Rotation of the Plane of Polarization of Light. 339 



problem (48) the displacement-current has gone, so that the 

 existence of H appears to rest merely upon the assumption 

 that moving electrification is true current. But if the plane 

 be not infinite, though large, we shall have (48) nearly true 

 near it, and away from the edges ; whilst the displacement- 

 current will be strong near the edges and almost nil where 

 (48) is nearly true. 



But in some cases of rotating electrification, there need be 

 no displacement anywhere, except during the setting up of 

 the final state. This brings us to the rather curious question 

 whether there is any difference between the magnetic field of 

 a convection-current produced by the rotation of electrification 

 upon a good nonconductor and upon a good conductor re- 

 spectively, other than that due to diffusion in the conductor. 

 For in the case of a perfect conductor, it is easy to imagine 

 that the electrification could be at rest, and the moved con- 

 ductor merely slip past it. Perhaps Professor Rowland''s 

 forthcoming experiments on convection-currents may cast 

 some light upon this matter. 



December 27, 1888. 



XL. The Rotation of the Plane of Polarization of Light hy 

 the Discharge of a Ley den Jar. By Dr. Oliver Lodge*. 



THE current produced by the discharge of a Lej^den jar is 

 so violent while it lasts, that those phenomena which 

 depend upon the value of a current independently of its 

 duration are well excited by it. Such are the induction of 

 currents, the production of magnetism, and the rotation of 

 the plane of polarization. 



Nothing is easier than to wind a quantity of thin gutta- 

 percha-covered wire round a piece of heavy glass, and to 

 witness the bright flashing of a dark field between polarizer 

 and analyser whenver a large Leyden jar is sparked 

 through the coil, the source of light being a paraffin-lamp or 

 gas-flame. The suddenness of the effect suggests^ of course 

 erroneously^ that it is an illumination caused by the light of 

 the spark which one is looking at. 



The fact that the discharge is oscillatory, and that the 

 restoration of light in the dark field is osciflatory too, is 

 proved by the fact that an adjustment of the analyser to one 

 side or the other of complete darkness has just the same effect 

 on the result. It is proved also by the fact that a biquartz 



* Communicated by the Physical Society : read March 9, 1889. 



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