1299 lines
103 KiB
Plaintext
1299 lines
103 KiB
Plaintext
James Clerk MAXWELL
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20 Quaternion Equations
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rexresearch.com
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James Clerk Maxwell, "A Dynamical Theory of the Electromagnetic Field" ( Royal Society Transactions, Vol. CLV, 1865, p 459 ); Orally read Dec. 8, 1864. [ MS-Word.doc ] Andre Waser : On the Notation of Maxwell's Field Equations [ PDF ]
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The 1873 edition of A Treatise on Electricity & Magnetism contains the 20 Quaternion Equations that later were rewritten --- censored --- by Oliver Heaviside, et al.. These equations reconcile relativity with modern quantum physics and help to explain "free energy" and anti-gravity. Table of Contents: Volume 1: 1 ~ 2 ~ 3 ~ 4 ~ 5 ~ 6 ~ 7 ~ 8 ~9 ~ 10 ~ 11 ~ 12 ~ 13 Volume 2: 1 ~ 2 ~ 3 ~ 4 ~ 5 ~ 6 ~ 7 ~ 8 ~ 9 ~ 10 ~ 11 ~ 12 ~ 13 ~ 14 ~ 15 ~ 16 ~ 17 ~ 18 ~ 19 Links to the complete copies in the Posner Collection at Carnegie Mellon University: http://posner.library.cmu.edu/Posner/books/book.cgi?call=537_M46T_1873_V OL._1 http://posner.library.cmu.edu/Posner/books/book.cgi?call=537_M46T_1873_V OL._2 The complete copies also are included on Rex Research website CD.
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Maxwell's Quaternion Equations
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by
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Col. Tom Bearden
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Maxwell's original theory was published as:
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James Clerk Maxwell, "A Dynamical Theory of the Electromagnetic Field", Royal Society Transactions, Vol. CLV, 1865, p 459. The paper was orally read Dec. 8, 1864.
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[ MS-Word.doc ]
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It is also published in The Scientific Papers of James Clerk Maxwell, 2 vols. bound as one, edited by W. D. Niven, Dover, New York, 1952, Vol. 1, p. 526-597. Two errata are given on the unnumbered page prior to page 1 of Vol. 1.
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In this paper Maxwell presented his seminal theory of electromagnetism, containing 20 equations in 20 unknowns. His equations of the electromagnetic field are given in Part III, General Equations of the Electromagnetic Field, p. 554-564. On p. 561, he lists his 20 variables. On p. 562, he summarizes the different subjects of the 20 equations, being three equations each for magnetic force, electric currents, electromotive force, electric elasticity, electric resistance, total currents; and one equation each for free electricity and continuity. In the paper, Maxwell adopts the approach of first arriving at the laws of induction and then deducing the mechanical attractions and repulsions.
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A copy of the original Maxwell paper can easily be obtained for about $15 from Amazon etc. It is:
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James Clerk Maxwell, The Dynamical Theory of the Electromagnetic Field, edited by Thomas F. Torrance, Wipf and Stock Publishers, Eugene, Oregon, 1996. This booklet, which sells for about $15, contains Maxwell's original 1865 dynamical theory paper and some additional commentaries.
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Here's what Barrett --- a nationally known electrodynamicist and one of the co-founders of ultrawideband radar --- has to say about Maxwell's theory:
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"In the case of electromagnetism, the theory was first simplified before being frozen. Maxwell expressed electromagnetism in the algebra of quaternions and made the electromagnetic potential the centerpiece of his theory. In 1881 Heaviside replaced the electromagnetic potential field by force fields as the centerpiece of electromagnetic theory. According to him, the electromagnetic potential field was arbitrary and needed to be "assassinated" (sic). A few years later there was a great debate between Heaviside and Tate about the relative merits of vector analysis and quaternions. The result was the realization that there was no need for the greater physical insights provided by quaternions if the theory was purely local, and vector analysis became commonplace.
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The vast applications of electromagnetic theory since then were made using vector analysis. Although generations of very effective students were trained using vector analysis, more might be learned physically by returning, if not to quaternions, to other mathematical formulations in certain well-defined circumstances. As examples, since the time when the theoretical design of electromagnetism was frozen, gauge theory has been invented and brought to maturity and topology and geometry have been introduced to field theory. Although most persons view their subject matter through the filter of the mathematical tools in which they are trained, the best mathematical techniques for a specific analysis depend upon the best match between the algebraic logic and the underpinning physical dynamics of a theoretical system." [Terence W. Barrett and Dale M. Grimes, Preface, p. vii-viii, in Advanced Electromagnetism: Foundations, Theory and Applications, Terence W. Barrett and Dale M. Grimes (eds.), World Scientific, Singapore, 1995.]
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Maxwell died in 1879 of stomach cancer.
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In the 1880s, several scientists --- Heaviside, Gibbs, Hertz etc. --- strongly assaulted the Maxwellian theory and dramatically reduced it, creating vector algebra in the process. Then circa 1892 Lorentz arbitrarily symmetrized the already seriously constrained Heaviside-Maxwell equations, just to get simpler equations easier to solve algebraically, and thus to dramatically reduce the need for numerical methods (which were a "real bear" before the computer). But that symmetrization also arbitrarily discarded all asymmetrical Maxwellian systems - the very ones of interest to us today if we are seriously interested in usable EM energy from the vacuum.
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So anyone seriously interested in potential systems that accept and use additional EM energy from the vacuum, must first violate the Lorentz symmetry condition, else all his efforts are doomed to failure a priori.
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We point out that quaternion algebra has a higher group symmetry than either vector algebra or tensor algebra, and hence it reveals much more EM phenomenology and dynamics than does EM in vector or tensor form.
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Today, the tremendously crippled Maxwell-Heaviside equations --- symmetrized by Lorentz --- are taught in all our universities in the electrical engineering (EE) department. Note that the EE professors still dutifully symmetrize the equations, following Lorentz, and thus they continue to arbitrarily discard all asymmetrical Maxwellian systems. Hence none of them has the foggiest notion of how to go about developing an "energy from the vacuum" system, which is asymmetrical a priori.
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The resulting classical electromagnetics and electrical engineering (CEM/EE) model taught in all our university EE departments also contains very serious falsities. Most of modern physics, such as special and general relativity, quantum field theory, etc., has been developed since the 1880s and 1890s fixating of the symmetrized Maxwell-Heaviside equations. A paper gathering together a listing these serious flaws and giving proper citations, is T. E. Bearden, "Errors and Omissions in the CEM/EE Model," available for free downloading at:
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http://www.cheniere.org/techpapers/CEM%20Errors%20%20final%20paper%20complete%20w%20longer%20abstract4.doc .
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This paper also shows a magnetic Wankel engine (suppressed from the world market) that can be built by any electrical engineering department or physics department, and then tested at COP>1.0 to one's heart's content. The magnetic Wankel system is also easily close-looped for self-powering (where all its input energy is freely furnished by the vacuum, and the operator need furnish none of the input energy at all --- thus providing fuel free, continuous use of the energy from the vacuum, at will.
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In the hard physics literature, rigorous proof that eliminating the arbitrary Lorentz condition provides systems having free additional energy currents from the vacuum is given by M. W. Evans et al., "Classical Electrodynamics without the Lorentz Condition: Extracting Energy from the Vacuum," PhysicaScripta, Vol. 61, 2000, p. 513-517. Evans' own O(3) model is very advanced, and it also directly specifies mechanisms for an EM system receiving and using excess energy freely from the vacuum.
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Fortunately, today some scientists have turned again to higher group symmetry algebras in which EM is expressed. These higher group symmetry electrodynamics theories then show far more EM phenomenology than the standard CEM/EE model used in electrical power engineering.
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Anyway, that gives you a brief overview of the Maxwell theory, and the rather sharp curtailment of it that has become the accepted but very crippled model for electrical engineering. Specifically, it is that crippled model and its continued propagation and use that is directly responsible for the increasing energy crisis worldwide, and our dependence on conventional fuels etc.
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We do point out that the original Maxwell quaternion and quaternion-like theory of 1865 also contained errors, by the physics that has been learned since then. One of those errors was Maxwell's assumption of the material ether, an ether which was falsified experimentally in 1887 after Maxwell was already dead. But the present CEM/EE model still assumes that same old material ether, more than a century later.
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Also, after Maxwell published the first edition of his famous "Treatise.", not much happened. He was soundly criticized for using the quaternion approach, and even his own editor chastised him rather unmercifully for it. His attachment to the potentials as primary was also roundly criticized, since almost all theorists of the day believed that the potentials were simply mathematical conveniences having no physical reality whatsoever. To them, the force fields were the only physical reality in Maxwell's theory. Today, of course, we know in the quantum theory that it is the potentials that are primary, and the fields are derived from changes in the potentials.
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The history of Maxwell's famous treatise is as follows: The publications are James Clerk Maxwell, A Treatise on Electricity and Magnetism, Oxford University Press, Oxford, 1873, Second Edition 1881 (Maxwell was already dead), Third Edition, Volumes 1 and 2, 1891.
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Foreword to the second edition was by Niven, who finished the work as Maxwell had dramatically rewritten the first nine chapters, much new matter added and the former contents rearranged and simplified. Maxwell died before finishing the rest of the second edition. The rest of the second edition is therefore largely a reprint from the first edition. The third edition edited by J. J. Thomson was published in 1892, by Oxford University Press, and later was published unabridged, Dover Publications, New York, 1954. J. J. Thomson finished the publication of the third edition, and wrote a "Supplementary Volume" with his notes. A summary of Maxwell's equations is given in Vol. II, Chapter IX of the third edition. However, Maxwell had gone (in his second edition) to some pains to reduce the quaternion expressions himself, and not require the students to know the calculus of quaternions (so stated on p. 257). We note that Maxwell did not finish the second edition, but died before that. He actually had no hand at all in the third edition as to any further changes. The Second edition (unfinished by Maxwell) was later finished by Niven by simply adding the remaining material from the previous first edition approved by Maxwell to that part that Maxwell had revised. The printing of the first nine chapters of the third edition was already underway when J. J. Thomson was assigned to finish the editing of the manuscript.
