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CHAPTER XX.

ACTION OF MAGNETISM ON LIGHT.

438. THE electromaguetic theory of light. so far as it has been developed in the preceding chapter, depends upon the hypothesis that a medium exists, whose special function is to propagate electrostatic and electromagnetic effects; and that when electromagnetic waves, whose periods lie between certain limits, are transmitted through the medium, the sensation of light is produced. If therefore light is the effect of an electromagnetic disturbance, the natural inference is, that an intimate connection exists between electricity and light; and that when a wave of light passes through an electromagnetic field, it ought to experience certain modifications during its passage, and to emerge from the field in a different condition from that in which it entered.

439. The conviction that a direct relation exists between electricity and light, led Faraday to attempt many experiments for the purpose of discovering some mutual action between the two classes of phenomena; but it was not until 1845, that he made the important discovery, that a field of magnetic force possesses the power of rotating the plane of polarization of light. During recent years, much attention has been devoted to this subject, and numerous experiments have been made by Kerr, Kundt and others, which have greatly extended our knowledge. We shall therefore commence by giving an account of the principal experimental results, and then proceed to enquire, how far they may be explained by theoretical considerations.

FARADAY'S DISCOVERY.

381

Faraday's Experiments.

440. The first experiment described by Faraday1, consisted in placing a plate composed of a variety of heavy glass, called silicated borate of lead, between the poles of an electromagnetic; and he found that when a ray of plane polarized light was transmitted through the glass in the direction of the lines of magnetic force, the plane of polarization was rotated in the same direction as that of the amperean current which would produce the force.

441. Further experiments upon a variety of other transparent media led to the following law:-In diamagnetic substances the direction of rotation of the plane of polarization is positive; that is to say, it is in the same direction as a positive current must circulate round the ray, in order to produce a magnetic force in the same direction, as that which actually exists in the medium.

The amount of rotation depends upon the nature of the medium and the strength of the magnetic force. No rotation has been observed, when the magnetic force is perpendicular to the direction of the ray.

442. Verdet however discovered, that certain ferromagnetic media, such as a strong solution of perchloride of iron in wood spirit or ether, produced a rotation in the opposite direction to that of the current, which would give rise to the magnetic force.

Kerr's Experiments.

443. Between the years 1875 and 1880, two very important series of experiments were made by Dr Kerr of Glasgow, upon the connection between light and electricity. The first series relate to the effect of electrostatic force, and the second to magnetic force.

Experiments on the Effect of Electrostatic Force.

444. In these experiments, a transparent dielectric was subjected to the action of electrostatic force, and the effect of the latter upon light was observed.

1 Phil. Trans. 1845, p. 1; Exp. Res. XIXth series §§ 2146-2242.

2 Ante, p. 159.

3 Phil. Mag. Nov. 1875, p. 339.

We shall first consider the case in which the dielectric is a plate of glass placed in a vertical plane, and shall suppose that the electrostatic force is horizontal.

Polarized light was transmitted at normal incidence through the plate of glass, and the analyser was placed in the position of extinction; and it was found, that when an electrostatic force was made to act upon the dielectric, the light reappeared, and disappeared after the force was removed. The effect was most marked, when the plane of polarization of the incident light was inclined at an angle of about 45° to the force; but when the incident light was polarized in or perpendicularly to the direction of the force, no effect was observed.

The light restored by electrostatic action was elliptically polarized, and could not therefore be extinguished by any rotation of the analyser.

It was also found that the optical effect was independent of the direction of the force; that is to say, its intensity remained unchanged when the direction of the force was reversed.

The optical effect did not acquire its maximum intensity at the instant the force commenced to act, but gradually increased during a period of about thirty seconds, at the end of which it attained its full effect. Also when the force was removed, the effect did not immediately disappear, but faded away at first rapidly, and then more gradually to perfect extinction.

