BOOK XIII.
VOLTAIC ELECTRICITY.
CHAPTER VII.
Magneto-electric Induction.
FARADAY’S discovery that, in combinations like those in which a voltaic current was known to produce motion, motion would produce a voltaic current, naturally excited great attention among the scientific men of Europe. The general nature of his discovery was communicated by letter[26] to M. Hachette at Paris, in December, 1831; and experiments having the like results were forthwith made by MM. Becquerel and Ampère at Paris, and MM. Nobili and Antinori at Florence.
[26] Ann. de Chimie, vol. xlviii. (1831), p. 402.
It was natural also that in a case in which the relations of space which determine the results are so complicated, different philosophers should look at them in different ways. There had been, from the first discovery by Oersted of the effect of a voltaic current upon a magnet, two rival methods of regarding the facts. Electric and magnetic lines exert an effort to place themselves transverse to each other (see [chapter iv.] of this Book), and (as I have already said) two ways offered themselves of simplifying this general truth:—to suppose an electric current made up of transverse magnetic lines; or to suppose magnetic lines made up of transverse electric currents. On either of these assumptions, the result was expressed by saying that like currents or lines (electric or magnetic) tend to place themselves parallel; which is a law more generally intelligible than the law of transverse position. Faraday had adopted the former view; had taken the lines of magnetic force for the fundamental lines of his system, and defined the direction of the magneto-electric current of induction by the relation [619] of the motion to these lines. Ampère, on the other hand, supposed the magnet to be made up of transverse electric currents ([chap. vi.]); and had deduced all the facts of electro-dynamical action, with great felicity, from this conception. The question naturally arose, in what manner, on this view, were the new facts of magneto-electric induction by motion to be explained, or even expressed?
Various philosophers attempted to answer this question. Perhaps the form in which the answer has obtained most general acceptance is that in which it was put by Lenz, who discoursed on the subject to the Academy of St. Petersburg in 1833.[27] His general rule is to this effect: when a wire moves in the neighborhood of an electric current or a magnet, a current takes place in it, such as, existing independently, would have produced a motion opposite to the actual motion. Thus two parallel forward currents move towards each other:—hence if a current move towards a parallel wire, it produces in it a backward current. A moveable wire conducting a current downwards will move round the north pole of a magnet in the direction N., W., S., E.:—hence if, when the wire have in it no current, we move it in the direction N., W., S., E., we produce in the wire an upward current. And thus, as M. de la Rive remarks,[28] in cases in which the mutual action of two currents produces a limited motion, as attraction or repulsion, or a deviation right or left, the corresponding magneto-electric induction produces an instantaneous current only; but when the electrodynamic action produces a continued motion, the corresponding motion produces, by induction, a continued current.
[27] Acad. Petrop. Nov. 29, 1833. Pogg. Ann. vol. xxxi. p. 483.
[28] Traité de l’Electricité, vol. i. p. 441 (1854).
Looking at this mode of stating the law, it is impossible not to regard this effect as a sort of reaction; and accordingly, this view was at once taken of it. Professor Ritchie said, in 1833, “The law is founded on the universal principle that action and reaction are equal.” Thus, if voltaic electricity induce magnetism under certain arrangements, magnetism will, by similar arrangements, react on a conductor and induce voltaic electricity.[29]
[29] On the Reduction of Mr. Faraday’s discoveries in Magneto-electric Induction to a General Law. Trans. of R. S. in Phil. Mag. N.S. vol. iii. 37, and vol. iv. p. 11. In the second edition of this history I used the like expressions.
There are still other ways of looking at this matter. I have elsewhere pointed out that where polar properties co-exist, they are [620] generally found to be connected,[30] and have illustrated this law in the case of electrical, magnetical, and chemical polarities. If we regard motion backwards and forwards, to the right and the left, and the like, as polar relations, we see that magneto-electric induction gives us a new manifestation of connected polarities.
[30] Phil. Ind. Sc. B. v. c. ii.
Diamagnetic Polarity.
But the manifestation of co-existent polarities which are brought into view in this most curious department of nature is not yet exhausted by those which we have described. I have already spoken ([chap. vii.]) of Dr. Faraday’s discovery that there are diamagnetic as well as magnetic bodies; bodies which are repelled by the pole of a magnet, as well as bodies which are attracted. Here is a new opposition of properties. What is the exact definition of this opposition in connexion with other polarities? To this, at present, different philosophers give different answers. Some say that diamagnetism is completely the opposite of ordinary magnetism, or, as Dr. Faraday has termed it for the sake of distinction, of paramagnetism. They say that as a north pole of a magnet gives to the neighboring extremity of a piece of soft iron a south pole, so it gives to the neighboring extremity of a piece of bismuth a north pole, and that the bismuth becomes for a time an inverted magnet; and hence, arranges itself across the line of magnetised force, instead of along it. Dr. Faraday himself at first adopted this view;[31] 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.
[31] Faraday’s Researches, Art. 2429, 2430.
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,[32] 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 [621] directions:—these hypothetical currents being in non-conducting diamagnetic, as in magnetic bodies, not in the mass, but round the particles of the matter.
[32] Poggendorf’s Ann. Jou. 1848.
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, 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.[33] 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.
[33] Silicated borate of lead. See Researches, § 2151, &c. Also flint glass, rock salt, water (2215).
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,[34] 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 [622] 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.
[34] Researches, Art. 2231.
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,[35] of the University of Bonn, using a powerful magnetic apparatus, similar to Faraday’s, found that crystals in general are magnetic, in this 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.
[35] Taylor’s Scientific Memoirs, vol. v.
Soon afterwards (in 1848) Faraday also discovered[36] 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[37] 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 him[38] 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 calcspar does; or a combination of calcspar crystals which would behave as a crystal of bismuth does.
[36] Researches, Art. 2454, &c.
[37] Art. 2469.
[38] Art. 2593, 2601.
And thus we have fresh examples to show that the Connexion of coexistent Polarities is a thought deeply seated in the minds of the [623] profoundest and most sagacious philosophers, and perpetually verified and illustrated, by unforeseen discoveries in unguessed forms, through the labors of the most skilful experimenters.
Magneto-electric Machines.
The discovery that a voltaic wire moved in presence of a magnet, has a current generated in it, was employed 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 current. This was pointed out by M. De la Rive in 1843.[39] 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.
[39] Traité de l’Elect. i. 391.
Applications of Electrodynamic Discoveries.
The great series of discoveries of which I have had to speak have been applied in many important ways 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 [624] 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 civilised 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.[40]
[40] Researches, Art. 2892.