Although Dr Yuakawa did not refer to other Maxwell's original writings, the prefaces or introductions of other Maxwell's original writings are also worth reading. Why don't we try whole writings if we have time and interests we can get more from him.
On Physical Lines of Force (published in March, 1861)
No title of introduction but Part I stats with introductory story, which does mean.
Part I - The Theory of Molecular Vortices applied to Magnetic Phenomena
In all phenomena involving attractions and repulsions, or any forces depending on the relative positions of bodies, we have to determine the magnitude and direction of the force which would act on a given body, if placed in a given position.
In the case of a body acted on by the gravitation of a sphere, this force is inversely as the square of the distance, and in a straight line to the centre of the sphere. In the case of two attracting spheres, or of a body not spherical, the magnitude and direction of the force vary according to more complicated laws. In electric and magnetic phenomena, the magnitude and direction of the resultant force at any point is the main subject of investigation. Suppose that the direction of the force at any point is known, then, if we draw a line so that in every part of its course it coincide in direction with the force at that point, this line may be called a line of force, since it indicates the direction of the force in every part of its course.
By drawing a sufficient number of lines of force we may indicates the direction of the force in every part of the space it acts.
Thus if we strew iron filings on paper near a magnet, each filing will be magnetized by induction, and the consecutive filings will unite by their opposite poles, so as to form fibers, and these fibers will indicate the direction of the lines of force. The beautiful illustration of the presence of magnetic force afforded by this experiment, naturally tends to make us think of the lines of force as something real, and as indicating something more than the mere resultant of the two forces, whose seat of action is at a distance, and which do not exist there at all until a magnet is placed in that part of the field. We are dissatisfied with the explanation founded on the hypothesis of attractive and repellent forces directed towards the magnetic poles, even though we may have satisfied ourselves that the phenomenon is in strict accordance withe hypothesis, we cannot help thinking that in every place where we find these lines of force, some physical state or action must exist in sufficient energy to produce the actual phenomena.
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My Internal Note - 1 (underlined by sptt)
This part verbally but clearly states what Dr Yukawa said in the lecture.
"
Maxwell wrote in the preface (of Treaties on Electricity and Magnetism (published in 1973)) is that he takes a standpoint of "Near Force" like Faraday. And by this he thoroughly explained the then Electromagnetic theory based on Action-at-distance in terms of "Near Force", plus he drew a conclusion the light being Electromagnetic phenomenon. This is written in this book. This story is just an opposite direction of the development of mechanics. Thus was what actually happened.
"
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On Physical Lines of Force - continued
My object in this paper is to clear the way for speculation in this direction, by investigating the mechanical results of certain states of tension and motion to a medium, and comparing these with the observed phenomena of magnetism and electricity. By pointing out the mechanical consequences of such hypotheses, I hope to be of some use to those who consider the phenomena as due to the action of a medium, but are in doubt as to the relation of this hypothesis to the experimental laws already established, which have generally been expressed in the language of other hypotheses.
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My Internal Note - 2 (underlined by sptt)
Other hypotheses are supposed to be those of "action at distance" as Dr Yukawa lectured in Day One - 13.遠隔力と近接力 Action-at-distance and "Near Force".
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On Physical Lines of Force - continued
I have in a former paper *(* See a paper "On Faraday's Lines of Forces") endeavoured to lay before the mind of the geometer a clear conception of the relation of the lines of force to the space in which they are traced. By making use of the conception of currents in a liquid, I showed how to draw lines of force, which should indicate by their number the amount of force, so that each line may be called a unit-line of force (see Faraday's 'Researches,' 8122); and I have investigated the path of the lines where they pass from one medium to another.
In the same paper I have found the geometrical significance of the "Electronic State," and have shown how to deduce the mathematical relations between the electronic state, magnetism, electric currents, and the electromotive force, using mechanical illustrations to assist the imagination, but not to account for the phenomena.
I propose now to examine magnetic phenomena form mechanical point of view, and to determine what tensions in, or motions of, a medium are capable of producing the mechanical phenomena observed. If, by the same hypothesis, we can conduct the phenomena of magnetic attraction with electromagnetic phenomena and with those of induced currents, we shall have found a theory which, if not true, can only be proved to be erroneous by experiments which will greatly enlarge our knowledge of this part of physics.
The mechanical conditions of a medium under magnetic influence have been variously conceived of, as currents, undulations, or states of displacement or strain, or pressure or stress.
Currents, issuing from the north pole and entering the south pole of a magnet, or circulating round an electric current, have the advantage of representing correctly the geometrical arrangement of the lines of force, if we could account on mechanical principles for the phenomena of attraction, or for the currents themselves, or explain their continued existence.
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My Internal Note - 3
I will try the introductory part of "On Faraday's Lines of Forces" layer in the next post.The key word is of course " force ". Dr Yukawa mentioned the importance of force in physics repeatedly especially in the lecture - Day One and Day Two. Please see below.
