William Whewell to Faraday   9 September 1835

Trin. Coll. Cambridge, Sept. 9, 1835

My dear Sir

I have been reading your ninth series of "Researches"¹ with the strongest interest, as I read all of them, and especially those which relate to your "Induc‑tion." I have a wish to ask a question or two which I dare say you will be good natured enough to accept as an excuse for my troubling you. In Art. 1077², you say that the supposition of momentum or inertia is at once set aside by your expts. (1069) and (1089)³[.] Now will you allow that there is any force in the following remarks on this? I do not think that it follows that the effects on breaking contact are not produced by inertia, because they are modified by the effect of the wire on itself. Such an effect may alter the distribution of the fluid, so as to affect even its mechanical properties. The mechanical effect of the impulse of the same quantity of water in a water-course will be different when it flows smoothly and when it runs in waves. Then as to (1089) it appears to me to comprise the notion of momentum. When the contact is broken the momen‑tum of the long wire current overpowers that of the shorter one and turns it back. Does not this represent the fact?

I have been strongly impressed with the notion of the inertia of the electro‑dynamic current, ever since I read your first series of researches⁴. I state it thus. "A substance when put in motion by another substance, produces, at the first instant, an impulse opposite to that of the motion; if the velocity be uniform, no further effect is perceived till the motion is stopped; and at that instant, an impulse is produced in the direction of the motion." Now this description is a full and exact account of the laws, alike, of electrical currents and mechanical collision. How then can I help identifying the two cases?

I will also take the liberty of making a remark on phraseology, which you know I am used to do. In your first researches you spoke of volta-induction ⁵, a term which you have since not used. You then employed the term ‑magneto- electric induction ⁶, which you still, I think, retain. It appears to me that as Ampere did well to introduce the word electrodynamic action, to include electro-mag‑netic, voltaic, and magnetic, after he had shown that the three are identical, so you ought to use a term of like extent for the like reason and call your induction electrodynamic induction.

But I have something more to say of the same kind. The facts which occur at making or breaking contact are not induction, if induction means, as by analogy it should mean, the condition produced by the neighbourhood of an electric circuit. These facts mark the beginning and end of induction. Whether there is a permanent condition produced by a neighbouring circuit or not, this class of facts has no right to the name. But allowing the analogy of mechanical action to obtain in the action of electrodynamic wires (I have shown above how strong the analogy is) we see clearly what these facts of instantaneous action are. They are the reaction of the current in which motion is produced by induction. You have yourself so called them (1114)[.] You say "a current sustains a state which when allowed to react, at the cessation of the original current, produces a second current."⁷ I think therefore that in consistence with your own views, all the facts produced by the beginning and end of electrodynamical proximity should be ascribed to electrodynamical reaction.

I have always admired how you contrived to get <(114)>⁸ so soon at the very difficult law which governs the direction of an electrodynamical current pro‑duced by motion. But I have also wondered, that accepting Ampere's theory, as I think you do, you have never expressed that law and some other of your results, in terms of his theory: but have, on the contrary, determined everything by reference to magnetical directions and curves which according to him are derivative elements. I mention this because I have found that it gave me some trouble to refer your results to his theory.

According to his theory the convertibility of electricity and magnetism, of which you speak (1114)⁹ as a problematical point, is a fundamental fact. And it seems to me that taking his theory, and supposing the electric fluid to have inertia, the only point in this part of the subject, which remains obscure, is the law of electrodynamic induction properly so called, that is the force by which a current produces a current in a neighbouring substance, whether a current be a vibration, a strain, or any thing else¹⁰.

I shall be glad if you have time to give me any light as to your views on these points. I consider the train of discovery in which you are engaged as so great a thing for us and for posterity that I cannot help wishing to understand the progress of it as clearly as possible. Believe me, my dear Sir,

Yours very truly | W. Whewell

Address: Dr Faraday | Royal Institution | Albemarle Street | London