445. It is known that compressed glass acts like a negative. uniaxal crystal, whose axis is parallel to the direction of compression; whilst stretched glass acts like a positive uniaxal crystal, whose axis is parallel to the direction of extension. Accordingly Kerr introduced a slip of glass, called a compensator, and found that when the slip was compressed in a direction parallel to the lines of electrostatic force, the optical effect produced by the latter was strengthened, but when the slip was stretched in that direction, the effect was weakened.

From these experiments Kerr concluded, that the effect of electrostatic stress on glass is to transform it into a medium, which possesses the optical properties of a negative uniaxal crystal, whose axis is parallel to the direction of the force. Under these circumstances, it ought to follow, that glass, when under the action of electrostatic force, should be capable of producing the

KERR'S EXPERIMENTS.

383

rings and brushes of uniaxal crystals, but no experiments elucidating this point appear to have been performed.

446. From experiments made on resin, it appeared that the effect of electrostatic force upon this substance was to convert it into a medium, which is optically equivalent to a positive uniaxal crystal.

447. A few experiments were made on a plate of quartz, whose axis was perpendicular to the direction of the force; and these experiments indicated, that the optical effects were of a similar kind to those produced upon glass. It is to be hoped, that more elaborate experiments upon quartz will be attempted; for if the optical effects are of a similar kind to those produced upon glass, it would follow that the effect of electrostatic force would be to convert a plate of quartz, whose axis is perpendicular to the force, into a biaxal crystal, which is capable of producing rotatory polarization. Since the principal wave velocity in the direction of the force, is capable of being varied at pleasure by increasing or diminishing the force, we should anticipate that some very curious phenomena would be observed in connection with coloured rings, and also possibly in connection with conical refraction.

448. Experiments made on liquids' showed, that disulphide of carbon, benzol, paraffin and kerosin oils, and spirits of turpentine act, when subjected to electrostatic force, like a positive uniaxal crystal, whose axis is parallel to the direction of the force; whilst olive oil acts like a negative uniaxal crystal. Turpentine, as is known, produces rotatory polarization, some specimens being right-handed and others left-handed; and therefore in experimenting upon this liquid, two samples of contrary photogyric power were mixed together, in such proportions as to destroy the rotatory properties of the mixture.

449. Further experiments led to the following law:-The effect of electrostatic force upon an isotropic transparent dielectric, is to render it optically equivalent to a uniaxal crystal, whose axis is parallel to the direction of the force; and the difference between the retardations of the ordinary and extraordinary rays, is proportional to the product of the thickness of the dielectric, and the square of the resultant electric force.

1 Phil. Mag. December, 1875, p. 446.

2 Ibid. March, 1880, p. 157.

Kerr's Experiments on Reflection from a Magnet.

450. Shortly after the experiments described in the preceding sections had been made, Kerr commenced a series of experiments upon light reflected from an electromagnet. In the first series of experiments, the light was reflected from the pole of the magnet; whilst in the second series, a bar of soft iron was laid across the poles of an electromagnet, so that the lines of magnetic force were parallel to the reflecting surface.

451. We shall now describe the first series of experiments, and the apparatus employed1.

B

M

N'

E

L

An electromagnet M, consisting of a solid core of soft iron surrounded by a wire making 400 turns, was worked by a Grove's battery of six cells; and the poles of the magnet were carefully polished, so as to form a good reflecting surface. The source of light was a narrow paraffin flame L, which was polarized by a Nicol N, and the reflected light was analysed by a second Nicol N'. A wedged-shape piece of soft iron B with a well-rounded edge, called a submagnet, was placed in close proximity to the reflecting surface, with its rounded edge perpendicular to the plane of incidence, so as to leave a space of about th of an inch between the two. The object of the submagnet was to intensify the magnetic force in the neighbourhood of the mirror, when the circuit was closed; and Kerr found that without it, he never obtained any optical effect. The preceding arrangement was employed, when the angle of incidence lay between 60° and 80°; but when the incidence was perpendicular, a different arrangement was adopted, which will be explained later on.

1 Phil. Mag. May, 1887.

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