STS3301 Paper Week 4
Response to Week 4 Readings, Masters of Theory ST&S 3301
On lines of force:
I noticed, in On Some New Electro-Magnetical Motions and on the Theory of Magnetism, that Faraday considered the experience that magnetism around a straight wire sparked his curiosity, or seemed to him a proof that something invisible turns as it flows through the wire. I later read, a first for me, Patricia Fara's account of Cartesian magnetism, how it survived on both sides of the Channel even as Cartesians and Newtonians were at loggerheads over other forces. [Ford, P. Fatal Attraction, ch. 8. Icon Books, Cambridge, 2005] Fara cites a B. Langrish quote from 1773, essentially that the iron filings are clearly the tracks of magnetic effluvia. She writes that a late 18th century Englishman, Robison, sought to purge Cartesian Magnetism by considering all forces "Newtonian," but Faraday can be considered a skipping of generations, having followed more the thinking of Gowin Knight. Evidence includes Fara's mention of Faraday "taking over Knight's [magnet-making] machine." [p. 196]
Faraday's electromotive apparatus, the wire stirring in mercury seen in Plate XVIII of the reading also evokes a sense of a stirring force. One could counter the idea of magnetism being invisible when one is staring at such a machine for hours or days in a lab. i.e., what's so invisible about something that is clearly moving? So, to Faraday, perhaps, what makes the lines of force real and physical, is, in part, the clear motions.
But there are several other pieces of evidence of how Faraday came to see the lines of force are real and physical.
1) Faraday, we see, came to have a picture of the lines juxtaposed with his vision of experiments, as evidenced in his paragraph 2168 [p. 598], where he shows he's at the point where he can predict how they act: "This was because certain lines of magnetic force, which at first passed through the glass parallel to the ray, now crossed the glass and the ray, the iron giving two contrary poles opposite the poles of the magnet, and thus determining a new course for a certain portion of the magnetic power, and that across the polarized ray."
2) His paragraph 2172: "...any difference in volume occured when the magnetic curves passed through them." By this time, they're always there, the curves, in his vision.
3) Since by Ampère's convention, Ampère's current flows in the same direction as the that current imparts rotation on polarized light, it stands to follow that it is doing something mechanical or at least physical to the light somewhat directly.
4) In On the Physical Lines of Magnetic Force, F. compares magnetism to gravity, which, since it (seemingly) depends not on time, must be acting merely on an "ideal line representing the direction in which the power is exerted." [p. 817] On that same page, Faraday explains that electricity and magnetism having polarity separates them from gravity, the lines of force having a sense of direction.
5) Faraday, arguing that since the electric and magnetic force lines are curved around poles and around intervening plates, that they interact with their environment, unlike gravity lines, that they are real in space. I don't see how a Huyghens or Young would have agreed with him, but it seems good enough for Faraday. [p. 818]
6) Faraday concludes that paper saying that magnetic field lines are independent of ponderable matter. [p. 819]
7) Hertz, writing in 1884, wrote that F. believed he showed that lines of force were independent of how they were produced. [p. 274, H. Hertz, Miscellaneous Papers, London, Macmillan and Co. 1896.]
On Maxwell's new view, especially of EM:
Maxwell walks a line between Faraday's excellent work and Scottish purist thinking. He points out that Faraday learned action at a distance by rote, not by labor in astronomy and math as did most stricter physicists [Lecture on FLOF, p. 801] He says that Faraday might think of lines of force through a medium. While Maxwell works on the mechanism represented/representing that medium, he cares more about the flux of the field, the motion of the surface perpendicular to the lines of force, rather than the lines themselves. [OFLOF, p. 159]
For Maxwell, a valid physical explanation may just be a scaffolding of facts, a "physiological explanation," as opposed to an "ordinary scientific explanation." [LOFLOF p. 803]
Maxwell is creating a new method. Since magnetism as well as gravity work through interstitial bodies, he creates a new concept of a "physical hypothesis": It could be there, might not be, let's just use it until it breaks.
A few notes on Dear's presentation:
François Arago said Oersted was able to see what no French (Laplacian) scientists would ever see; the French would never "play" with magnets and current together before Oersted. Ampére was not a prototypical Laplacian, but did have an analytic, "Newtonian" view of the magnetic field. (Interestingly, Patricia Fara publishes Newton's accomodation that there might me some sort of medium for all forces.)
As Dear puts it, "over a bit of time...Faraday gets so attached to the idea of field lines," that he sees them as real. He also argues that curvy chains of electrolytes between electrodes, in Faraday's chemical conceptualization of decomposition, are a model for curvy lines of electric and also magnetic fields. Dear also pointed to the evidence that since a material interposed between capacitor plates affected the electrostatic force between the plates, that there must be something of a medium carrying lines of force between things, not action-at-a-distance like gravity.
Dear claims that Faraday's paper of magnetic effects on polarization is the establishment of F.'s belief in the physical reality of magnetic lines of force. I feel that there were 4 or five pieces of evidence published before that paper, as I note above (items 1, 2, 3, and the preceding paragraphs).
I unhesitatingly agree with Dear's claim that Maxwell promotes physical models that push the envelop, ultimately unprovable, but allowing mechanical thinking. Hertz, in reconciling Helmholtz and Maxwell, argues that the German's analytic method has hit a dead end of a certain proof that Maxwell's methods help Hertz un-jam. [op cit, p. 289]
Harman gives several pieces of evidence of selection/influence on Maxwell's developing methods : Stokes' emphasizing "the independence from physical hypothesis of his theory of elasticity" [p. 206]; Fourier "avoiding physical hypotheses and deriving his theory from empirical laws of the temperature distribution in bodies" [p. 206]; Maxwell's early work as "congruent with the geometry and 'mixed mathematics'" of Cambridge [p. 207]; Maxwell's hydrodynamical arguments in theorizing tubes formed by lines of force [p. 211]; and the relegation of "metaphysical foundations" to merely providing "justificatory sanction for a physical and mathematical worldview." [p. 214]
Siegel's work was based more closely on Maxwell's writings, as opposed to Harman's more social comparison. Siegel shows Maxwell holding his physical theory cards close, in dealing with his "basic allegiance to the notion that reality is ultimately mechanical..." [p. 180-182] From Siegel, writing about Maxwell's perspective, one gets a view of vortices going from obscurity in 1830, to the height of popularity around 1860, back to near-obscurity by the 1880s. It was only by reading other authors that I learned of much earlier vortex hypotheses.