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Indeed, as an example of a major error in the present CEM/EE model, we know today that matter is a component of force, and therefore the EM force fields prescribed in matter-free space by Maxwell and his followers (and by all our electrical engineering departments today), do not exist. The EM field in massless space is force-free, and is a "condition of space" itself, as pointed out by Feynman in his three volumes of sophomore physics. Specifically, speaking of the electric field Feynman states:
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"...the existence of the positive charge, in some sense, distorts, or creates a "condition" in space, so that when we put the negative charge in, it feels a force. This potentiality for producing a force is called an electric field." [Richard P. Feynman, Robert B. Leighton, and Matthew Sands, The Feynman Lectures on Physics, Addison-Wesley, Reading, MA, Vol. 1, 1964, p. 2-4].
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He further states:
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"We may think of E(x, y, z, t) and B(x, y, z, t) as giving the forces that would be experienced at the time t by a charge located at (x, y, z), with the condition that placing the charge there did not disturb the positions or motion of all the other charges responsible for the fields." [ibid, vol. II, p. 1-3.]
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But the CEM/EE texts still teach that old force field in empty space. However, Jackson --- a superb classical electrodynamicist of international reknown --- at least points out that this dramatic error in the model is just ignored. Jackson states:
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"Most classical electrodynamicists continue to adhere to the notion that the EM force field exists as such in the vacuum, but do admit that physically measurable quantities such as force somehow involve the product of charge and field." [J. D. Jackson, Classical Electrodynamics, Second Edition, Wiley, 1975, p. 249].
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Jackson does admit it and point out that this logical problem is just ignored, for which he is to be highly commended. Most textbooks simply do not even discuss it.
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So at his death in 1879, Maxwell had already laboriously simplified some 80% of his "Treatise" himself, to comply with the severe demands of the publisher. The second edition of his book thus has the first 80% considerably changed by Maxwell himself. The third edition contained the same theory as the second edition essentially, but just with additional commentary. It is this third edition that is widely available and usually referred to as "Maxwell's theory".
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Today, there is still a widespread belief that the third edition represents Maxwell's original EM work and theory, in pristine form just as created originally by Maxwell.
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It doesn't.
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Best wishes,
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Tom Bearden
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[459]
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VIII A Dynamical Theory of the Electromagnetic Field. By J. Clerk Maxwell, F.R.S
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Received October 27, -- Read December 8, 1864
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(note: Maxwell's 1873 work see http://posner.library.cmu.edu/Posner/books/CALL5/537_M46T_1873_VOL._1/vol0/part0/copy0/jpg/sm /(*)-sm.jpg )
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(Transcribers note: For this document to properly display you must have Times New Roman Font and Code 2000 installed and Asian fonts enabled. For clarity some formulas are repeated in common computer style notation (as found in GW Basic) using blue font. This document was transcribed from a .pdf version found on the internet.pak, 20070821, Ione)
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PART I. -- INTRODUCTORY.
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(I) The most obvious mechanical phenomenon in electrical and magnetical experiments is the mutual action by which bodies in certain states set each other in motion while still at a sensible distance from each other. The first step, therefore, in reducing these phenomena into a scientific form, is to ascertain the magnitude and direction of the force acting between the bodies, and when it is found that this force depends in a certain way upon the relative position of the bodies and on their electric or magnetic condition, it seems at first sight natural to explain the facts by assuming the existence of something either at rest or in motion in each body, constituting its electric or magnetic state, and capable of acting at a distance according to mathematical laws.
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These theories assume, more or less explicitly, the existence of substances the particles of which have the property of acting on one another at a distance by attraction or repulsion. The most complete development of a theory of this kind is that of M.W. WEBER*, who has made the same theory include electrostatic and electromagnetic phenomena.
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In doing so, however, he has found it necessary to assume that the force between two electric particles depends on their relative velocity, as well as on their distance.
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This theory, as developed by MM. W. WEBER and C. NEUMANNΏ, is exceedingly ingenious, and wonderfully comprehensive in its application to the phenomena of statical electricity, electromagnetic attractions, induction of currents and diamagnetic phenomena; and it comes to us with the more authority, as it has served to guide the speculations of one who has made so great an advance in the practical part of electric science, both by introducing a consistent system of units in electrical measurement, and by actually determining electrical quantities with an accuracy hitherto unknown.
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* electrodynamiacheMaassbestimmung. Leipzic Trans, vol. i. 1840, and TAYLOR'SScientific Memorirs, vol. v. art.xiv.
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Ώ "Explicaretentaurquomodofiat ut luciaplanumpolarizationis per vires electricasvolmagneticasdeclinetur." -- HalisSaxonum, 1838.
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MDCCCLXC
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3R
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460 PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
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(2) The mechanical difficulties, however, which are involved in the assumption of particles acting at a distance with forces which depend on their velocities are such as to prevent me from considering this theory as an ultimate one, though it may have been, and may yet be useful in leading to the coordination of phenomena.
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I have therefore preferred to seek an explanation of the fact in another direction, by supposing them to be produced by actions which go on in the surrounding medium as well as in the excited bodies, and endeavoring to explain the action between distant bodies without assuming the existence of forces capable of acting directly at sensible distances.
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(3) The theory I propose may therefore be called a theory of the Electromagnetic Field, because it has to do with the space in the neighbourhood of the electric or magnetic bodies, and it may be called a Dynamical Theory, because it assumes that in that space there is matter in motion, by which the observed electromagnetic phenomena are produced.
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(4) The electromagnetic field is that part of space which contains and surrounds bodies in electric or magnetic conditions.
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It may be filled with any kind of matter, or we may endeavor to render it empty of all gross matter, as in the case of GEISSLER'S tubes and other so-called vacua.
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There is always, however, enough of matter left to receive and transmit the undulations of light and heat, and it is because the transmission of these radiations is not greatly altered when transparent bodies of measurable density are substituted for the so-called vacume, that we are obliged to admit that the undulations are those of an aethereal substance, and not of the gross matter, the presence of which merely modifies in some way the motion of the aether.
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We have therefore some reason to believe, from the phenomena of light and heat, that there is an aethereal medium filling space and permeating bodies, capable of being set in motion and of
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transmitting that motion from one part to another, and of communicating that motion to gross matter so as to heat it and affect it in various ways.
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(5) Now the energy communicated to the body in heating it must have formerly existed in the moving medium, for the undulations had left the source heat some time before they reached the body, and during that time the energy must have been half in the form of motion of the medium and half in the form of elastic resilience. From these considerations Professor W. THOMSON has argued*, that the medium must have a density capable of comparison with that of gross matter, and has even assigned an inferior limit to that density.
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(6) We may therefore receive, as a datum derived from a branch of science independent of that with which we have to deal, the existence of a pervading medium, of small but real density, capable of being set in motion, and of transmitting motion from one part to another with great, but not infinite, velocity.
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Hence the parts of this medium must be so connected that the motion of one part
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* "On the Possible Density of the Luminiferous Medium, and on the Mechanical Value of a Cubic Mile of Sunlight," Transactions of the Royal Society of Edinburgh (1854), p. 57.
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PROFESSOR CLERK MAXWELL ON THE ELECROMAGNETIC FIELD.
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461
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depends in some way on the motion of the rest; and at the same time these connexions must be capable of a certain kind of elastic yielding, since the communication of motion is not instantaneous, but occupies time.
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The medium is therefore capable of receiving and storing up two kinds of energy, namely, the "actual" energy depending on the motions of its parts, and "potential" energy, consisting of the work which the medium will do in recovering from displacement in virtue of its elasticity.
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The propagation of undulations consists in the continual transformation of one of these forms of energy into the other alternately, and at any instant the amount of energy in the whole medium is equally divided, so that half is energy of motion, and half is elastic resilience.
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(7) A medium having such a constitution may be capable of other kinds of motion and displacement than those which produce the phenomena of light and heat, and some of these may be of such a kind that they may be evidenced to our senses by the phenomena they produce.
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(8) Now we know that the luminiferous medium is in certain cases acted on by magnetism; for FARADAY * discovered that when a plane polarized ray traverses a transparent diamagnetic medium in the direction of the lines of magnetic force produced by magnets or currents in the neighbourhood, the plane of polarization is caused to rotate. This rotation is always in the direction in which positive electricity must be carried round the diamagnetic body in order to produce the actual magnetization of the field. M. VERDET has since discovered that if a paramagnetic body, such as solution of perchloride of iron in either, be substituted for the diamagnetic body, the rotation is in the opposite direction.
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Now Professor W. THOMSON has pointed out that no distribution of forces acting between the parts of a medium whose only motion is that of the luminous vibrations, is sufficient to account for the phenomena, but that we must admit the existence of a motion in the medium depending on the magnetization, in addition to the vibratory motion which constitutes light.
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It is true that the rotation by magnetism of the plane of polarization has been observed only in media of considerable density; but the properties of the magnetic field are not so much altered by the substitution of one medium for another, or for a vacuum, as to allow us to suppose that the dense medium does anything more than merely modify the motion of the ether. We have therefore warrantable grounds for inquiring whether there may not be a motion of the ethereal medium going on wherever magnetic effects are observed, and we have some reason to suppose that this motion is one of rotation, having the direction of the magnetic force of its axis.
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(9) We may now consider another phenomenon observed in the electromagnetic
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* Experimental Researches, Series 19. ComptesRendux (1856, second half year, p. 329, and 1857, first half year, p. 1209). Procedings of the Royal Society, June 1856 and June 1861.
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3R2
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462
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PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
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field. When a body is moved across the lines of magnetic force it experiences what is called an electromotive force; the two extremities of the body tend to become oppositely electrified, and an electric current tends to flow through the body. When the electromotive force is sufficiently powerful, and is made to act on certain compound bodies, it decomposes them, and cause one of their components to pass towards one extremity of the body, and the other in the opposite direction.