From Wiki <Line of Force>
"
Historian Nancy J. Nersessian in her paper "Faraday's Field Concept" distinguishes between the ideas of Maxwell and Faraday:[5]
The specific features of Faraday's field concept, in its 'favourite' and most complete form, are that force is a substance, that it is the only substance and that all forces are interconvertible through various motions of the lines of force. These features of Faraday's 'favourite notion' were not carried on. Maxwell, in his approach to the problem of finding a mathematical representation for the continuous transmission of electric and magnetic forces, considered these to be states of stress and strain in a mechanical aether. This was part of the quite different network of beliefs and problems with which Maxwell was working."
I want to quote what Dr Yukawa wrote in his book Invisible things (published in 1946).
Chapter 3 Force and Energy
........ there is a close relation between electromagnetic force and photon. Looking at the electric force between electron and nucleus from a different perspective we can say that electron and nucleus always exchange energy by means of photon. In our visible world force and matter are totally different concepts but in invisible micro world this difference between force and matter become quite unclear. In this micro world the exchange of matter like photon and force working are the front side and the back side of the same fact.
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On Physical Lines of Force - continued
Undulations issuing from a centre world, according to the calculations of Professor Challis, produce an effect similar to attraction in the direction of the centre; but admitting this to be true, we know that two series of undulations traversing the same space do not combine into one resultant as two attractions do, but produce an effect depending on relations of phase as well as intensity, and if allowed to proceed, they diverge from each other without any mutual action. In fact the mathematical laws of attractions are not analogous in any respect to those undulations, while they have remarkable analogies with those of currents, of the conduction of heat and electricity and of elastic bodies.
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My Internal Note - 4
This part is difficult to understand without some knowledge of the background in this year and some writing of Professor Challis. "Undulations" seem to be "waves".
https://books.google.com.hk/books?id=zfM8AAAAIAAJ&pg=PA693&lpg=PA693&dq=Challis+undulations&source=bl&ots=kmbmqGi5O8&sig=sGh1xCzlYvFEbHa0ZHVjCUqyiAg&hl=en&sa=X&ei=PTAWVeeDF4K3mwW2z4DwDA&redir_esc=y#v=onepage&q=Challis%20undulations&f=false
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On Physical Lines of Force - continued
In the Cambridge and Dublin Mathematical Journal for January 1847, Professor William Thomson has given a "Mechanical Representation of Electric, Magnetic and Galvanic Forces," by means of the displacements of the particles of an elastic solid in a state of strain. In this representations we must make the angular displacement at every point of the solid proportional to the magnetic force at the corresponding point of the magnetic field, the direction of the axis of rotation of the displacement corresponding to the direction of the magnetic force. The absolute displacement of any particle will then correspond in magnitude and direction to that which I have identified with the electrotonic state; and the relative displacement of any particle, considered with reference to the particle in its immediate neighbourhood, will correspond in magnitude and direction to the quantity of electric current passing through the corresponding point of the magneto-electric field. The author of this method of representations does not attempt to explain the origin the observed forces by the effects due to these strains in the elastic solid, but makes use of the mathematical analogies of the two problems to assist the imagination in the study of both.
My Internal Note - 5 (underlined by sptt)
This part is also difficult to understand without knowing what Professor William Thomson says in "Mechanical Representation of Electric, Magnetic and Galvanic Forces" as well as also the background in this ear. (* 1) Ref Note at the end.
The underlined part suggests an idea of 'curl' or 'rotation'. Wiki once stated that <The name "curl" was first suggested by James Clerk Maxwell in 1871> but later changed as follows:
Wiki "Curl (Mathematics)
"
The name "curl" was first suggested by James Clerk Maxwell in 1871[1] but the concept was apparently first used in the construction of an optical field theory by James MacCullagh in 1839.[2]
"
Mathematical concept of curl is important to understand electromagnetic phenomena. Curl is not a physics law but a mathematician invention or definition, which however explains electromagnetic phenomena well or mathematically in a compact form, at least better than by language if you understand it (to some extent at least). You can find in this writing (On Physical Lines of Force) the direction of rotation is (not a circular but) an axis, which is a definition like 'cross product'.
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On Physical Lines of Force - continued
We come now to consider the magnetic influence as existing in the form of some kind of pressure or tension, or more generally, of stress in the medium.
Stress is action or reaction between the consecutive parts of a body, and consists in general of pressure or tension different in different directions at the same point of the medium.
The necessary relations among these forces have been investigated by mathematicians; and it has been shown that the most general type of a stress consists of a combination of three principal pressures or tensions, in directions at right angles to each other.
When two of the principal pressures are equal, the third becomes an axis of symmetry, either greatest or least pressure, the pressure at right angle to this axis being all equal.
When the three principal pressures are equal, the pressure is equal in every direction, and there results a stress having no determinate axis of direction, of which we have an example of in simple hydrostatic pressure.
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My Internal Note - 6
This part also will be easy to understand if you have knowledge of curl to a certain extent.
