Faraday to George Biddell Airy   29 April 1846¹

Royal Institution | 29 April 1846

My dear Sir

I will make the experiment you desire² as soon as I can but it will be 7 or 8 days first. In the mean time I may tell you that early in my experiments I made the plane of polarization (using a reflecting surface glass) revolve about the line passing from the polarizer to the analyser & eye. Thus consider first that the polarized ray passed in an invariable horizontal line in which was the heavy glass or other diamagnetic then the lamp was placed on the right & on the left & above & below & at intervals of 45˚ with these positions the polarizing reflector being turned in a corresponding degree round the horizontal line as axis. So here were 8 planes of polarization but in all the phenomena were precisely the same i.e if the action of the magnetic force was in any one case to cause a rotation of a certain number of degrees to the right hand, it had precisely the same effect in any of the other position[s.] It so happened that partly for convenience with different apparatus & partly on purpose, I did when using magnets make them as it were revolve about this line of the polarized ray, placing the body of the horse shoe or other magnet used sometimes beneath sometimes above sometimes to the right & sometimes to the left but the effect always was the same.

All these cases may be considered as included in the case of a helix & Electric current without a magnet see paragraph 2195³, because here no one plane of polarization can be considered as in any way different from another all are exactly alike.

I do not know whether these facts answer your inquiries if they do not then I have probably not understood your question and may not make exactly the experiment you want - at all events I will make it as soon as I can unless you think it now unnecessary[.]

Ever Dear Sir | Yours Most Truly | M. Faraday

G.B. Airy Esq | &c &c &c