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Here we have evidence of a force causing an electric current in spite of resistance; electrifying the extremities of a body in opposite ways, a condition which is sustained only by the action of the electromotive force, and which, as soon as that force is removed, tends, with an equal and opposite force, to produce a counter current through the body to restore the original electrical state of the body; and finally, if strong enough, tearing to pieces chemical compounds and carrying their components in opposite directions, while their natural tendency is to combine, and to combine with a force which can generate and electromotive force in the reverse direction.
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This, then, is a force acting on a body caused by its motion through the electromagnetic field, or by changes occurring in that field itself; and the effect of the force is either to produce a current and heat the body, or to decompose the body, or, when it can do neither, to put the body in a state of electric polarization, -- a state of constraint in which opposite extremities are oppositely electrified, and from which the body tends to relieve itself as soon as the disturbing force is removed.
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(10) According to the theory which I propose to explain, this "electromotive force" is the force called into play during the communication of motion from one part of the medium to another, and it is by means of this force that the motion of one part causes motion in another part. When electromotive force acts on a conducting circuit, it produces a current, which, as it meets with resistance, occasions a continual transformation of electrical energy into heat, which is incapable of being restored again to the form of electrical energy by any reversal of the process.
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(11) But when electromotive force acts on a dielectric it produces a state of polarization of its parts similar in distribution to the polarity of the parts of a mass of iron under the influence of a magnet, and like the magnetic polarization, capable of being described as a state in which every particle has its opposite poles in opposite conditions*.
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In a dielectric under the action of electromotive force, we may conceive that the electricity in each molecule is so displaced that one side is rendered positively and the other negatively electrical, but that the electricity remains entirely connected with the molecule, and does not pass from one molecule to another. The effect of this action on the whole dielectric mass is to produce a general displacement of electricity in a certain direction. This displacement does not amount to a current, because when it has attained to a certain value it remains constant, but it is the commencement of a current, and its variations constitute currents in the positive or the negative direction according
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* FARADAY, Exp. Res. Series XL; Mossotti, Mem. della Soc. Italiann (Modena), vol. xxiv. part 2. p. 49.
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PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
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463
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as the displacement is increasing or decreasing. In the interior of the dielectric there is no indication of electrification, because the electrification of the surface of any molecule is neutralized by the opposite electrification of the surface of the molecules in contact with it; but at the bonding surface of the dielectric, where the electrification is not neutralized, we find the phenomena which indicate positive or negative electrification.
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The relation between the electromotive force and the amount of electric displacement it produces depends on the nature of the dielectric, the same electromotive force producing generally a greater electric displacement in solid dielectrics, such as glass or sulphur, than in air.
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(12) Here, then, we perceive another effect of electromotive force, namely, electric displacement, which according to our theory is a kind of elastic yielding to the action of the force, similar to that which takes place in structures and machines owing to the want of perfect rigidity of the connexions.
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(12) Here, then, we perceive another effect of electromotive force, namely, electric displacement, which according to our theory is a kind of elastic yielding to the action of the form, similar to that which takes place in structures and machines owing to the want of perfect rigidity of the connexions.
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(13) The practical investigation of the inductive capacity of dielectrics is rendered difficult on account of two disturbing phenomena. The first is the conductivity of the dielectric, which, though in many cases exceedingly small, is not altogether insensible. The second is the phenomenon called electric absorption*, in virtue of which, when the dielectric is exposed to electromotive force, the electric displacement gradually increases, and when the electromotive force is removed, the dielectric does not instantly return to its primitive state, but only discharges a portion of its electrification, and when left to itself gradually acquires electrification on its surface, as the interior gradually becomes depolarized. Almost all solid dielectrics exhibit this phenomenon, which gives rise to the residual charge in the Leyden jar, and to several phenomena of electric cables described by Mr. F. JENKIN .
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(14) We have here two other kinds of yielding besides the yielding of the perfect dielectric, which we have compared to a perfectly elastic body. The yielding due to conductivity may be compared to that of a viscous fluid (that is to say, a fluid having great internal friction), or a soft solid on which the smallest force produces a permanent alteration of figure increasing with the time during which the force acts.
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The yielding due to electric absorption may be compared to that of a cellular elastic body containing a thick fluid in its cavities. Such a body, when subjected to pressure, is compressed by degrees on account of the gradual yielding of the thick fluid; and when the pressure is removed it does not at once recover, its figure, because the elasticity of the substance of the body has gradually to overcome the tenacity of the fluid before it can regain complete equilibrium.
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Several solid bodies in which no such structure as we have supposed can be found, seem to possess a mechanical property of this kind ; and it seems probable that the
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* Faraday, Exp. Res. 1233-1250 Reports of British Association, 1859, p. 248; and Report of Committee of Board of Traded on Submarine Cables,
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pp. 136 & 464 As, for instance, the composition of glue, treacle, &c., of which small plastic figures are made, which after being
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distorted gradually recover their shape.
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464
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PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
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same substances, if dielectrics, may poses the analogous electrical property, and if magnetic, may have corresponding properties relating to the acquisition, retention, and loss of magnetic polarity.
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(15) It appears therefore that certain phenomena in electricity and magnetism lead to the same conclusion as those of optics, namely, that there is an aethereal medium pervading all bodies, and modified only in degree by their presence; that the parts of this medium are capable of being set in motion by electric currents and magnets; that this motion is communicated from one part of the medium to another by forces arising from the connexions of those parts; that under the action of these forces there is a certain yielding depending on the elasticity of these connexions; and that therefore energy in two different forms may exist in the medium, the one form being the actual energy of motion of its parts, and the other being the potential energy stored up in the connexions, in virtue of their elasticity.
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(16) Thus, then, we are led to the conception of a complicated mechanism capable of a vast variety of motion, but at the same time so connected that the motion of one part depends, according to define relations, on the motion of other parts, these motions being communicated by forces arising from the relative displacement of the connected parts depends, in virtue of their elasticity. Such a mechanism must be subject to the general laws of Dynamics, and we ought to be able to work out all the consequences of its motion, providing we know the form of the relation between the motions of the parts.
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(17) We know that when an electric current is established in a conducting circuit, the neighbouring part of the field is characterized by certain magnetic properties, and that if two circuits are in the field, the magnetic properties of the field due to the two currents are combined. Thus each part of the field is in connexion with both currents, and the two currents are put in connexion with each other in virtue of their connexion with the magnetization of the field. The first result of the connexion that I propose to examine is the induction of one current by another, and by the motion of conductors in the field.
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The second result, which is deduced from this, is the mechanical action between conductors carrying currents. The phenomenon of the induction of currents has been deduced from their mechanical action by HELMHOLTZ* and THOMSON . I have followed the reverse order, and deduced the mechanical action from the laws of induction. I have then described experimental methods of determining the quantities L, M, N, on which these phenomena depend.
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(18) I then apply the phenomena of induction and attraction of currents to the exploration of the electromagnetic field, and the laying down systems of lines of magnetic force which indicate its magnetic surfaces, cutting the lines of force at right angles.
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* "Conservation of Force," Physical Society of Berlin, 1847; and Taylor's Scientific Memoirs, 1853, p. 114. Reports of the British Association, 1848; Philosophical Magazine, Dec. 1851.
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
465
|
||
|
||
In order to bring these results within the power of symbolical calculation, I then express them in the form of the General Equations of the Electromagnetic Field.
|
||
|
||
These equations express --
|
||
|
||
(A) The relation between electric displacement, true conduction, and the total current, compounded of both.
|
||
|
||
(B) The relation between the lines of magnetic force and the inductive coefficients of a circuit, as already deduced from the laws of induction.
|
||
|
||
(C) The relation between the strength of a current and its magnetic effects, according to the electromagnetic system of measurement.
|
||
|
||
(D) The value of the electromotive force in a body, as arising from the motion of the body in the field, the alteration of the field itself, and the variation of the electric potential from one part of the field to another.
|
||
|
||
(E) The relation between electric displacement, and the electromotive force which produces it.
|
||
|
||
(F) The relation between an electric current, and the electromotive force which produces it.
|
||
|
||
(G) The relation between the amount of free electricity at any point, and the electric displacements in the neighbourhood.
|
||
|
||
(H) The relation between the increase or diminution of free electricity and the electric currents in the neighbourhood.
|
||
|
||
There are twenty of these equations in all, involving twenty variable quantities.
|
||
(19) I then express in terms of these quantities the intrinsic energy of the Electro-magnetic Field as depending partly on its magnetic and partly on its electric polarization at every point.
|
||
From this I determine the mechanical force acting, 1st on a moveable conductor carrying an electric current: 2ndly, on a magnetic pole; 3rdly, on and electrified body.
|
||
The last result, namely, the mechanical force acting on an electrified body, gives rise to an independent method of electrical measurement founded on its electrostatic effects. The relation between the units employed in the two methods is shown to depend on what I have called the "electric elasticity" of the medium, and to be a velocity, which has been experimentally determined by MM. WEBER and KOHLRAUSCH.
|
||
I then show how to calculate the electrostatic capacity of a condenser, and the specific inductive capacity of a dielectric.
|
||
The case of a condenser composed of parallel layers of substances of different electric resistances and inductive capacities is next examined, and it is shown that the phenomenon called electric absorption will generally occur, that is, the condenser, when suddenly discharged, will after a short time show signs of a residual charge.
|
||
(20) The general equations are next applied to the case of a magnetic disturbance propagated through a non-conducting field, and it is shown that the only disturbances which can be so propagated are those which are transverse to the direction of propagation, and that the velocity of propagation is the velocity v, found from experiments such
|
||
|
||
466
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
as those of WEBER, which expresses the number of electrostatic units of electricity which are contained in one electromagnetic unit.
|
||
|
||
This velocity is so nearly that of light, that it seems we have strong reason to conclude that light itself (including radiant heat, and other radiations if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws. If so, the agreement between the elasticity of the medium as calculated from the rapid alterations of luminous vibrations, and as found by the slow processes of electrical experiments, shows how perfect and regular the elastic properties of the medium must be when not encumbered with any matter denser than air. If the same character of the elasticity is retained in dense transparent bodies, it appears that the square of the index of refraction is equal to the product of the specific dielectric capacity and the specific magnetic capacity. Conducting media are shown to absorb such radiations rapidly, and therefore to be generally opaque.