－－－＞
[]
ーーー＞
－－－＞
[]
＜ーーー
－－－ーー＞
[]
ーーー＞
Consider and visualize the rotation of []. The axis is <into or from the screen>.
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On Physical Lines of Force - continued
The general type of a stress is not suitable as a representation of a magnetic force, because a line of magnetic force has direction and intensity, but has no third quality including any difference between the sides of the line, which would be analogous to that observed in the case of polarized light*.
* See Faraday's 'Researches' : 3252
We must therefore represent the magnetic force at one point by a stress having a single axis of greatest of least pressure, and all the pressures at right angle to this axis is equal. It may be objected that it is inconsistent to present a line of force, which is essentially dipolar, by an axis of stress, which is necessarily isotropic; but we know that every phenomenon of action and reaction is isotropic in its results, because the effects of the force on the bodies between which it acts are equal and opposite, while the nature and origin of the force may be dipolar, as in the attraction between a north and a south pole.
Let us next consider the mechanical effect of a state of stress symmetrical about an axis. We may resolve it, in all case, into a simple hydrostatic pressure, combined with a simple pressure or tension along the axis. When the axis is that of greatest pressure, the force along the axis will be a pressure. When the axis is that of least pressure, the force along the axis will be a tension.
If we observe the lines of force between two magnets, as indicated by iron flings, we shall see that whenever the lines of force pass from one pole to another, there is attraction between those poles; and where the lines of force from the poles avoid each other and are dispersed into space, the poles repel each other, so that in both cases they are drawn in the direction f the resultant of the lines of force.
It appears therefore that the stress in the axis of a line of magnet force is a tension, like that of a rope.
If we calculate the lines of force in the neighbourhood of two gravitating bodies, we shall find them the same direction as those near two magnetic of the same name; but we know that the mechanical effect is that of attraction instead of repulsion. The lines of force in this case do not run between the bodies, but avoid each other, and are dispersed over space. In order to produce the effect of attraction, the stress along the lines of gravitating force must be a pressure.
Let us now suppose that the phenomena of magnetism depend on the existence of a tension in the direction of the lines of force, combined with a hydrostatic pressure; or in other words, a pressure greater in the equatorial than in the axial direction; the next question is what mechanical explanation can we give of this inequality of pressure in a fluid or mobile medium? The explanation which most readily occurs to the mind is that the excess of pressure in the equatorial direction arises from the centrifugal force of vortices or eddies in the medium having their axes in the directions parallel to the lines of force.
The explanation of the cause of the inequality of pressure at once suggests the means of representing the dipolar character of the line of force. Every vortex is essentially dipolar, the two extremities of its axis being distinguished by the direction of its revolution as observed from those points.
We also know that when electricity circulates in a conductor, it produces line of magnetic force passing through the circuit, the direction of the lines depending on the direction of the circulation. Let us suppose that the direction of the revolution of our vortices is that in which vitreous electricity must revolve in order to produce lines of force whose direction within the circuit is the same as ttat of the given lines of force.
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My Internal Note - 6
This part is difficult to follow without some knowledge of fluid dynamics, equations and drawings.
Refer to "right- hand rule", which is, as the name shows, a rule, not a law of nature.
We shall suppose at present that all the cortices in any one part of the field are revolving in the same direction about axes nearly parallel, but that in passing from one part of the field to another, the direction of the axes, the velocity of rotation, and the density of the substance of the vortices are subject to change. We shall investigate the resultant mechanical effect upon an element of the medium, and from the mathematical expression of this resultant e shall deduce the physical character of its different component parts.
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My Internal Note -7
Please remind that the title of Part I is "The theory of Molecular Vortices applied to Magnetic Phenomena". Maxwell tries to explain the source of magnetic forces by using the model of Molecular Vortices which are revolving.
Note at the end
(*1) Evolution of Electromagnetics in the 19th Century
I. V. Lindell
Helsinki Univ. Tech., Otakaari 5A, Espoo 02015HUT, Finland
3.2 Thomson’s analogies
William Thomson (Kelvin) (1824–1907) had read Fourier’s
book when entering the University of Cambridge in 1840.
He wrote a paper showing that Fourier’s stationary flow of
heat was mathematically analogous to Coulomb’s force law
even if the former applied contiguous transfer of heat and the
latter action over a distance. The lines of heat flow appeared
to follow exactly Faraday’s electric lines of force. This gave
Thomson the idea to represent the electric field in terms of
a flux of electricity starting from the charge point. Another
set of analogies was found between electrostatic polarization
in insulating media and displacements in elastic solids due to
stress. In 1856 he made an attempt to explain the Faraday rotation
in terms of molecular vortices caused by the magnetic
field.
The underlined part (made by sptt) may closely related this Maxwell paper, especially Part I "The Theory of Molecular Vortices applied to Magnetic Phenomena. (1861)"
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"Evolution of Electromagnetics in the 19th Century" is an outline and treat "the Continental Theories" and "British Theories' equally or rather neutrally.
sptt
