Faraday to George Biddell Airy   12 March 1860

[Royal Institution embossed letterhead] | 12 March 1860.

My dear Sir

Hard steel, which has been strongly magnetized at the first, in a certain direction, tends to keep up a condition in that direction when the bar is demagnetized or reversed; and I think it very possible that the condition you refer to¹, may occur, & perhaps even not infrequently. There are circumstances under which a magnetic bar may be considered as having two sets of particles within it magnetized in opposite directions. I had some bars which were harder along one side than the other, as from A to B. When these were very highly charged by the electro magnet, they soon fell somewhat in power, and I had reason to believe by the assumption on the softer side of a magnetism in the contrary direction to that in the hard part[.]

I believe this case could easily be made to occur. Let N.S. be a hard magnet, well magnetised in the first instance in that direction;- and let N.S. be a like hard steel magnet, equally magnetised in the direction indicated;- & let these be put together as figured: the system will shew little or no power. But if the system be magnetized, with the intention of making the Sn’ end north, the axis will be in the direction n’s’; whereas, if it be changed and magnetised in the opposite direction, the axis will be in the direction N.S. It is not difficult to perceive how the parts of a magnet may have more or less of this relation and how its heterogeneity, either in composition, or hardness, or application of the dominant magnet at the first on one side, or along an oblique line, may produce a certain amount of effect such as you are investigating:

Ever My dear Sir | Truly Yours | M. Faraday

G.B. Airy Esqr | &c &c &c