|
||
|
||
The conception of the propagation of transverse magnetic disturbances to the exclusion of normal ones is distinctly set forth by Professor FARADAY* in his "Thoughts on Ray Vibrations." The electromagnetic theory of light, as proposed by him, is the same in substance as that which I have begun to develop in this paper, except that in 1846 there were no data to calculate the velocity of propagation.
|
||
|
||
(21) The general equations are then applied to the calculation of the coefficients of mutual induction of two circular currents and the coefficient of self-induction in a coil. The want of uniformity of the current in the different parts of the section of a wire at the commencement of the current is investigated, I believe for the first time, and the consequent correction of the coefficient of self-induction is found.
|
||
|
||
These results are applied to the calculation of the self-induction of the coil used in the experiments of the Committee of the British Association on Standards of Electric Resistance, and the value compared with that deduced from the experiments.
|
||
|
||
PART II -- ON ELECTROMAGNETIC INDUCTION
|
||
|
||
Electromagnetic Momentum of a Current.
|
||
|
||
(22) We may begin by considering the state of the field in the neighbourhood of an electric current. We know that magnetic forces are excited in the field, their direction and magnitude depending according to known laws upon the form of the conductor carrying the current. When the strength of the current is increased, all the magnetic effects are increased in the same proportion. Now, if the magnetic state of the field depends on motions of the medium, a certain force must be exerted in order to increase or diminish these motions, and when the motions are excited they continue, so that the effect of the connexion between the current and the electromagnetic field surrounding it, is to endow the current with a kind of momentum, just as the connexion between the driving-point of a machine and a fly-wheel endows the driving-point with an additional
|
||
* Philosophical Magazine, May 1846, or Experimental Resarches, iii. p. 447.
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
467
|
||
|
||
momentum, which may be called the momentum of the fly-wheel reduced to the driving-point. The unbalanced force acting on the driving-point increases this momentum, and is measured by the rate of its increase.
|
||
|
||
In the case of electric currents, the resistance to sudden increase or diminution of strength produces effects exactly like those of momentum, but the amount of this momentum depends on the shape of the conductor and the relative position of its different parts.
|
||
|
||
Mutual Action of two Currents.
|
||
|
||
(23) If there are two electric currents in the field, the magnetic force at any point is that compounded of the forces due to each current separately, and since the two currents are in connexion with every point of the field, they will be in connexion with each other, so that any increase or diminution of the one will produce a force acting with or contrary to the other.
|
||
|
||
Dynamical Illustration of Reduced Momentum
|
||
|
||
(24) As a dynamical illustration, let us suppose a body C so connected with two independent drivingpoints A and B that its velocity is p times that of A together with q times that of B. Let u be the velocity of A, v that of B, and w that of C, and let òx, òy, òz be their simultaneous displacements, then by the general equation of dynamics*,
|
||
|
||
C dwdtòz=Xòx+Yòy [C*(dw/dt)*òz=Xòx+Yòy]
|
||
|
||
where X and Y are the forces acting at A and B. But
|
||
dwdt=p dudt+ q dvdt
|
||
[dw/dt=P*(du/dt)+q*(dv/dt)] and òz=pòx+qòy [ò*z=p*ò*x+q*ò*y] Substituting, and remembering thatòx, andòy are independent,
|
||
|
||
=X ddt(Cp2u+Cpqv), =Y ddt(Cpqu+Cq2v),
|
||
|
||
............... (1)
|
||
|
||
[X =(d/dt)*(C*p^2*u+C*p*q*v)] [Y=(d/dt)*(C*p*q*u+c*p^2*v)]
|
||
We may call Cp2+Cpqv the momentum of C referred to A, and Cpqu+Cq2v is momentum referred to B; then we may say that the effect of the force X is to increase the momentum of C referred to A, and that of Y to increase its momentum referred to B.
|
||
If there are many bodies connected with A and B in a similar way but with different values of p and q, we may treat the question in the same way assuming
|
||
|
||
L=Ĺ(Cp2), M=Ĺ(Cpq), andN=Ĺ(Cq2)
|
||
|
||
[L=Ĺ*(C*p^2), M=Ĺ*(C*p*q), andN=Ĺ*(C*q^2)]
|
||
|
||
* Lagrange, Mee. Anal.ii, w, & 5.
|
||
|
||
MDCCCLXV
|
||
|
||
3s
|
||
|
||
468
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
where the summation is extended to all the bodies with their proper values of C, p, and q.
|
||
|
||
Then the momentum of the system referred to A is Lu + Mv
|
||
|
||
and referred to B, Mu + Nv
|
||
|
||
and we shall have
|
||
=X ddt(Lu+Mv),
|
||
=Y ddt(Mu+Nv),
|
||
|
||
............... (2)
|
||
|
||
[X =d/dt* (Lu+Mv)] [Y =d/dt*(Mu+Nv)]
|
||
where X and Y are the external forces acting on A and B.
|
||
(25) To make the illustration more complete we have only to suppose that the motion of A is resisted by a force proportional to its velocity, which we may call Ru,and that of B by a similar force, which we may call Sv, R and S being coefficients of resistance.
|
||
Then if \ and L are the forces on A and B
|
||
|
||
\ = X+Ru = Ru +ddt(Lu + Mv) ............... (3)
|
||
L = Y + Sv = Sv+ddt (Mu +Nv)
|
||
[\ = X+Ru = Ru +d/dt*(Lu + Mv)] [L = Y + Sv = Sv +d/dt*(Mu +Nv)]
|
||
If the velocity of A be increased at the rate dudt,then in order to prevent B from moving a force, L=+ddt(Mu) must be applied to it.
|
||
This effect on B, due to an increase of the velocity of A, corresponds to the electromotive force on one circuit arising from an increase in the strength of a neighbouring circuit.
|
||
This dynamical illustration is to be considered merely as assisting the reader to understand what is meant in mechanics by Reduced Momentum. The facts of the induction of currents as depending on the variations of the quantity called Electromagnetic Momentum, or Electrotonic State, rest on the experiments of FARADAY*, FELICI , &c.
|
||
Coefficients of Induction for Two Circuits.
|
||
(26) In the electromagnetic field the values of L, M, N depend on the distribution of the magnetic effects due to the two circuits, and this distribution depends only on the form and relative position of the circuits. Hence L, M, N are quantities depending on the form and relative position of the circuits, and are subject to variation with the motion of the conductors. It will be presently seen that L, M, N are geometrical quantities of the nature of lines, that is, of one dimension in space; L depends on the form of the first conductor, which we shall call A, N on that of the second, which we shall call B, and M on the relative position of A and B.
|
||
(27) Let \ be the electromotive force acting on A, G the strength of the current, and
|
||
|
||
* ExperimentalResearches, Series I., IX. Annales de Chimie, sér. 3. xxxiv (1852) p. 64
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
469
|
||
|
||
R the resistance, then RG will be the resisting force. In steady currents the electromotive force just balances the resisting force, but in variable currents the resultant force \ = RG is expended in increasing the "electromagnetic momentum," using the word momentum merely to express that which is generated by a force acting during a time, that is, a velocity existing in a body.
|
||
|
||
In the case of electric currents, the force in action is not ordinary mechanical force, at least we are not as yet able to measure it as common force, but we call it electromotive force, and the body moved is not merely the electricity in the conductor, but something outside the conductor, and capable of being affected by other conductors in the neighbourhood carrying currents. In this it resembles rather the reduced momentum of a driving-point of a machine as influenced by its mechanical connexions, than that of a simple moving body like a cannon ball, or water in a tube.
|
||
|
||
Electromagnetic Relations of two Conducting Circuits.
|
||
|
||
(28) In the case of two conducting circuits, A and B, we shall assume that the electromagnetic momentum belonging to A is
|
||
|
||
LG+My
|
||
|
||
and that belong to B,
|
||
|
||
MG+Ny
|
||
|
||
where L, M, N correspond to the same quantities in the dynamical illustration, except that they are supposed to be capable of variation when the conductors A or B are moved.
|
||
|
||
Then the equation of the current is G in A will be
|
||
|
||
\ = RG +ddt(LG + My), ............... (4) L = Sy+ ddt (MG +Ny), ............... (5)
|
||
|
||
[\ = R*G+d/dt* (L*G + M*y)] [L = S*y+d/dt* (M*G+ N*y)]
|
||
|
||
where\ and L are the electromotive forces, G and y the currents, and R and S the resistances in A and B respectively.
|
||
|
||
Induction of one Current by another.
|
||
|
||
(29) Case 1st. Let there be no electromotive force on B, except that which arises from the action of A, and let the current of A increase from 0 to the value G, then
|
||
|
||
Sy+ddt (Mx+Ny)=0
|
||
|
||
[Sy+d/dt(M*x+N*y)=0]
|
||
|
||
whence
|
||
|
||
ю - to
|
||
|
||
m
|
||
|
||
Y =
|
||
|
||
ydt = S x
|
||
|
||
[Y = integral t o *y*d*t=- m/S*x]
|
||
|
||
that is, a quantity of electricity Y, being the total induced current, will low through B when x rises from 0 to x. This induction by variation of the current in the primary
|
||
3 s 2
|
||
|
||
470
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
conductor. When M is positive, the induced current due to increase of the primary current is negative.
|
||
|
||
Induction by Motion of Conductor.
|
||
|
||
(30) Case 2nd. Let x remain constant, and let M change from M to M', then
|
||
|
||
Y = - M'-Mx; S
|
||
[Y= - (M' - M) / X * x]
|
||
|
||
so that if M is increased, which it will be by the primary and secondary circuits approaching each other, there will be a negative induced current, the total quantity of electricity passed through B being Y.
|
||
|
||
This induction by the relative motion of the primary and secondary conductors.
