496 On the Magneto- Optical Generation of Electricity. 



effect. The proper precaution for this was of course the 

 screening off of the light while the machinery was kept going, 

 and the circuit &c. the same as before. If, then, the sound 

 ceased, the inference would be that the disturbance in the 

 telephone was caused by the motion of the beam of light. 



My own experiments were made to test a conclusion I had 

 come to, that the passage of a beam of circularly-polarized 

 light along the axis of a solenoid, included in the circuit of a 

 sensitive galvanometer, would, by instituting a magnetic field, 

 cause an induced current to flow in one direction in the circuit, 

 and the quenching of the beam by annulling the field would 

 cause an induced current to flow in the opposite direction. I 

 therefore produced a beam of circularly-polarized light, and 

 tried the effect of introducing and removing a screen, by 

 which the light was alternately cut off from the coil and 

 allowed to pass through it. As I have stated, on neither of 

 the two occasions on which I made these experiments did I 

 find the introduction or the removal of the screen produce any 

 effect. The failure, I am certain, was due to the utter in- 

 adequacy of my apparatus to detect the effect of the extremely 

 feeble magnetic field produced. It might be possible perhaps 

 to detect the effect looked for by improved arrangements, 

 employing as powerful a coil and as intense a beam of light 

 as possible, alternately cutting off and restoring the light by 

 a rapidly rotating perforated screen, and using in the circuit 

 of the coil a telephone instead of a galvanometer. 



My conclusion that such a field would be produced I was 

 glad to find corroborated by an investigation in Prof. J. J. 

 Thomson's c Applications of Dynamics to Physics and Che- 

 mistry ' (p. 77), where it is shown that a circularly-polarized 

 beam of strong sunlight ought to produce a magnetic field 

 of intensity not greater than 2xl0~ 18 C.G.S. units. Prof. 

 Thomson states that this is much too small to be detected by 

 experiment. It seems to me just possible that the above 

 mode of experimenting might show some result ; at any rate 

 the plan may be worthy of a trial. 



A similar mode of experimenting might be used, I think, 

 to test whether the twisting of the plane of polarization of 

 ordinary plane-polarized light produces any magnetic effect. 

 Supposing the substance (for example, a bar of heavy glass, 

 or a tube containing carbon disulphide) placed along the lines 

 of force between the poles of an electromagnet with perforated 

 pole-pieces for Faraday's magneto-optic experiment, and sur- 

 rounded with a coil in circuit with a sufficiently sensitive 

 galvanometer, any reaction on the field produced by the twist- 

 ing of the plane of polarization would be shown by an induced 



