Sir John Herschel

The true path was first indicated by Sir John Herschel who discovered the connexion between the outward form of quartz crystals and their property of rotating the plane of polarization of light.

He remarked that a rectilinear electric current, deflecting a needle to right and left all round it, possesses a helicoidal dissymmetry similar to that displayed by the crystals.

“Therefore,” he wrote,[65] “induction led me to conclude that a similar connexion exists, and must turn up somehow or other, between the electric current and polarized light, and that the plane of polarization would be deflected by magneto-electricity.”

The nature of this connexion was discovered by Faraday, who so far back as 1834[66] had transmitted polarized light through an electrolytic solution during the passage of the current, in the hope of observing a change of polarization, This early attempt failed; but in September, 1845, he varied the experiment by placing a piece of heavy glass between the poles of an excited electro-magnet; and found that the plane of polarization of a beam of light was rotated when the beam traveiled through the glass parallel to the lines of force of the magnetic field.[67]

In the year following Faraday’s discovery, Airy[68] suggested a way of representing the effect analytically; as might have been expected, this was by modifying the equations which had been already introduced by MacCullagh for the case of naturally active bodies. In MacCullagh’s equations

the terms

and

change sign with x, so that the rotation of the plane of polarization is always right-handed or always left-handed with respect to the direction of the beam. This is the case in naturally-active bodies; but the rotation due to a magnetic field is in the same absolute direction whichever way the light is travelling, so that the derivations with respect to x must be of even order. Airy proposed the equations

where μ denotes a constant, proportional to the strength of the magnetic field which is used to produce the effect. He remarked, however, that instead of taking

and

as the additional terms, it would be possible to take

and

or

and

or any other derivates in which the number of differentiations is odd with respect to t and even with respect to x. It may, in fact, be shown by the method previously applied to MacCullagh’s formulae that, if the equations are

where (r + s) is an odd number, the angle through which the plane of polarization rotates in unit length of path is a numerical multiple of

where τ denotes the period of the light. Now it was shown by Verdet[69] that the magnetic rotation is approximately proportional to the inverse square of the wave-length; and hence we must have

so that the only equations capable of correctly representing Faraday’s effect are either

or

The former pair arise, as will appear later, in Maxwell’s theory of rotatory polarization: the latter pair, which were suggested in 1868 by Boussinesq.[70] follow from that physical theory of the phenomenon which is generally accepted at the present time.[71]

Airy’s work on the magnetic rotation of light was limited in the same way as MacCullagh’s work on the rotatory power of quartz; it furnished only an analytical representation of the effect, without attempting to justify the equations. The earliest endeavour to provide a physical theory seems to have been made in 1858, in the inaugural dissertations of Carl Neumann, of Halle.[72] Neumann assumed that every element of an electric current exerts force on the particles of the aether; and in particular that this is true of the molecular currents which constitute magnetization, although in this case the force vanishes except when the aethereal particle is already in motion. If e donote the displacement of the aethereal particle m, the force in question may be represented by the term

where K denotes the imposed magnetic field, and k denotes a magneto-optic constant characteristic the body. When this term is introduced into the equations of motion of the aether, they take the form which had been suggested by Airy; whence Neumann’s hypothesis is seen to lead to the incorrect conclusion that the rotation is independent of the wave-length.

The rotation of plane-polarized light depends, as Fresnel had shown,[73] on a difference between the velocities of propagation of the right-handed and left-handed circularly polarized waves into which plane-polarized light may be resolved.

In the case of magnetic rotation, this difference was shown by Verdet to be proportional to the component of the magnetic force in the direction of propagation of the light; and Cornu[74] showed further that the mean of the velocities of the right-handed and left-handed waves is equal to the velocity of light in the medium when there is no magnetic field.

From these data, by Fresnel’s geometrical method, the wave-surface in the medium may be obtained; it is found to consist of two spheres (one relating to the right-handed and one to the left-handed light), each identical with the spherical wave-surface of the unmagnetized medium, displaced from each other along the lines of magnetic force.[75]

The discovery of the connexion between magnetism and light gave interest to a short paper of a speculative character which Faraday published[76] in 1846, under the title “Thoughts on Ray-Vibrations.” In this it is possible to trace the progress of Faraday’s thought towards something like an electro-magnetic theory of light.

Considering first the nature of ponderable matter, he suggests that an ultimate atom may be nothing else than a field of force—electric, magnetic,and gravitational—surrounding a point-centre; on this view, which is substantially that of Michell and Boscovich, an atom would have no definite size, but ought rather to be conceived of as completely penetrable, and extending throughout all space; and the molecule of a chemical compound would consist not of atoms side by side, but of “spheres of power mutually penetrated, and the centres even coinciding."[77]

All space being thus permeated by lines of force, Faraday suggested that light and radiant heat might be transverse vibrations propagated along these lines of force. In this way he proposed to “dismiss the aether,” or rather to replace it by lines of force between centres, the centres together with their lines of force constituting the particles of material substances.

If the existence of a luminiferous aether were to be admitted, Faraday suggested that it might be the vehicle of magnetic force; " for,” he wrote in 1851,[78] “it is not at all unlikely that if there be an aether, it should have other uses than simply the conveyance of radiations.”

This sentence may be regarded as the origin of the electro-magnetic theory of light.

At the time when the “Thoughts on Ray-Vibrations” were published, Faraday was evidently trying to comprehend everything in terms of lines of force. His confidence in which had been recently justified by another discovery. A few weeks after the first observation of the magnetic rotation of light, he noticed[79] that a bar of the heavy glass which had been used in this investigation, when suspended between the poles of an electro-magnet, set itself across the line joining the poles: thus behaving in the contrary way to a bar of an ordinary magnetic substance, which would tend to set itself along this line.