|
||
|
||
Equation of Work and Energy
|
||
|
||
(31) To form the equation between work done and energy produced, multiply (1) by x and (2) by y, and add
|
||
\x+Ly = Rx2+Sy2 + xddt (Lx +My) +y ddt(Mx+Ny)
|
||
|
||
[\x+Ly= Rx^2+Sy^2 +x*d/d*t*(L*x +M*y) +y*d/d*t*(M*x+N*y)]
|
||
|
||
Here \x is the work done in unit of time by the electromotive force \ acting on the current x and maintaining it, and Ly is the work done by the electromotive force L. Hence the left-hand side of the equation represents the work done by the electromotive forces in unit of time.
|
||
Heat produced by the Current.
|
||
(32) On the other side of the equation we have, first,
|
||
Rx2 + Sy2 = H
|
||
[R*x^2 +S*y^2 = H]
|
||
which represents the work done in overcoming the resistance of the circuits in units of time. This is converted into heat. They may be written
|
||
½ddt (Lx2+2Mxy+ny2) + ½ dLdt x2 +dMdtxy+ ½ dNdty2
|
||
Intrinsic Energy of the Currents.
|
||
(33) If L, M, N are constant, the whole work of the electromotive forces which is not spent against resistance will be devoted to the development of the currents. The whole intrinsic energy of the currents is therefore
|
||
½Lx2 + Mxy + ½ Ny2 = E ............ (10)
|
||
[1/2*L*x^2 + M*x*y*1/2*N*y^2=E]
|
||
|
||
This energy exists in a form imperceptible to our senses, probably as actual motion, the seat of this motion being not merely the conducting circuits, but the space surrounding them.
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
471
|
||
|
||
Mechanical Action between Conductors.
|
||
|
||
(34) The remaining terms,
|
||
½ dLdt x2 + dMdtxy+ ½ dNdty2 = W . . . . . . . . . . . . . . . . . . . (11)
|
||
|
||
[1/2*d*L/d*t *x^2+d*M/d*t*x*y+1/2*d*N/d*t*y^2 = W]
|
||
represent the work done in unit of time arising from the variations of L, M, and N, or what is the same thing, alterations in the form and position of the conducting circuits A and B.
|
||
Now if work is done when a body is moved, it must arise from ordinary mechanical force acting on the body while it is moved. Hence this part of the expression shows that there is a mechanical force urging every part of the conductors themselves in that direction in which L, M, and N will be most increased.
|
||
The existence of the electromagnetic force between conductors carrying currents is therefore a direct consequence of the joint and independent action of each current on the electromagnetic field. If A and B are allowed to approach a distance ds, so as to increase M from M to M' while the currents are x and y, then the work done will be
|
||
(M'-M) xy,
|
||
and the force in the direction of ds will be
|
||
dMdsxy
|
||
|
||
and this will be an attraction x and y are the same sign, and if M is increased as A and B approach.
|
||
|
||
It appears, therefore, that if we admit that the unresisted part of the electromotive force goes on as long as it acts, generating a self-persistent state of the current, which we may call (from mechanical analogy) its electromagnetic momentum, and that this momentum depends on circumstances external to the conductor, then both induction of currents and electromagnetic attractions may be proved by mechanical reasoning.
|
||
|
||
What I have called electromagnetic momentum is the same quantity which is called by FARADAY* the electronic state of the circuit, every change of which involves the action of an electromotive force, just as change of momentum involves the action of mechanical force.
|
||
|
||
If, therefore, the phenomena described by FARADAY in the Ninth Series of his Experimental Researches were the only known facts about electric currents, the laws of Ampere relating to the attraction of conductors carrying currents, as well as those of Faraday about the mutual induction of currents, might be deduced by mechanical reasoning.
|
||
|
||
In order to bring these results within the range of experimental verification, I shall next investigate the
|
||
|
||
case of a single current, of two currents, and of the six currents in the electric balance, so as to enable
|
||
|
||
the experimenter to determine the values of L, M, N.
|
||
|
||
* Experimental Researches, Series
|
||
|
||
I. 60 &c.
|
||
|
||
472
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
Case of a single Circuit.
|
||
|
||
(35) The equation of the current x in a circuit whose resistance is R, and whose coefficient of selfinduction is L, acted on by an external electromotive force \, is
|
||
|
||
[\-R*x =d/dt*L*x]
|
||
|
||
\-Rx =ddtLx. . . . . . . . . . . (13)
|
||
|
||
When \ is constant, the solution is of the form
|
||
-R -t
|
||
x=b+(a-b)e L where a is the value of the current at the commencement, and b is the final value; The total quantity of electricity which passes in time t, where t is great, is
|
||
t
|
||
юL o xdt = bt+(a-b) R . . . . . . . . . . . (14)
|
||
|
||
the integral of x2 with respect to the time is
|
||
|
||
t
|
||
|
||
ю ( ) L 3b+a o x2dt = b2 t +(a-b) R 2
|
||
|
||
. . . . . . . . . (15)
|
||
|
||
The actual current changes gradually from the initial value a to the final value b, but the values of the integrals of x and x2 are the same as if a steady current of intensity 1/2(a+b) were to flow for a time 2L/R, and where then succeeded by the steady current b. The time 2L/r is generally so minute a fraction of a second, that the effects on the galvanometer and dynamometer may be calculated as if the impulses were instantaneous.
|
||
If the circuit consists of a battery and a coil, then, when the circuit is first completed, the effects are the same as if the current had only half of its final strength during the time 2L/R. This diminution of the current, due to induction, is sometimes called the countercurrent.
|
||
(36) If an additional resistance r is suddenly thrown into the circuit, as by breaking contact, so as to \
|
||
force the current to pass through a thin wire of resistance r, then the original current is a= R , and the final current is b = \ / R+r.
|
||
The current of induction is then 1/2 \ (2R+r)/R(R+r) , and continues for a time 2 (L/R+r). This current is greater than that which the battery can maintain in the two wires R and r, and may be sufficient to ignite the thin wire r.
|
||
When contact is broken by separating the wires in air, this additional resistance is given by the interposed air, and since the electromotive force across the new resistance is very great, a spark will be forced across.
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
473
|
||
|
||
If the electromotive force is of the form E sin pt, as in the case of a coil revolving in a magnetic field, then
|
||
|
||
x = E/\ sin (pt-a)
|
||
|
||
where g^2 = r^2 +L^2*p^2, and tan a = Lp/R.
|
||
|
||
Case of two Circuits.
|
||
|
||
(37) Let R be the primary circuit and S the secondary circuit, then we have a case similar to that of the induction coil.
|
||
|
||
The equations of currents are those marked A and B, and we may here assume L, M, N as constant because here is no motion of the conductors. The equations then become
|
||
|
||
Rx + L(dy/dt)
|
||
|
||
+M(dy/dt) = \
|
||
|
||
. . . . . . . . . . . . . (13*) Sy + M(dx/td)+N(dy/dt) = 0
|
||
|
||
To find the total quantity of electricity which passes, we have only to integrate these equations with respect to t; then if xo, yo be the strengths of the currents at time 0 and x1, y1 at the time t1 and if X, Y be the quantities of electricity passed through each circuit during time t,
|
||
|
||
X = 1/R {\t+L(xo- x1)+M(yo- y1)},
|
||
. . . . . . . . . . . . . Y = 1/S{M(xo- x1)+N(yo- y1)},
|
||
|
||
(14*)
|
||
|
||
When the circuit R is completed, then the total currents up to time t, when t is great, are found by making
|
||
|
||
xo= 0 x1 = \/R, yo = 0, y1 = 0 ;
|
||
|
||
then
|
||
|
||
X = x1(t-L/R), Y = - (M/S) x1 . . . . . . . . . . . . . (15*)
|
||
The value of the total counter-current in R is therefore independent of the secondary circuit, and the induction current in the secondary circuit depends only on M, the coefficient of induction between the coils, S the resistance of the secondary coil, and x1 the final strength of the current in R.
|
||
|
||
When the electromotive force \ ceases to act, there is an extra current in the primary circuit, and a positive induced current in the secondary circuit, whose values are equal and opposite to those produced on making contact.
|
||
|
||
(38) All questions relating to the total quantity of transient currents, as measured by the impulse given to the magnet of the galvanometer, may be solved in this way without the necessity of a complete solution of the equations. The heating effect of
|
||
|
||
474
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
the current, and the impulse it gives to the suspended coil of Weber's dynamometer, depend on the square of the current at every instant during the short time it lasts. Hence we must obtain the solution of the equations, and from the solution we may find the effects both on the galvanometer and dynamometer; and we may then make use of the method of Weber for estimating the intensity and duration of a current uniform while it lasts which would produce the same effects.
|
||
|
||
(39) Let n1, n2 be the roots of the equation
|
||
|
||
(LN-MN^2)n^2+(RN+LS)n + RS = 0
|
||
|
||
and let the primary coil be acted on by a constant electromotive force Rc so that c is the constant current it could maintain; then the complete solution of the equations for making contact is
|
||
|
||
x=c/S * (n1n2/n1-n2){(s/n1+N)*e^(n1*t) - (S/n2+N)*e^(n2*t) + S*(n1-n2/n1*n2)}, . . . . . (17)
|
||
|
||
y = (c*M/S)*( n1n2/ n1-n2) {e^(n1*t) - e^( n2*t)} . . . . . . . . . . . . . . . . . . . (18)
|
||
|
||
From these we obtain for calculating the impulse on the dynamometer,
|
||
|
||
юx^2dt = c^2{t-3/2*L/R-1/2*(m^2/RN+LS)}, юy^2dt = c^2*1/2(M^2*R/(S(RN+LS)) )
|
||
|
||
. . . . . . . . . . . . . (19) . . . . . . . . . . . . . (20)
|
||
|
||
The effects of the current in the secondary coil on the galvanometer and dynamometer are the same as those of a uniform current
|
||
|
||
-1/2*c*(MR/RN+LS))
|
||
|
||
for a time
|
||
L 2*(L/R+N/S). (note: original shows L/R as R )
|
||
(40) The equation between work and energy may be easily verified. The work done by the electromotive force is
|
||
\юxdt = c^2*(Rt - L). Work done in overcoming resistance and producing heat,
|
||
Rюx^2*dt+Sюy^2*dt = c^2*(Rt-3/2*L).
|
||
Energy remaining in the system,
|
||
= 1/2c^2*L
|
||
(41) If the circuit R is suddenly and completely interrupted while carrying a current c1 then the equation of the current in the secondary coil would be
|
||
y = c*(M/N)*e^(-(S/N)*t) .
|
||
This current begins with a value c * (M/N) and gradually disappears.
