FARADAY’S discovery that, in combinations like those in which a voltaic current was known to produce motion, motion would produce a voltaic current, naturally excited great attention among the scientific men of Europe. The general nature of his discovery was communicated by letter[26] to M. Hachette at Paris, in December, 1831; and experiments having the like results were forthwith made by MM. Becquerel and Ampère at Paris, and MM. Nobili and Antinori at Florence.
[26] Ann. de Chimie, vol. xlviii. (1831), p. 402.
It was natural also that in a case in which the relations of space which determine the results are so complicated, different philosophers should look at them in different ways. There had been, from the first discovery by Oersted of the effect of a voltaic current upon a magnet, two rival methods of regarding the facts. Electric and magnetic lines exert an effort to place themselves transverse to each other (see [chapter iv.] of this Book), and (as I have already said) two ways offered themselves of simplifying this general truth:—to suppose an electric current made up of transverse magnetic lines; or to suppose magnetic lines made up of transverse electric currents. On either of these assumptions, the result was expressed by saying that like currents or lines (electric or magnetic) tend to place themselves parallel; which is a law more generally intelligible than the law of transverse position. Faraday had adopted the former view; had taken the lines of magnetic force for the fundamental lines of his system, and defined the direction of the magneto-electric current of induction by the relation [619] of the motion to these lines. Ampère, on the other hand, supposed the magnet to be made up of transverse electric currents ([chap. vi.]); and had deduced all the facts of electro-dynamical action, with great felicity, from this conception. The question naturally arose, in what manner, on this view, were the new facts of magneto-electric induction by motion to be explained, or even expressed?
Various philosophers attempted to answer this question. Perhaps the form in which the answer has obtained most general acceptance is that in which it was put by Lenz, who discoursed on the subject to the Academy of St. Petersburg in 1833.[27] His general rule is to this effect: when a wire moves in the neighborhood of an electric current or a magnet, a current takes place in it, such as, existing independently, would have produced a motion opposite to the actual motion. Thus two parallel forward currents move towards each other:—hence if a current move towards a parallel wire, it produces in it a backward current. A moveable wire conducting a current downwards will move round the north pole of a magnet in the direction N., W., S., E.:—hence if, when the wire have in it no current, we move it in the direction N., W., S., E., we produce in the wire an upward current. And thus, as M. de la Rive remarks,[28] in cases in which the mutual action of two currents produces a limited motion, as attraction or repulsion, or a deviation right or left, the corresponding magneto-electric induction produces an instantaneous current only; but when the electrodynamic action produces a continued motion, the corresponding motion produces, by induction, a continued current.
[27] Acad. Petrop. Nov. 29, 1833. Pogg. Ann. vol. xxxi. p. 483.
[28] Traité de l’Electricité, vol. i. p. 441 (1854).
Looking at this mode of stating the law, it is impossible not to regard this effect as a sort of reaction; and accordingly, this view was at once taken of it. Professor Ritchie said, in 1833, “The law is founded on the universal principle that action and reaction are equal.” Thus, if voltaic electricity induce magnetism under certain arrangements, magnetism will, by similar arrangements, react on a conductor and induce voltaic electricity.[29]
[29] On the Reduction of Mr. Faraday’s discoveries in Magneto-electric Induction to a General Law. Trans. of R. S. in Phil. Mag. N.S. vol. iii. 37, and vol. iv. p. 11. In the second edition of this history I used the like expressions.
There are still other ways of looking at this matter. I have elsewhere pointed out that where polar properties co-exist, they are [620] generally found to be connected,[30] and have illustrated this law in the case of electrical, magnetical, and chemical polarities. If we regard motion backwards and forwards, to the right and the left, and the like, as polar relations, we see that magneto-electric induction gives us a new manifestation of connected polarities.
[30] Phil. Ind. Sc. B. v. c. ii.