A simpler manifestation of the effect was obtained when a cube or sphere of the substance was used; in such forms it showed a disposition to move from the stronger to the weaker places of the magnetic field. The pointing of the bar was then seen to be merely the resultant of the tendencies of each of its particles to move outwards into the positions of weakest magnetic action.

Many other bodies besides heavy glass were found to display the same property; in particular, bismuth.[80] The name diamagnetic was given to them.

Faraday remarked:

“Theoretically, an explanation of the movements of the diamagnetic bodies might be offered in the supposition that magnetic induction caused in them a contrary state to that which it produced in magnetic matter.

If a particle of each kind of matter were placed in the magnetic field, both would become magnetic, and each would have its axis parallel to the resultant of magnetic force passing through it. ut the particle of magnetic matter would have its north and south poles opposite, or facing toward the contrary poles of the inducing magnet, whereas with the diamagnetic particles the reverse would be the case; and hence would result approximation in the one substance, recession in the other.

Upon Ampère’s theory, this view would be equivalent to the supposition 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 diamagnetic bodies, the currents induced are in the contrary direction."[81][errata 1]

This explanation became generally known as the “hypothesis of diamagnetic polarity”.

It represents diamagnetism as similar to ordinary induced magnetism in all respects, except that the direction of the induced polarity is reversed. It was accepted by other investigators, notably by W. Weber, Plücker, Reich, and Tyndall; but was afterwards displaced from the favour of its inventor by another conception, more agreeable to his peculiar views on the nature of the magnetic field.

In this second hypothesis, Faraday supposed an ordinary magnetic or para- magnetic[82] body to be one which offers a specially easy passage to lines of magnetic force, so that they tend to crowd into it in preference to other bodies; while he supposed a dia- magnetic body to have a low degree of conducting power for the lines of force, so that they tend to avoid it.

“If, then,” he reasoned,[83] “a medium having a certain conducting power occupy the magnetic field, and then a portion of another medium or substance be placed in the field having a greater conducting power, the latter will tend to draw up towards the place of greatest force, displacing the former.”

There is an electrostatic effect to which this is quite analogous ; a charged body attracts a body whose specific inductive capacity is greater than that of the surrounding medium, and repels a body whose specific inductive capacity is less; in either case the tendency is to afford the path of best conductance to the lines of force.[84]

For some time the advocates of the “polarity” and “conduction” theories of diamagnetism carried on a contro- versy which, indeed, like the controversy between the adherents of the one-fluid and two-fluid theories of electricity, persisted after it had been shown that the rival hypotheses were mathe- matically equivalent, and that no experiment could be suggested which would distinguish between them.

Meanwhile new properties of magnetizable bodies were being discovered. In 1847 Julius Plücker (b. 1801, d. 1868), Professor of Natural Philosophy in the University of Bonn, while repeating and extending Faraday’s magnetic experiments, observed[85] that certain uniaxal crystals, when placed between the two poles of a magnet, tend to set themselves so that the optic axis has the equatorial position.

At this time Faraday was continuing his researches; and, while investigating the diamagnetic properties of bismuth, was frequently embarrassed by the occurrence of anomalous results. In 1848 he ascertained that these were in some way connected with the crystalline form of the substance, and showed[86] that when a crystal of bismuth is placed in a field of uniform magnetic force (so that no tendency to motion arises from its diamagnetism) it sets itself so as to have one of its crystalline axes directed along the lines of force.

At first he supposed this effect to be distinct from that which had been discovered shortly before by Plücker. “The results,” he wrote,[87] “are altogether very different from those produced by diamagnetic action.

They are equally distinct from those discovered and described by Plücker, in his beautiful researches into the relation of the optic axis to magnetic action; for there the force is equatorial, whereas here it is axial. So they appear to present to us a new force, or a new form of force, in the molecules of matter, which, for convenience sake, I will conventionally designate by a new word, as the magnecrystallic force.”

Later in the same year, however, he recognized[88] that “the phaenomena discovered by Plücker and those of which I have given an account have one common origin and cause.”

The idea of the “conduction” of lines of magnetic force by different substances, by which Faraday had so successfully explained the phenomena of diamagnetism, he now applied to the study of the magnetic behaviour of crystals. “If,” he wrote[89] “the idea of conduction be applied to these magnecrystallic bodies, it would seem to satisfy all that requires explanation in their special results.

A magnecrystallic substance would then be one which in the crystallized state could conduct onwards, or permit the exertion of the magnetic force with more facility in one direction than another; and that direction would be the magnecrystallic axis. Hence, when in the magnetic field, the magnecrystallic axis would be urged into a position coincident with the magnetic axis, by a force correspondent to that difference, just as if two different bodies were taken, when the one with the greater conducting power displaces that which is weaker.”

This hypothesis led Faraday to predict the existence of another type of magnecrystallic effect, as yet unobserved. “If such a view were correct,” he wrote,[90] “it would appear to follow that a diamagnetic body like bismuth ought to be less diamagnetic when its magnecrystallic axis is parallel to the magnetic axis than when it is perpendicular to it. In the two positions it should be equivalent to two substances having different conducting powers for magnetism, and therefore if submitted to the differential balance ought to present differential phaenomena.” This expectation was realized when the matter was subjected to the test of experiment.[91]

The series of Faraday’s “Experimental Researches in Electricity” end in the year 1855. The closing period of his life was quietly spent at Hampton Court, in a house placed at his disposal by the kindness of the Queen; and here on August 25th, 1867, he passed away.

Among experimental philosophers Faraday holds by universal consent the foremost place.