KERR'S EXPERIMENTS.

385

452. Experiment I. Light polarized in or perpendicularly to the plane of incidence is allowed to fall on the pole of the electromagnet, and the analyser is placed in the position of extinction. When the circuit is closed, so that the reflector becomes magnetized, the light immediately reappears; when the circuit is broken, the light disappears, and again reappears when the current is reversed.

The light reflected whilst the circuit is closed is elliptically polarized, since it cannot be extinguished by rotating the analyser.

Experiment II. The arrangements are the same as in the last experiment, and the analyser is turned from the position of extinction, through a small angle towards the right hand of an observer, who is looking through it at the point of incidence, giving a faint restoration of light. When the circuit is closed, so that the reflector becomes a negative pole, the intensity is increased; but when the current is reversed, so that the reflector becomes a positive pole, the intensity is diminished. The weakening effect of the second operation is always less than the strengthening effect of the first, and its effect diminishes as the angle through which the analyser is turned is diminished.

In these two experiments the angle of incidence lay between 60° and 80°; and Kerr does not appear to have observed the effect produced, when the angle of incidence lay between 80° and 90°. See § 463.

453. We must now describe the arrangements, when the incidence is perpendicular.

Instead of employing a wedge for the submagnet, Kerr substituted a block of soft iron BB, rounded at one end into the frustum of a cone. A small boring was drilled through the block, narrowing towards the conical ends, and the block was placed next the magnet M. The surface of the boring was well coated with lampblack. Above the block, a thin sheet of glass C was placed at an angle of 45° to the horizon, which received the horizontal beam from the first Nicol N, and reflected it downwards through the boring, perpendicularly to the surface of the reflector. The reflected beam then proceeded back again through the thin sheet of glass and the second Nicol N', which served as the analyser.

454. Experiment III. The polarizer and analyser are first placed in the position of extinction, and the analyser is then turned through a small angle, towards the right hand of a person who is looking through it at the point of incidence, giving a faint restoration of light. The circuit is now closed; and it is found, that when the reflector is negatively magnetized, the intensity is increased; and that when it is positively magnetized, the intensity is diminished.

When the analyser is turned towards the left, the results are the same, provided the operations of positive and negative magnetization are reversed.

455. As the result of his experiments, Kerr deduced the following general law, viz.;—

When plane polarized light is reflected from the pole of an electromagnet, the plane of polarization of the reflected light is turned through a sensible angle, in a direction contrary to that of the amperean current, which would produce the magnetic force; so that a positive pole of polished iron acting as a reflector, turns the plane of polarization towards the right hand of an observer, looking at the point of incidence along the reflected ray.

456. In the preceding experiments, the reflector was supposed to be magnetized perpendicularly to its surface. We shall now describe the second series of experiments made by Kerr, for the purpose of investigating the effect of a reflector, which is magnetized parallel to its surface'.

1 Phil. Mag. March, 1878.

KERR'S EXPERIMENTS.

387

457. The electromagnet stands upright upon a table, and a rectangular prism of soft iron, one of whose faces is carefully planed and polished, lies upon the poles of the magnet, with its polished face vertical. The two Nicols N, N', and the lamp L, stand upon the same table as the magnet, and at the same height as the mirror.

The arrangement is shown in the figure; AB is the reflector, E is the eye of the observer, and the dotted lines represent the poles of the magnet. P is a metallic screen, containing a slit th of an inch wide, placed between the first Nicol and the lamp.

In the above arrangement, the magnetic force is very nearly parallel to the reflector, and may be conceived to be produced by currents circulating spirally round the prism AB from one pole to the other. Such a current will be considered right-handed, when its direction is towards the right hand of an observer viewing it from F; and a rotation of the analyser N', which is in the direction of the hands of a watch, when viewed from E, will be considered right-handed.

458. In the following two experiments the plane of incidence is parallel to the direction of magnetization.

Experiment IV. The incident light is polarized in the plane of incidence, and the analyser is initially placed in the position of extinction, and is then turned through a small angle. The circuit is now closed; and it is found, that the light restored from extinction by a small right-handed rotation of the analyser, is always strengthened by a right-handed magnetizing current, and always weakened by a left-handed current. Conversely the light restored by a small left-handed rotation, is always weakened by a righthanded current, and strengthened by a left-handed one.