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
475
|
||
|
||
The total quantity of electricity is c*(M/S), and the value of юy^2dt is c^2 *(M^2/2SN). The effects on the galvanometer and dynamometer are equal to those of a uniform current 1/2*c*(M/N) for a time 2(N/S).
|
||
|
||
The heating effect is therefore greater than that of the current on making contact.
|
||
|
||
(42) If an electromotive force of the form \=E cospt acts on the circuit R, then if the circuit S is removed, the value of x will be
|
||
|
||
x=(E/A) sin (pt-a), where A^2 = R^2+L^2p^2, and tan a=Lp/R.
|
||
|
||
The effect of the presence of the circuit S in the neighbourhood is to alter the value of A anda, to that which they would be if R become R+ p^2*(MS/S^2+p^2*N^2).
|
||
|
||
Hence the effect of the presence of the circuit S is to increase the apparent resistance and diminish the apparent self-induction of the circuit R.
|
||
|
||
On the Determination of Coefficients of Induction by the Electric Balance.
|
||
|
||
(43) The electric balance consists of six conductors joining four points, A C D E, two and two. One pair, A C, of these points is connected through the battery B. The opposite pair, D E, is connected through the galvanometer G. Then if the resistances of the four remaining conductors are represented by P, Q, R, S, and the currents in them by x, x-z, y and y+z, the current through G will be z. Let the potentials at the four points be A, C, D, E. Then the conditions of steady currents may be found from the equations
|
||
|
||
Px= A - D Ry = A - E Gz = D - E
|
||
|
||
Q(x-z) = D - C S(y+z) = E - C B(x+y) = - A+C+F.
|
||
|
||
. . . . . . . . . . . . . . . (21)
|
||
|
||
Solving these equations for z, we find
|
||
|
||
z {1/p+1/Q+1/R+1/S+B*(1/P+1/R)*(1/Q+1/S)+G*(1/P+1/Q)*(1/R+1/S) + BG/PQRS*(P+Q+R+S)} = F(1/PS1/QR).
|
||
|
||
MDCCCLXV.
|
||
|
||
3 T
|
||
|
||
476
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
In this expression F is the electromotive force of the battery, z the current through the galvanometer when it has become steady. P, Q, R, S the resistances in the four arms.B that of the battery and electrodes, and G that of the galvanometer.
|
||
|
||
(44) If PS=QR, then z=0, and there will be no steady current, but a transient current through the galvanometer may be produced on making or breaking circuit on account of induction, and the indications of the galvanometer may be used to determine the coefficients of induction, provided we understand the actions which take place. We shall suppose PS=QR, so that the current z vanishes when sufficient time is allowed, and
|
||
|
||
x (P+Q) = y(R+S)= (F(P+Q)*(R+S))/(P+Q)(R+S)+B(P+Q)(R+S)
|
||
|
||
Let the induction coefficients between P, Q, R, S, be given by the following Table, the coefficient of induction of P on itself beingp, between P and Q, h, and so on.
|
||
|
||
PQ R S P ph k l Qhq mn
|
||
|
||
Let g be the coefficient of induction of the galvanometer
|
||
|
||
on itself, and let it be out of reach of the inductive
|
||
|
||
influence of P, Q, R, S (as it must be in order to avoid
|
||
|
||
direct action of P, Q, R, S on the needle). Let X, Y, Z be the integrals of x, y, z with respect to t. At making contact x, y, z are zero. After a time z disappears, and x and y reach constant values. The equations for each conductor will therefore be
|
||
|
||
PX +(p+h) x +(k+l)y = юAdt-юDdt, Q(X-Z)+(h+q)x+(m+n)y = юDdt-юCdt, RY +(k+m)x+(r+o)y = юAdt-юEdt, (24) S(Y+Z)+(l+n)x+(o+s)y = юEdt-юCdt,
|
||
|
||
. . . . . . .
|
||
|
||
GZ = юDtd - юEdt
|
||
Solving these equations for Z we find
|
||
Z{1/P+1/Q+1/R+1/S+B(1/P+1/R)(1/Q+1/S)+G(1/P+1/Q)(1/R+1/S) (BG/PQRS(P+Q+R+S))} = - F(1/PS){p/P-q/Q-r/R+S/S+h(1/P-1/Q)+k(1/R-1/P)+l(1/R+1/Q)-m(1/P+1/S)+n(1/Q-1/S)+o(1/S-1/R)}
|
||
(45) Now let the deflection of the galvanometer by the instantaneous current whose intensity is Z be a. Let the permanent deflection produced by making the ration of PS to QR, ʌ instead of unity, be ɽ. Also let the time of vibration of the galvanometer needle from rest to rest be T.
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
477
|
||
|
||
Thencallingthequantity
|
||
|
||
p/P-q/Q-r/R+s/S+h(1/P-1/Q)+k(1/R-1/P)+l(1/R+1/Q)-m(1/P+1/S)+n(1/Q-1/S)+o(1/S-1/R) = X, (26)
|
||
|
||
we find
|
||
|
||
Z/z = (2sin1/2a/tan ɽ) T/ʋ = X/1 - ʎ
|
||
|
||
(this letter looks more like, but it is not listed under Greek in any font style.)
|
||
|
||
In determining ʏ by experiment, it is best to make the alteration of resistance in one of the arms by means of the arrangement described by Mr. JENKINS in the Report of the British Association for 1863, by which any value of ʌ from 1 to 1.01 can be accurately measured.
|
||
|
||
We observe (ɴ) the greatest deflection produced by the permanent current when the resistance of one of the arms is increased in the ration of 1 to ʌ , the galvanometer not being in circuit till a little while after the connexion is made with the battery.
|
||
|
||
In order to eliminate the effects of resistance of the air, it is best to vary ʌ till ɴ = 2ɲ nearly; then
|
||
|
||
ʏ=T* 1/ʋ(1-ʌ)((2 sin 1/2* ɲ)/(tan 1/2* ɴ)) . . . . . . . . . (28)
|
||
|
||
If all the arms of the balance except P consist of resistance coils of very fine wire of no great length and doubled before being coiled, the induction coefficients belonging to these coils will be insensible, and ʏ will be reduced to p/P . The electric balance therefore affords the means of measuring the selfinduction of any circuit whose resistance is known.
|
||
|
||
(46) It may also be used to determine the coefficient of induction between two circuits, as for instance, that between P and S which we have called m ; but it would be more convenient to measure this by directly measuring the current, as in (37), without using the balance. We may also ascertain the equality of p/P and q/Q by there being no current of induction, and thus, when we know the value of p, we may determine that of q by a more perfect method than the comparison of deflections.
|
||
Exploration of the Electromagnetic Field.
|
||
(47) Let us now suppose the primary circuit A to be of invariable form, and let us explore the electromagnetic field by means of the secondary circuit B, which we shall suppose to be variable in form and position.
|
||
We may begin by supposing B to consist of a short straight conductor with its extremities sliding on two parallel connecting rails, which are put in connexion at some distance from the sliding-piece.
|
||
3 T 2
|
||
|
||
478
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
Then, if sliding the moveable conductor in a given direction increases the value of M, a negative electromotive force will act in the circuit B, tending to produce a negative current in B during the motion of the sliding-piece.
|
||
|
||
If a current be kept up in the circuit B, then the sliding-piece will itself tend to move in that direction, which causes M to increase. At every point of the field there will always be a certain direction such that a conductor moved in that direction does not experience any electromotive force in whatever direction its extremities are turned. A conductor carrying a current will experience no mechanical force urging it in that direction or the opposite.
|
||
|
||
This direction is called the direction of the line of magnetic force through that point. Motion of a conductor across such a line produces electromotive force in the direction perpendicular to the line and to the direction of motion, and a conductor carrying a current is urged in a direction perpendicular to the line and to the direction of the current.
|
||
|
||
(48) We may next suppose B to consist of a very small plane circuit capable of being placed in any position and of having its plane turned in any direction. The value of M will be greatest when the plane of the circuit is perpendicular to the line of magnetic force. Hence if a current is maintained in B it will tend to set itself in this position, and will of itself indicate, like a magnet, the direction of the magnetic force.
|
||
|
||
On Lines of Magnetic Force.
|
||
|
||
(49) Let any surface be drawn, cutting the lines of magnetic force, and on this surface let any system of lines be drawn at small intervals, so as to lie side by side without cutting each other. Next, let any line be drawn on the surface cutting all these lines and let a second line be drawn near it, its distance from the first being such that the value of M for each of the small spaces enclosed between these two lines and the lines of the first system is equal to unity.
|
||
|
||
In this way let more lines be drawn so as to form a second system, so that the value of M for every reticulation formed by the intersection of the two systems of lines is unity. Finally, from every point of
|
||
|
||
intersection of these reticulations let a line be drawn through the field, always coinciding in direction with the direction of magnetic force.
|
||
(50) In this way the whole field will be filled with lines of magnetic force at regular intervals, and the properties of the electromagnetic field will be completely expressed by them.
|
||
For, 1st, If any closed curve be drawn in the field, the value of M for that curve will be expressed by the number of lines of force which pass through that closed curve. 2ndly. If this curve be a conducting circuit and be moved through the field, and electromotive force will act in it, represented by the rate of decrease of the number of lines passing through the curve. 3rdly. If a current be maintained in the circuit, the conductor will be acted on by forces tending to move it so as to increase the number of lines passing through it, and
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
479
|
||
|
||
the amount of work done by these forces is equal to the current in the circuit multiplied by the number of additional lines.
