William Whewell to Faraday   12 February 1855

Trinity Lodge, Cambridge, | Feb. 12 1855

My Dear Dr Faraday

I have received papers containing your speculations on magnetism and especially the paper in the Phil. Mag. on Magnetic Philosophy¹. I have read them with great interest, as I always read your speculations; but they require more time and leisure before I can fully possess myself of your views. At this imperfect stage of thought, and at the risk of proposing difficulties which your former papers have solved, will you allow me to make a remark on your notions as there given.

Your lines of magnetic force whether or not they contain the true theory, are an admirable way of exhibiting the facts; and I have always ascribed the success with which you have unravelled so many very complex phenomena to your starting from these lines. I do not say that they do not contain the true theory, or come nearest to it; for I think we are arriving at a point when the other two theories which you mention² not only cannot explain but cannot express the facts. And if the lines of physical force come to be the only way of expressing the laws of phenomena they must be accepted at least till they are resolved into something else[.] Now what I have to say is this: I do not think that the facts of magnetism alone, even those in your new paper, are the strongest examples or any examples of this peculiar advantage of the physical lines of force. So long as we confine ourselves to magnetism alone (paramagnetism) all the facts can be explained by the existence of two fluids. All your facts of chambers in which the magnetic force vanishes flow easily from that theory: for a self-repelling fluid is necessarily concentrated at edges and points: and the theory of two mutually attracting self-repelling fluids includes, so far as I see, all the facts by which you reduce the universal duality of the forces. But when instead of confining ourselves to one kind of polarity magnetism, we take in the related polarities, electric or voltaic currents, then your lines of force become the only way, so far as I see, of exhibiting the facts. I can make nothing of the other theories in that case, and I have not seen any attempt to apply them coherently. And the same is the case with diamagnetism. I do not see that either of the other theories enables us to explain the most obvious facts. Now what I have to say is this; the advantage of the physical lines of force theory thus residing in its application to diamagnetism and voltaic currents in their relation to magnetism, it would be a great boon to the ordinary thinker if you could explain it more fully in these relations. Your application of it to voltaic currents always makes me wonder at the clearness and readiness with which you conceive the relations of space, but I fear is not intelligible to ordinary readers. It might be made so by figures, diagrams, of the physical lines copiously used, and exhibiting many kinds of examples of the application of your views. And the same is the case with the application of the lines of force in diamagnetic phenomena. We - ordinary readers - would like to see the lines of force drawn in such cases as you have given in page 10 of this last paper³, and in many other cases. It is probable that you have explained this matter is some of your previous papers; but it has I conceive a special bearing upon your present attempts to show the advantages of the lines of physical force. Your theorem (p.33) “pointing in one direction or another is a differential action due to the convergence or divergence of lines of force”⁴ &c is a very curious proposition; and seems to me, or rather is, so far as I see, the only way of exhibiting the facts; but it wants much development to make it intelligible to us; and I want you to give it this development, by diagrams, copious and various, as well as by experiment. Excuse my liberty; I want to have all possible light thrown upon us from your abundant internal light[.]

Yours always truly | W. Whewell