnetic effects of air at different temperatures: although, as I have already said, (Book xii.) the discovery of a decennial period in the diurnal changes of magnetic declination shows that any explanation of those changes which refers them to causes existing in the atmosphere must be very incomplete, 15 15 Researches, Art. 2892. |