|
||
4thly. If a small plane circuit be placed in the field, and be free to turn, it will place its plane perpendicular to the lines of force. A small magnet will place itself with its axis in the direction of the lines of force.
|
||
5thly. If a long uniformly magnetized bar is placed in the field, each pole will be acted on by a force in he direction of the lines of force. The number of lines of force passing through unit of area is equal to the force acting on a unit pole multiplied by a coefficient depending on the magnetic nature of the medium, and called the coefficient of magnetic induction. In fluids and isotropic solids the value of this coefficient ʅ is the same in whatever direction the lines of force pass through the substance, but in crystallized, strained, and organized solids the value of ʅ may depend on the direction of the lines of force with respect to the axes of crystallization, strain, or growth.
|
||
|
||
In all bodies ʅ is affected by temperature, and in iron it appears to diminish as the intensity of the magnetization increases.
|
||
|
||
On Magnetic Equipotential Surfaces.
|
||
|
||
(51) If we explore the field with a uniformly magnetized bar, so long that one of its poles is in a very weak part of the magnetic field, then the magnetic forces will perform work on the other pole as it moves about the field.
|
||
|
||
If we start from a given point, and move this pole from it to any other point, the work performed will be independent of the path of the pole between the two points; provided that no electric current passes between the different paths pursued by the pole.
|
||
|
||
Hence, when there are no electric currents but only magnets in the field, we may draw a series of surfaces such that the work done in passing from one to another shall be constant whatever be the path pursued between them. Such surfaces are called Equipotential Surfaces, and in ordinary cases are perpendicular to the lines of magnetic force.
|
||
|
||
If these surfaces are so drawn that, when a unit pole passes from any one to the next in order, unity of work is done, then the work done in any motion of a magnetic pole will be measured by the strength of the pole multiplied by the number of surfaces which it has passed through in the positive direction.
|
||
(52) If there are circuits carrying electric currents in the field, then there will still be equipotential surfaces in the parts of the field external to the conductors carrying the currents, but the work done on a unit pole in passing from one to another will depend on the number of times which the path of the pole circulates round any of these currents. Hence the potential in each surface will have a series of values in arithmetical progression, differing by the work done in passing completely round one of the currents in the field. The equipotential surfaces will not be continuous closed surfaces, but some of them
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
480
|
||
|
||
will be limited sheets, terminating in the electric circuit as their common edge or boundary. The number of these will be equal to the amount of work done on a unit pole in going round the current, and this by the ordinary measurement = 4ʋ ɶ, where ɶ is the value of the current.
|
||
|
||
These surfaces, therefore, are connected with the electric current as soap-bubbles are connected with a ring in M. PLATEAU'S experiments. Every current ɶ has 4ʋ ɶ surfaces attached to it. These surfaces have the current for their common edge, and meet it at equal angles. The form of the surfaces in other parts depends on the presence of other currents and magnets, as well as on the shape of the circuit to which they belong.
|
||
|
||
PART III -- GENERAL EQUATIONS OF THE ELECTROMAGNETIC FIELD.
|
||
|
||
(53) Let us assume three rectangular directions in space as the axes of x, y, and z, and let all quantities having direction to be expressed by their components in these tree directions.
|
||
|
||
Electrical Currents (p, q, r).
|
||
|
||
(54) An electrical current consists in the transmission of electricity from one part of a body to another. Let the quantity of electricity transmitted in unit of time across unit of area perpendicular to the axis of x be called p, then p is the component of the current at that place in the direction of x.
|
||
|
||
We shall use the letters p, q, r to denote the components of the current per unit of area in the directions of x, y, z.
|
||
|
||
Electrical Displacements (f, g, h).
|
||
|
||
(55) Electrical displacement consists in the opposite electrification of the sides of a molecule or particle of a body which may or may not be accompanied with transmission through the body. Let the quantity
|
||
|
||
of electricity which would appear on the faces dy.dz of an element dx, dy, dz cut from the body be f.dy.dz, then f is the component of electric displacement parallel to x. We shall use f, g, h to denote he electric displacements parallel to x, y, z respectively.
|
||
|
||
The variations of the electrical displacement must be added to the currents p, q, r to get the total motion of electricity, which we may calp', q', r', so that
|
||
|
||
p' = p + df/dt , q' = q + dg/dt , r' =r + dh/dt,
|
||
|
||
. . . . . . . (A)
|
||
|
||
Electromotive Force (P, Q, R).
|
||
|
||
(56) Let P, Q, R represent the components of the electromotive force at any point. Then P represents the difference of potential per unit of length in a conductor
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
481
|
||
|
||
placed in the direction of x at the given point. We may suppose an indefinitely short wire placed parallel to x at a given point and touched, during the action of the force P, by two small conductors, which are then insulated and removed from the influence of the electromotive force. The value of P might then be ascertained by measuring the charge of the conductors.
|
||
|
||
Thus if l be the length of the wire, the difference of potential at its ends will be Pl, and if C be the capacity of each of the small conductors the charge on each will be 1/2CPl .Since the capacities of moderately large conductors, measured on the electromagnetic system, are exceedingly small, ordinary electromotive forces arising from electromagnetic actions could hardly be measured in this way. In practice such measurements are always made with l on conductors, forming closed or nearly closed circuits.
|
||
|
||
Electromagnetic Momentum (F, G, H).
|
||
|
||
(57) Let F, G, H represent the components of electromagnetic momentum at any point of the field, due to any system of magnets or currents.
|
||
|
||
Then F is the total impulse of the electromotive force in the direction of x that would be generated by the removal of these magnets or currents from the field, that is, if P be the electromotive force at any instant during the removal of the system.
|
||
|
||
юF= Pdt.
|
||
|
||
Hence the part of the electromotive force which depends on the motion of magnets or currents in the field, or their alteration of intensity, is
|
||
P = -dF/dt, Q = dG/dt, R = - dH/dt . . . . . . . . . . . . . . . . . (29)
|
||
|
||
Electromagnetic Momentum of a Circuit.
|
||
(58) Let s be the length of the circuit, then if we integrate
|
||
ю(F(dx/ds)+G(dy/ds)+H(dz/ds)ds . . . . . . . . . . . . . . . . . . (30)
|
||
round the circuit, we shall get the total electromagnetic momentum of the circuit, or the number of lines of magnetic force which pass through it, the variations of which measure the total electromotive force in the circuit. This electromagnetic momentum is the same thing to which Professor Faraday has applied the name of the Electrotonic State. If the circuit be the boundary of the elementary area dydz, then its electromagnetic momentum is
|
||
(dH/dy - dG/dz) dydz ,
|
||
and this is the number of lines of magnetic force which pass through the area dy dz.
|
||
Magnetic Force (ɲ, ɴ, ɶ). (59) Let ɲ, ɴ, ɶ represent the force acting on a unit magnetic pole placed at the given point resolved in the direction of x, y and z.
|
||
|
||
482
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
Coefficient of Magnetic Induction (ʅ).
|
||
|
||
(60) Let ʅ be the ratio of the magnetic induction in a given medium to that in air under an equal magnetizing force, then the number of lines of force in unit of area perpendicular to ʖ will be ʅɲ (ʅ is a quantity depending on the nature of the medium, its temperature, the amount of magnetization already produced, and in crystalline bodies varying with the direction).
|
||
|
||
(61) Expressing the electric momentum of small circuits perpendicular to the three axes in this notation, we obtain the following
|
||
|
||
Equations of Magnetic Force.
|
||
|
||
ʅɲ = (dH/dy - dG/dz) , ʅɴ = (dF/dz - dH/dx) , . . . . . . . . . . . . . (B) ʅɶ = (dG/dx - dF/dy) .
|
||
|
||
Equations of Currents.
|
||
|
||
(62) It is known from experiment that the motion of a magnetic pole in the electromagnetic field in a closed circuit cannot generate work unless the circuit which the pole describes passes round an electric current. Hence, except in the space occupied by the electric currents,
|
||
|
||
ɲdx + ɴdy + ɶdz = dʔ . . . . . . . . . . . . . . . (31)
|
||
|
||
a complete differential of ʔ, the magnetic potential.
|
||
|
||
The quantity ʔ may be susceptible of an indefinite number of distinct values, according to the number of times that the exploring point passes round electric currents in its course, the difference between successive values of ʔ corresponding to a passage completely round a current of strength c being 4ʋc.
|
||
|
||
Hence if there is no electric current,
|
||
|
||
dy - d ɴ / dz = 0;
|
||
|
||
but if there is a current of p',
|
||
|
||
dɶ/dy - d/dz = 4ʋp'.
|
||
|
||
Similarly,
|
||
|
||
d ɲ /dz - d ɶ /dx = 4ʋq' ,
|
||
|
||
d ɴ /dx - d ʖ /dy = 4ʋr'.
|
||
|
||
We may call these the Equations of Currents.