The intensity of these optical effects of magnetization varies with the angle of incidence. At an incidence of 85° the effects

are very faint; at 75° they are stronger; at incidences from 65o to 60°, they are clear and strong; at 45° they are fairly strong, though fainter than at 60°; at 30° they are again very faint, and much the same as at 85°.

Experiment V. The incident light is polarized perpendicularly to the plane of incidence, and the arrangements are the same as in the last experiment. At an incidence of 85°, the light restored by a right-handed rotation of the analyser is strengthened by a right-handed current, and weakened by a left-handed one; and the effects are undistinguishable from those of the fourth experiment, except that they are considerably weaker. At 80°, the effects are of the same kind, but a good deal fainter; and at 75° they disappear. At 70°, they reappear faintly, but the phenomena are now of a contrary character; for the light restored by a righthanded rotation, is now weakened by a right-handed current, and strengthened by a left-handed one. At incidences of 65°, 60°, 45°o, 30o, the effects are of the same kind as at 70°; and at 60° they are comparatively clear and strong, though sensibly fainter than those obtained in the last experiment at the same incidence. At 30° they are faint, but stronger than the contrary effects obtained at 85°.

459. The results of the last two experiments may be summed up as follows:

(i) When the incident light is polarized in the plane of incidence, the plane of polarization of the reflected light is always rotated in the opposite direction to that of the amperean current, which would produce the magnetic force.

(ii) When the incident light is polarized perpendicularly to the plane of incidence, the rotation of the plane of polarization of the reflected light is in the opposite direction to that of the current, so long as the angle of incidence lies between 90° and 75o; and in the same direction, when it lies between 75° and 0o.

460. Experiment VI. In this experiment, the plane of incidence was perpendicular to the direction of magnetization; and it was found that no optical effect was produced by magnetization.

Experiment VII. In this experiment, the incidence was normal, and the inclination of the plane of polarization to the direction of magnetization was varied from 0° to 90°; and it was found, that no optical effect was produced by magnetization.

KUNDT'S EXPERIMENTS.

389

461. Dr E. H. Hall' of Baltimore has examined the effects produced, when the electromagnet is composed of nickel and of cobalt ; and he found that in both metals, the sign of Kerr's effect was the same as in iron.

462. The experiments of Kerr would lead us to anticipate, that when light is reflected from a conductor, which is strongly charged with electricity, the reflected light would experience certain modifications; but no experiments of this character appear to have been performed.

Kundt's Experiments.

463. The experiments of Kerr were repeated by Kundt, and were completely confirmed by the latter with one exception, viz. that when light polarized perpendicularly to the plane of incidence is reflected at the pole of an iron electromagnet, the direction of rotation is reversed at an incidence of about 82°; that is to say, the rotation is in the contrary direction to that of the current so long as the angle of incidence lies between 0° and 82o, and in the same direction when it lies between 82° and 90o.

464. When light was reflected from the pole of a nickel electromagnet, it was found that the rotation was more feeble than that produced by iron. When the light was polarized in the plane of incidence, the rotation was always negative (that is in the contrary direction to that of the current); but when the light was polarized perpendicularly to the plane of incidence, the rotation was negative from 0° to 50°, and changed sign between 50° and 60°.

465. Kundt also made experiments upon the rotation produced, when light is transmitted through films of iron, cobalt and nickel, which were so thin as to be semi-transparent; and he found, that all these metals, when maguetized perpendicularly to the surface of the film, produced a powerful rotation of the plane of polarization of the transmitted light; and that the rotation takes place in the direction of the magnetizing current. The rotation produced by iron upon the mean rays of the spectrum, is

Phil. Mag. Sep. 1881, p. 171.

2 Berlin Sitzungsberichte, July 10th, 1884; translated Phil. Mag. Oct. 1884, p. 308.