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XXXY. On the Electromagnetic Rotation of the Plane of Pola- 

 rization of Light by means of Iron, Cobalt, and Nickel. By 



A. KUNDT*. 



DR. KE RR has shown f that the plane of polarization of 

 the light reflected normally from the polished pole of 

 an electromagnet suffers a rotation. The direction of the 



o 



rotation is opposed to that of the magnetizing current, and 

 consequently to that of the amperian molecular currents of 

 the magnet. Kerr has further examined the reflection of 

 light at the polished side-face of a magnet. He found that 

 here also a rotation of the plane of polarization takes place 

 when the electromagnet is excited or has its magnetism 

 reversed. The phenomena are, however, somewhat complicated. 



Plane-polarized light which is reflected at any angle from 

 a metallic surface retains its plane of polarization only in the 

 cases in which that of the incident light is coincident with 

 the plane of incidence, or at right angles to it. In all other 

 positions of the plane of polarization the reflected light is 

 elliptically polarized ; Kerr therefore confined his investiga- 

 tions to the first two cases. He finds that if the polarizing 

 and analyzing nicols are at right angles, then when the 

 magnet is excited the field of view becomes bright, which he 

 interprets as a rotation of the plane of polarization produced 

 by the magnetization. The direction and the amount of 

 this rotation are not dependent simply upon the angle of 

 incidence, but are different also according as the plane of 

 polarization of the incident light (the long diagonal of the 

 section of the nicol) is coincident with, or at right angles to, 

 the plane of incidence. 



If the light is polarized in the plane of incidence, the direc- 

 tion of rotation is the same for ail angles of incidence, and 

 opposed to that of the amperian molecular currents of the 

 reflecting magnet. 



If the plane of polarization is at right angles to the plane 

 of incidence, then for angles of incidence from 0° to about 80° 

 the direction of rotation is the same as that of tne amperian 

 currents, but opposed to it for incidences between 80° and 90°. 

 The amount of rotation is in both cases a maximum for "an 

 angle of incidence of about 65°. 



There is also a rotation with oblique incidence of the light 

 upon the polished pole-surface of an electromagnet. The 

 direction of the rotation according to Kerr, so far as I can 

 gather from his data, is the same for all angles of incidence 



* Translated from the Berlin Sitzunqsberichte, July 10, 1884. 

 f Phil. Mag. [5] 1877, vol. iii. p. 321, and 1878, vol. v. p. 161. 



