Talk:On Physical Lines of Force

The 1856 experiment by Weber and Kohlrausch

Headbomb, this experiment is actually crucial in Maxwell's 1861 paper. Without the result of this experiment, Maxwell would not have been able to have linked his equations to the speed of light. The purpose of the 1861 paper builds up in stages. The punchline comes at equation (132) where he uses the Weber/Kohlrausch experimental result in Newton's equation for the speed of sound, and on establishing the link to the speed of light, he makes his conclusion that light is a wave in the same medium as is the cause of electric and magnetic phenomena.

Take a look and see for yourself. The link to the original paper is here [1]. Go to page 48 of the pdf file in the link. That will take you to pages 21 and 22 in the actual paper. Read the bottom part of page 21 and the top two thirds of page 22. There you will see exactly how the Weber/Kohlrausch experiment is crucial to the entire purpose of the paper. I think that you ought to restore that material in order to be fair to the readers. David Tombe (talk) 00:40, 20 November 2010 (UTC)

All I'm saying is that this is an article about the paper, not about the experiment (which really should get its own article). All that needs to be here is the result of the experiment, and not much else. Headbomb {talk / contribs / physics / books} 01:47, 20 November 2010 (UTC)

Looking back to the old version, it's not that it doesn't belong here, more like it was mentioned out of sequence. Everything from "On Physical Lines of Force ..." to "...vector calculus format" should be treated as the lead. After it could come a "results" or "experimental motivations" or something like it, containing Maxwell's use of the Leyden jar result (and other things introduced in this paper), followed by an "impact" section with Einstein' and Feynman's quotes. How does that sound to you David? Headbomb {talk / contribs / physics / books} 01:51, 20 November 2010 (UTC)

Headbomb, Well of course the Einstein and the Feynman quotes are excellent and to the point. But that very point has been removed from the article. The crucial aspect which made Maxwell's 1861 paper so excellent was the fact that he used the Weber/Kohlrausch result in order to link his equations to the speed of light. Without the Weber/Kohlrausch result, his paper would have been second rate. I was first compelled to read this original paper because of a summary in a secondary source 'The Dictionary of Scientific Biography' (article on James Clerk-Maxwell). It was there that I read how Maxwell made analogies between the elastic quantities and the electromagnetic constants, and how he was then able to insert the Weber/Kohlrausch ratio into the equation for the speed of a mechanical wave in an elastic solid, and that in doing so, he obtained the speed of light. It's that crucial piece of information which drew me to read the original Maxwell paper. And now we have it available on-line, and you will see Maxwell's famous quote about 2/3 the way down page 22. You can see how the Weber/Kohlrausch result is central to the whole business.

I do think that this needs to be mentioned in the lead, otherwise we will be removing the icing from the cake. The article as it stands is very good. The sources and the quotes are excellent. But it is a very short article, and there is easily room to expand the lead to incorporate the singularly most important aspect of the topic. David Tombe (talk) 10:17, 20 November 2010 (UTC)

I'm gonna give it a try, tell me what you think. Headbomb {talk / contribs / physics / books} 15:11, 20 November 2010 (UTC)

Headbomb, There has always been conflicting accounts surrounding the extent to which Maxwell was initially aware of the 1856 Weber/Kohlrausch experiment. Some sources say that the speed of light was not explicit in Weber and Kohlrausch's result because it was masked by a factor of the square root of two, and that as such, they didn't notice it. On the other hand, I find it hard to believe that the connection hadn't been established at least by the next year, 1857, when Kirchhoff did his famous paper connecting the speed of light to the propagation of a signal along a wire. Anyway, even prior to the 1856 Weber/Kohlrausch experiment, Maxwell had already developed his elastic modelling of Faraday's lines of force to a considerable degree, but of course without any connection to the speed of light. As to when Maxwell first heard about the Weber/Kohlrausch result, I have never really been sure. I once read a source which claimed that following the 1856 result, Maxwell knew immediately that he should be working towards a wave equation for light, and that the 1861 paper was the first serious attempt in that direction. On the other hand, the Dictionary of Scientific Biography, which has an excellently sourced article on Maxwell, tells us that Maxwell didn't look up Weber and Kohlrausch's results until he was well into part III of his 1861 paper, and that when he did so, he had to fiddle around with it a bit to get it into a compatible format. That would lead one to believe that Maxwell hadn't made the connection to the speed of light until his trip to London in late 1861. On the other hand, I find it hard to believe that he didn't already have a pretty good idea about what he was leading up to, and what he was going to find when he looked up Weber and Kohlrausch's experiment.

