William Thomson to Faraday   24 July 1849

9 Barton Street, Westminster | July 24, 1849

My dear Sir

In the conversation which we had, about the beginning of this month, I mentioned several objects of experimental research which occurred to me as of much importance with reference to a theory of Diamagnetic, and, still more, of Magnecrystallic, action. I now take the liberty of addressing to you a few memoranda on the subject.

1. If a ball, cut out of a crystal of bismuth, be placed so as to be repelled by a magnet, will the repulsion not be stronger when the magnecrystallic axis is held perpendicular to the lines of force than when it is held in the direction of these lines? (Reference to § 2552 of your Researches¹).

2. It would be a valuable acquisition to our experimental elements if a ball cut from a crystal of bismuth were suspended in the manner described by you in § 2551², and experiments were made by varying the length of the lever, and altering the general dispositions, so as to perceive cases in wh the tendency to move, due to the repulsive action, might be exactly balanced by a tendency to move in the contrary direction arising from the magnecrystallic action. A sketch, with dimensions, of the arrangements in any such case of equilibrium, would be most valuable.

3. In such a case as the preceding, if the strength of the magnet, (a pure electro magnet, without soft iron, would be the most satisfactory kind for such an experiment) be increased or diminished, will the equilibrium remain undisturbed?

4. Is the repulsion on a non-crystalline or crystalline diamagnetic ball or the attraction on a ferro-magnetic ball, exactly proportional to the square of the strength of the magnet? Thus in any case of pure repulsion, or of pure attraction, if the strength of the magnet be doubled, would the force be quadrupled; if the strength of the magnet be increased threefold, would the force be increased ninefold? In this investigation, as in the preceding, a pure electro-magnet would be the best, since in such a magnet the strength may be altered in any ratio, which ratio may be measured with much precision by a torsion galvanometer, while the character and form of the lines remains absolutely invariable.

5. How are crystals of magnetic iron ore related to other crystals in their magnetic properties? Are they intrinsically polar, or are they merely axial? For example, if, supposing that to be possible, a crystal of magnetic iron ore have its polarity reversed, will it remain permanently magnetized in this reverse way? or, if a crystal of magnetic iron be demagnetized, will it remain non-magnetic? Will it not, in virtue of an intrinsic tendency to magnetization, gradually become magnetized in its original way?

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I have a small ball of loadstone from the Island of Elba, which I have employed in place of a needle in a torsion galvanometer, & which appears to be susceptible of inductive action like soft iron (returning apparently to its primitive magnetic state when the inducing magnet is removed); and to be susceptible of this action to a greater degree when its axis is along the lines of inducing force than when it is perpendicular to them. My means of experimenting are however so very limited that I cannot be confident with reference to any such conclusions.

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My intended departure for Norway, of wh I spoke to you, has been necessarily delayed for a fortnight by important & unexpected business. I hope tomorrow however to be on my way to Copenhagen by steamer.

I hope you are at present enjoying a pleasant and refreshing tour, as I heard today at the Royal Institution that you are travelling.

Believe me, my dear Sir, Your’s very truly | William Thomson