Julius Plücker to Faraday   4 December 1849

Dear Sir!

A few days ago I received your long expected paper¹. Belonging to sulfate of iron I found my observations mentioned in the latin Memoir² in complete accordance with yours. Your arsenic is diamagnetic, mine magnetic; nevertheless in both cases the axis is attracted by the poles. Respecting Antimony our observations dont agree. I examined a great number of crystals and allways the optic axis, which is perpendicular to the cleavage plane, was repulsed like in calcareous Spar. A piece of crystallised Antimony I brought from Paris (containing there is no doubt a small quantity of iron), showed very complicated phenomena. There was diamagnetic magnetic and crystallamagnetic action: all these three actions varying differently with distance.

Being returned from Berlin, - where I lost a good time in fruitless deliberations on University reform - took up again my magnetic researches. During the last two years I have been much tantalized, not being able to imagine any force whatever producing the paradox phenomena presented by crystals. Now I may reduce all to the co<bar>mon law of magnetic attraction and repulsion and confirm my theoretical views as well by calcul[ati]ons as by direct experiment. I gave an incomplete notice of it to Mr Poggendorff³ and I am now writing a more elaborate Memoir⁴. I dare not believe you will adopt my views, but nevertheless let me beg to give in a few words an idea of it.

I got by experiment new proofs of diamagnetic polarity, induced by electric currents: Ampère would say there are in diamagnetic bodies induced currents going round the molecules in a direction opposite to that in magnetic bodies. (There is some analogy in this explication with that given by Fresnel⁵ of the phenomena of light passing along the axis t[h]rough rock crystal⁶ and in any direction whatever t[h]rough the well known fluids, there being circular rotations of the ether in opposite direction.) That is also I think the first explication you gave of diamagnetic action, which I adopted myself since I found, that there were no necessity to adopt for the diamagnetic and magnetic force a different law of diminution by distance, and since I thought it without doubt that Bismuth gets polarity by becoming diamagnetic. Join to this hypothesis the new one, that in magnetic crystals the polarity of the molecules be induced in different directions with a different facility, energy or stability; that the same take place belonging to the diamagnetic polarity in diamagnetic crystals - and you may explain all phenomena. The meaning of the new hypothesis will be completely understood, when for instance I say that a cylindrical bar of soft iron gets its polarity more easily along the axis of the bar, and with more stability and power, than in any other direction; quite the contrary takes place, when the bar will be reduced to a plate.

By putting thin iron bars into a piece of wood or copper, you may, by choosing properly their directions, imitate all experiments with magnetic crystals.

Allow me to explain a single case, for instance that of a prism of turmaline [sic]. By considerations taken from the elasticity of the ether, in accordance with its conducting power for Electricity, you may deduce, that the magnetic polarity of such a bar is most easily and strongly excited perpendicularly to its axis. Suppose it be so.

Now take a bar C of any indifferent substance, suspended horizontally between the two poles, to which ends a small iron bar AB is attached. Such a bar will point equatorially with great energy: the iron going away from the poles. In that way you may, in the case of turmaline reproduce the magnecrystallic action by small iron bars, perpendicular to its axis. You may too reproduce the magnetic action of its mass by a small iron wire passing through it in the direction of its greatest dimension i.e. its axis. Thus you will have two groups of forces, acting one oppositely to the other. You may take only two forces, one driving A towards the nearer pole, the other one driving

away from it the bar BC. By giving to the two bars BC and AA’ convenient relative thickness, you will, for a given distance of the poles, have a state of equilibrium; and, the poles being far enough, the equilibrium will subsist in every position of the system of the two bars. Then you may mathematically prove that the system will point either axially or equatorially if you go either farther or nearer with the poles. &c. &c. An iron helix will point equatorially &c &c.

Tis a consequence of my theoretic views, confirmed by experiment, that the attraction of a given crystal by the poles of the Magnet be only dependant of its exterior form, but independant of the direction of its optical or crystallographical axis with regard to the poles of the Magnet.

Some weeks ago, in repeating Boutigny’s⁷ experiments⁸, I put my hand into melted iron at 1500 C, without feeling within any heat whatever. Tis curious.

My best thanks for your paper. At the same time I got Mr. Grove’s⁹. When I write again I will have repeated his interesting experiments as well as Zantedchi’s¹⁰ very strange ones¹¹. Unhappily for myself nearly all my time is absorbed by public lectures at the University.

Very truly | Yours | Plücker

Bonn | 4th of December | 1849.

Address: Professor Faraday | &c &c &c | Royal Institution | London