Faraday to Carlo Matteucci   2 November 1855

November 2, 1855.

My dear Matteucci, - When I received your last, of October 23¹, I knew that Tyndall would return from the country in a day or two, and so waited until he came. I had before told him of your desire to have a copy of his paper², and I think he said he would send it to you; I have always concluded he did so, and therefore thought it best to continue the same open practice and show him your last letter, note and all. As I expected, he expressed himself greatly obliged by your consideration³, and I have no doubt will think on, and repeat, your form of experiment; but he wished you to have no difficulty on his account. I conclude he is quite assured in his own mind, but does not for a moment object to counter views, or to their publication: and I think feels a little annoyed that you should imagine for a moment that he would object to or be embarrassed by your publication⁴. I think in that respect he is of my mind, that we are all liable to error, but that we love the truth, and speak only what at the time we think to be truth; and ought not to take offence when proved to be in error, since the error is not intentional; but be a little humbled, and so turn the correction of the error to good account. I cannot help thinking that there are many apparent differences amongst us, which are not differences in reality. I differ from Tyndall a good deal in phrases, but when I talk with him I do not find that we differ in facts. That phrase polarity in its present undefined state is a great mystifier (3307, 3308)⁵. Well! I am content, and I suppose he is, to place our respective views before the world, and there leave them. Although often contradicted, I do not think it worth while reiterating the expressions once set forth; or altering them, until I either see myself in the wrong or misrepresented; and even in the latter case, I let many a misrepresentation pass. Time will do justice in all these cases.

One of your letters asks me, “What do you conceive the nature of the lines of magnetic force to be?” I think it wise not to answer that question by an assumption, and therefore have no further account to give of such physical lines than that is already given in my various papers. See that referred to already in the “Philosophical Magazine” (3301-3305)⁶; and I would ask you to read also 3299, the last paragraph in a paper in the “Philosophical Magazine,”⁷ June 1852, which expresses truly my present state of mind.

But a physical line of force may be dealt with experimentally, without our knowing its intimate physical nature. A ray of light is a physical line of force; it can be proved to be such by experiments made whilst it was thought to be an emission, and also by other experiments made since it has been thought to be an undulation. Its physical character is not proved either by the one view or the other (one of which must be, and both may be wrong), but it is proved by the time it takes in propagation, and by its curvatures, inflexions, and physical affections. So with other physical lines of force, as the electric current; we know no more of the physical nature of the electric lines of force than we do of the magnetic lines of force; we fancy, and we form hypotheses, but unless these hypotheses are considered equally likely to be false as true, we had better not form them; and therefore I go with Newton when he speaks of the physical lines of gravitating force (3305 note), and leave that part of the subject for the consideration of my readers⁸.

The use of lines of magnetic force (without the physical) as true representations of nature, is to me delightful, and as yet never failing; and so long as I can read your facts and those of Tyndall, Weber⁹, and others by them, and find they all come into one harmonious whole, without any contradiction, I am content to let the erroneous expressions, by which they seem to differ, pass unnoticed. It is only when a fact appears that they cannot represent that I feel urged to examination, though that has not yet happened. All Tyndall’s results are to me simple consequences of the tendency of paramagnetic bodies to go from weaker to stronger places of action, and of diamagnetic bodies to go from stronger to weaker places of action, combined with the true polarity or direction of the lines of force in the places of action. And this reminds me of a case you put in one of your letters, which to me presents no difficulty:- “a piece of bismuth on which the pole p

acts suffers an action on the part of the pole p’, which is the same as if the pole p’ did not act or was a pole of the contrary name.” p, being an S pole, repels b, and sends it from a stronger into a weaker part of the field, i.e. from

then p’ being brought up, and being also an S pole, B is no longer the weaker place of action but b; and hence the bismuth goes back. And that it is the weaker place of action can be shown by a minute magnetic needle or a crystal of bismuth, and in many other ways (3341, &c., especially 3350)¹⁰. But suppose p’ is selected, an N pole, then the lines of force between p’ and p are greatly strengthened in power, and the small needle, or crystal bismuth, shows it to be so; but still B is no longer a weaker place of power than b, and if the bismuth can only move along the line pp’ it must move from B to b, for b is the weakest place of action in that line; but this is a place of unstable equilibrium, and, as you know very well, if it can move in the line mn, it will move either towards m or towards n, as it happens to be on one or the other side of the axial line of the magnetic field.

These principles, or rather laws, explain to me all those movements obtained by Tyndall against which your note is directed, and therefore I do not see in his experiments any proofs of a defined or inverse polarity in bismuth, beyond what we had before. He has worked out well the antithetical relations of paramagnetic and diamagnetic bodies; and distinguished mixed actions, which by some have been much confused; but the true nature of polarity, and whether it is the same, or reversed in the two classes, is to my mind not touched. What a quantity I have written to you, all of which has no doubt been in your own mind, and tried by your judgment. Forgive me for intruding it. Ever truly yours,

M. Faraday

I am sorry to refer you to the “Philosophical Magazine.” I have a third volume of my “Experimental Researches”¹¹ on my desk waiting for you; it contains them all. I have not yet found a means of sending it.