So let's now examine your latest edits. Maxwell's theoretical work was largely independent of the Weber/Kohlrausch result, so it is not technically correct to say that his analogies between density and magnetic permeability, and between transverse elasticity and dielectric constant were closely connected to the 1856 result. Those analogies were largely derived from hydrodynamical modelling of the lines of force in part I in the case of density/permeability, and by a solid elastic analogy in part III in the case of elasticity/dielectric constant. Maxwell then showed how the Weber/Kohlrausch ratio was related to the ratio of dielectric constant to permeability. That of course brings us to the modern equation c^2 = 1/(mu)(epsilon). But in addition to this, Maxwell performed a lengthy elasticity analysis between equations (81) and (108) in which he showed that the dielectric/permeability ratio should be equal to the elasticity/density ratio. Finally he wheeled in the Weber/Kohlrausch result which he was now able to insert into Newton's equation for the speed of sound. It is at that point that the whole picture came together and he saw that light was an electromagnetic wave.

So we need to distance the Weber/Kohlrausch result from the majority of Maxwell's theoretical analysis. It is merely a final numerical experimental result which he brings in towards the end in order to reveal the entire agenda. It is the crucial number which puts the whole picture together. That's why, when I did my edit, I introduced the Weber/Kohlrausch result as if it were seemingly a totally unrelated issue, but then went on to point out how it linked in with Maxwell's theoretical work. Perhaps that is why you thought that my edit was tangential. But it was in fact very carefully thought out.

One final point, you mention about 20 equations. In the later 1865 paper there is a concise list of 20 equations for the electromagnetic field. It's actually really 8 equations, but 6 of them are expressed in each of their three cartesian components. In the 1861 paper, Maxwell has no definitive set of equations. In the 1861 paper, as compared to the modern Heaviside 4, Guass's law is at equation (115), Apmere's circuital law with displacement current is at (112), Faraday's law is at (54), and the zero divergence magnetic field law is at (56). Hence your mention of 20 equations is somewhat misleading as regards the 1861 paper. Once again, I was very careful to word it so as to merely say the Heaviside 4 had all appeared in Maxwell's 1861 paper David Tombe (talk) 16:50, 20 November 2010 (UTC)

Well, feel free to edit the article. I just laid out the structure I think it should have and added some notes. Headbomb {talk / contribs / physics / books} 17:28, 20 November 2010 (UTC)

Headbomb, I don't think that the factual accuracy of the article is in dispute. You were correct in stating that the Weber/Kohlrausch experiment is tangential to Maxwell's theoretical works. But you simply overlooked the fact that it is a tangent which intersects with Maxwell's theoretical works and leads to the golden result. We are dealing with the coming together of two distinct areas of research by two sets of people in different countries, and leading to the historical landmark in the history of electromagnetism which Einstein and Feynman are marvelling about in their quotes. It's clearly a page of history which as yet remains uncut, and hopefully this article which you have created will help to rectify the situation. How about adding the Maxwell quote alongside the Einstein quote and the Feynman quote? Ie. the quote that runs something like

"we can scarcely avoid the inference that light consists in the transverse undulations of the same medium that is the cause of electric and magnetic phenomena" James Clerk-Maxwell 1861. David Tombe (talk) 00:24, 21 November 2010 (UTC)