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LVI. On the Magneto- Optical Generation of Electricity. 

 By Professor A. G-kay, M.A.* 



IN the October number of the Philosophical Magazine is 

 an article describing some experiments made by Dr. 

 Samuel Sheldon on the magnetic effect of rotation of the 

 plane of polarization of a beam of light transmitted through a 

 helix of wire. Several years ago I made some experiments 

 on this subject myself without obtaining any result; and 

 since coming to the University College of North Wales, I 

 have resumed the investigation without obtaining any decisive 

 indication of the effect of which I was in search. My method 

 of proceeding was, however, quite different from that of Dr. 

 Sheldon, and failed, I feel sure, simply through the inadequacy 

 of my arrangements for the detection of what can never be 

 more than an exceedingly small magnetic force. But before 

 describing my own experiments, I wish to refer to one or two 

 of the statements which Dr. Sheldon makes. After describing 

 the direct experiment of the rotation of the plane of polariza- 

 tion of a beam of plane-polarized light by the creation of a 

 magnetic field, he says : — 



" Now if a difference of potential, under these conditions, 

 produces such a rotation of the plane, why should not a rapid 

 rotation of the plane under exactly the same conditions pro- 

 duce an inverse difference of potential between the terminals 

 of the coil? A continuous rotation should produce a con- 

 tinuous current of electricity, and an oscillating of the plane 

 an alternating current. The experiments which have been 

 performed verify the latter supposition." 



Dr. Sheldon then proposes to produce the "continuous 

 rotation " of the plane of polarization by rotating the polari- 

 zing prism, but abandons the idea on account of the difficulty 

 of producing and maintaining a sufficiently high angular 

 velocity to give any observable effect, and substitutes oscilla- 

 tion of the Nicol for continuous rotation. It seems to me that 

 the notion that "a continuous rotation should produce a 

 continuous current of electricity " involves a fallacy which it 

 may be desirable to point out. 



When a ray of plane-polarized light is subjected to the 

 action of a magnetic field the lines of force of which are 

 parallel to the direction of propagation of the beam, the plane 

 of polarization changes in the medium to a twisted surface. 

 If the medium be quite uniform, the surface has a uniform 

 twist from one face to the other of the stratum of substance 



* Communicated by the Author. 



