William Thomson to Faraday   27 June 1848

Borley Rectory, Sudbury, | Suffolk, June 27, 1848.

My dear Sir,

Since I had the conversation with you last week, I have been reconsidering the subject with some care and I am quite satisfied that the theoretical views which I then mentioned are correct. I should not however expect that it would be at all easy, or perhaps possible, to verify experimentally the result that a needle of a diamagnetic substance tends to arrange itself in the direction of the lines of force, when in a situation where there is no sensible variation of magnetic intensity through the space in which it is free to move; since this tendency arises from the mutual influence of the different portions of the diamagnetic substance itself, & is consequently excessively slight. An experiment showing that a diamagnetic needle will be sensibly astatic when the intensity in its neighbourhood is nearly constant, would be extremely interesting as so far confirming the conclusions deduced from theory; but I fear that a complete verification would be unattainable, on account of the excessive feebleness of the force of which the existence is to be tested.

The same process of mathematical reasoning enables us to infer that a needle of soft iron, and a needle of a diamagnetic substance would both rest stably, in the direction of the lines of force, or unstably in a perpendicular direction; but in the former case the directive tendency is extremely sensible, so much so that it may be easily verified by observing the position which a delicately suspended needle of soft iron will assume when acted upon only by the earth.

I have not yet had an opportunity of referring to Poggendorf[f] to find the account of the remarkable researches which you mentioned to me¹, but as soon as I return to Cambridge I shall read it with great interest.

My address till about the 20th of July will be St. Peter's College, Cambridge. Letters so addressed at any time to me would be forwarded to me.

I remain, My dear Sir, | Very truly your's | William Thomson