On the Magneto- Optical Generation of Electricity. 495 



across which the light is propagated. Calling 6 the angle 

 which the plane of polarization of the incident light makes 

 with that of the emergent light, and d the thickness of 

 the medium, the twist is numerically 0/d. The greater the 

 intensity of the field in the medium the greater is this twist. 



Now consider the inverse experiment proposed by Dr. 

 Sheldon. The field is non-existent, to begin with ; but a 

 beam of plane-polarized light produced by a Nicol prism is 

 passed along the axis of a helix, the ends of which are joined 

 so that the circuit is closed. The Nicol is then set into rota- 

 tion continuously in one direction. Clearly a twist will be 

 given to the plane of polarization depending on the velocity 

 of rotation. If v be the velocity of light, the time taken by 

 the beam to penetrate the medium is djv ; and if w be the 

 angular velocity of the polarizing prism, the twist given will 

 be the angle turned through in this time divided by the 

 thickness, that is co/v. 



But so long as co is constant this twist is constant, and the 

 surface simply turns as a whole in the medium with angular 

 velocity co. I do not think the latter motion can produce any 

 magnetic effect whatever. A magnetic field may perhaps be, 

 and I should say probably is, produced when the twist is created 

 by the setting of the Nicol into rotation, and annulled when the 

 Nicol is stopped. For a continuous current a continuous flux 

 of magnetic induction through the coil would be necessary, 

 and this could only be produced by increasing the twist of the 

 surface, that is by continually increasing the angular velocity 

 of rotation. What is to be looked for therefore is an induced 

 current when the Nicol is started, and an opposite current 

 when it is stopped ; in the interval there will exist with con- 

 stant rotation a constant field, and the current will be zero. 



If Dr. Sheldon's idea were correct, continuous rotation of 

 the Nicol prism would produce a continually increasing field- 

 intensity within the coil, and thus as intense a field as might 

 be named could be produced by only driving the Nicol long 

 enough. Or, again, an action on the magnetic field which 

 twists the plane of polarization in the direct experiment 

 would be produced by merely rotating the substance round 

 an axis in the direction of the magnetic force. 



With regard to the oscillation experiments, it seems pos- 

 sible that by this method the effect looked for may be obtained, 

 as the twist of the plane of polarization changes sign at each 

 swing of the polarizer. But in the account of his experiments 

 given by Dr. Sheldon, there is nothing to show that the 

 " sizzling " in the telephone which might be produced by the 

 moving machinery was not mistaken for the magneto-optic 