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
483
|
||
|
||
Electromotive Force in a Circuit.
|
||
|
||
(63) Let ʇ be the electromotive force acting round the circuit A, then
|
||
|
||
юʇ = (P(ds/dx)+Q(dy/ds)+R(dz/ds))ds , . . . . . . . . . . . . (32)
|
||
|
||
whereds is he element of length, and the integration is performed round the circuit.
|
||
Let the forces in he field be those due to the circuits A and B, then the electromagnetic momentum of A is
|
||
ю(F(dx/ds)+G(dy/ds)+H(dz/ds))ds= Lu + Mv , . . . . . . . . . . (33)
|
||
|
||
whereu and v are the currents in A and B and
|
||
|
||
ʇ = - d/dt(Lu+Mv) . . . . . . . . . . . . (34)
|
||
|
||
Hence, if there is no motion of the circuit A,
|
||
|
||
P = - dF/dt - dʗ/dx,
|
||
|
||
Q = - dG/dt -dʗ/dy,
|
||
|
||
.......... .
|
||
|
||
(35)
|
||
|
||
R = - dH/dt -dʗ/dz,
|
||
|
||
where ʗ is a function of x, y, z, and t, which is indeterminate as far as regards the solution of the above equations, because he terms depending on it will disappear on integrating round the circuit. The quantity ʗ can always, however, be determined in any particular case when we know the actual conditions of the question. The physical interpretation of ʗ is, that it represents the electric potential at each point of space.
|
||
Electromotive Force on a Moving Conductor.
|
||
(64) Let a short straight conductor of length a, parallel to the axis of x, move with a velocity whose components are dx/dt, dy/dt, dz/dt, and let its extremities slide along two parallel conductors with a velocity ds/dt. Let us find the alteration of the electromagnetic momentum of the circuit of which this arrangement forms a part.
|
||
In unit of time this moving conductor has traveled distances dx/dt, dy/dt, dz/dt along the directions of the three axes, and at the same time the lengths of the parallel conductors included in the circuit have each been increased by ds/dt.
|
||
Hence the quantity
|
||
ю(F(dx/ds)+G(dy/ds)+H(dz/ds) )ds
|
||
|
||
MDCCCLXV.
|
||
|
||
3U
|
||
|
||
485 484
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
will be increased by the following increments,
|
||
|
||
a (dF/dx dv/dt + dF/dydy/dt + dF/dzdz/dt), due to motion of conductor,
|
||
|
||
-a ds/dt (dF/dx dx/ds + dG/dx dy/ds + dH/dx da/ds), due to lengthening of circuit.
|
||
|
||
The total increment will therefore be
|
||
|
||
a (dF/dy - dG/dx)dy/dt - a(dH/dx - dF/dz)dz/dt;
|
||
|
||
or, by the equations of Magnetic Force (8),
|
||
|
||
- a(ʅɶdy/dt - ʅɴ dz/dt).
|
||
|
||
If P is the electromotive force in the moving conductor parallel to x referred to unit of length, then the actual electromotive force is Pa; and since this is measured by the decrement of the electromagnetic momentum of the circuit, the electromotive force due to the motion will be
|
||
|
||
P =ʅɶdy/dt- ʅɴ dz/dt . . . . . . . . . . . . . . . (36)
|
||
|
||
(65) The complete equations of electromotive force on a moving conductor may now be written as follows: --
|
||
|
||
Equations of Electromotive Force.
|
||
|
||
P = ʅ( ɶ dy/dt - ɴ dz/dt) - dF/dt - dʗ /dx , Q = ʅ(ɲ dz/dt - ɶ dx/dt) - dG/dt - dʗ /dy , R = ʅ( ɴ dx/dt - ɲ dy/dt) - dH/dt - dʗ /dz .
|
||
|
||
. . . . . . . . (D)
|
||
|
||
The first term on the right-hand side of each equation represents the electromotive force arising from the motion of the conductor itself. This electromotive force is perpendicular to the direction of motion and to the lines of magnetic force; and if a parallelogram be drawn whose sides represent in direction and magnitude the velocity of the conductor and the magnetic induction at that point of the field, then the area of the parallelogram will represent the electromotive force due to the motion of the conductor, and the direction of the force is perpendicular to the plane of the parallelogram.
|
||
|
||
The second term in each equation indicated the effects of changes in the position or strength of magnets or currents in the field.
|
||
|
||
The third term shows the effect of the electric potential ʗ. It has no effect in causing a circulating current in a closed circuit. It indicates the existence of a force urging the electricity to or from certain definite points in the field.
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
485
|
||
|
||
Electric Elasticity.
|
||
|
||
(66) When an electromotive force acts on a dielectric, it puts every part of the dielectric into a polarized condition, in which its opposite sides are oppositely electrified. The amount of this electrification depends on the electromotive force and on the nature of the substance, and, in solids having a structure defined by axes, on the direction of the electromotive force with respect to these axes. In isotropic substances, if k is the ratio of the electromotive force to the electric displacement, we may writ the
|
||
|
||
Equations of Electric Elasticity,
|
||
|
||
P = kf, Q = kg, R = kh.
|
||
|
||
. . . . . . . . . . . . . . . . . (E)
|
||
|
||
Electric Resistance.
|
||
|
||
(67) When an electromotive force acts on a conductor it produces a current of electricity through it. This effect is additional to the electric displacement already considered. In solids of complex structure, the relation between the electromotive force and the current depends on their direction through the solid. In isotropic substances, which alone we shall here consider, if ʌ is the specific resistance referred to unit of volume, we may write the
|
||
|
||
Equations of Electric Resistance,
|
||
|
||
P = - ʌ p, Q = - ʌ q, R = - ʌr.
|
||
|
||
. . . . . . . . . . . . . . . (F)
|
||
|
||
Electric Quantity.
|
||
(68) Let e represent the quantity of free positive electricity contained in unit of volume at any part of the field, then, since this arises from the electrification of the different parts of the field not neutralizing each other, we may write the
|
||
Equation of Free Electricity,
|
||
e + df/dx + dg/dy + dh/dz = 0 . . . . . . . . . . . . . . . (G)
|
||
(69) If the medium conducts electricity, then we shall have another condition, which may be called, as in hydrodynamics, the
|
||
Equation of Continuity,
|
||
de/dt + dp/dx + dg/dy + dr/dz = 0
|
||
(70) In these equations of the electromagnet field we have assumed twenty variable
|
||
3 U 2
|
||
|
||
486
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
quantities, namely
|
||
|
||
For Electromagnetic Momentum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F G H
|
||
|
||
"
|
||
|
||
Magnetic Intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ɲ ɴ ɶ
|
||
|
||
"
|
||
|
||
Electromotive Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P Q R
|
||
|
||
"
|
||
|
||
Current due to true conduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p q r
|
||
|
||
"
|
||
|
||
Electric Displacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . f g h
|
||
|
||
"
|
||
|
||
Total Current (including variation of displacement) . . . . . . . . . . . . . . . . . . . . . . . p' q' r'
|
||
|
||
"
|
||
|
||
Quantity of free Electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e
|
||
|
||
"
|
||
|
||
Electric Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ʗ
|
||
|
||
Between these twenty quantities we have found twenty equations, vis.
|
||
|
||
Three equations of Magnetic Force . . . . . . . . . . . . . . . . . (B)
|
||
|
||
"
|
||
|
||
Electric Currents . . . . . . . . . . . . . . . . . (C)
|
||
|
||
"
|
||
|
||
Electromotive Force . . . . . . . . . . . . . . . (D)
|
||
|
||
"
|
||
|
||
Electric Elasticity . . . . . . . . . . . . . . . . (E)
|
||
|
||
"
|
||
|
||
Electric Resistance . . . . . . . . . . . . . . . . (F)
|
||
|
||
"
|
||
|
||
Total Currents . . . . . . . . . . . . . . . . . (A)
|
||
|
||
One equation of Free Electricity . . . . . . . . . . . . . . . . . . (G)
|
||
|
||
"
|
||
|
||
Continuity . . . . . . . . . . . . . . . . . . . (H)
|
||
|
||
These equations are therefore sufficient to determine all the quantities which occur in them, provided we know the conditions of the problem. In many questions, however, only a few of the equations are required.
|
||
|
||
Intrinsic Energy of the Electromagnetic Field.
|
||
(71) We have seen (33) that the intrinsic energy of any system of currents if found by multiplying half the current in each circuit into its electromagnetic momentum.
|
||
This is equivalent to finding the integral
|
||
E 1/2 ɇ (Fp' + Gq' + Hr')dV . . . . . . . . . . (37)
|
||
over all the space occupied by currents, where p, q, r are the components of currents, and F, G, H the components of electromagnetic momentum.
|
||
Substituting the values of p', q', r' from the equations of Currents (C), this becomes
|
||
1/8ʋ * ɇ {F(dɶ/dy - d ɴ/dz) +G(dɲ/dz - dɶ /dx) + H(dɴ /dx - dɲ /dy)} dV . . . . . . (38)
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
487
|
||
|
||
where ɲ, ɴ, ɶ are the components of magnetic intensity or the force on a unit magnetic pole, and ʅɲ, ʅɴ, ʅɶ are the components of the quantity of magnetic induction, or the number of lines of force in unit of area.
|
||
|
||
In isotropic media the value of ʅ is the same in all directions, and we may express the result more simply by saying that the intrinsic energy of any part of the magnetic field arising from its magnetization is
|
||
|
||
(ʅ /ɸ ʋ) * I^2
|
||
|
||
per unit of volume, where I is he magnetic intensity.
|
||
|
||
(72) Energy may be stored up in the field in a different way, namely, by the action of electromotive force in producing electric displacement. The work done by a variable electromotive force, P, in producing a variable displacement, f, is got by integrating
|
||
|
||
юPdf
|
||
|
||
from P = 0 to the given value of P. Since P = kf, equation (E), this quantity becomes
|
||
|
||
юkfdf=1/2(kf^2)=1/2(Pf).
|
||
|
||
Hence the intrinsic energy of any part of the field, as existing in the form of electric displacement, is
|
||
|
||
1/2ɇ{1/8*ʋ(ɲʅɲ+ɴʅɴ+ ɶʅ ɶ)+1/2(Pf+qg+rh)}dV . . . . . . . (I)
|
||
The first term of this expression depends on the magnetization of the field, and is explained on our theory by actual motion of some kind. The second term depends on the electric polarization of the field, and is explained on our theory by strain of some kind in an elastic medium.
|
||
(73) I have on a former occasion * attempted to describe a particular kind of motion and a particular kind of strain, so arranged as to account for the phenomena. In the present paper I avoid any hypothesis of this kind; and in using such words as electric momentum and electric elasticity in reference to the know phenomena of the induction of currents and the polarization of dielectrics, I wish merely to direct the mind of the reader to mechanical phenomena which will assist him in understanding the electrical ones. All such phrases in the present paper are to be considered as illustrative, not as explanatory.
|
||
(74) In speaking of the Energy of the field, however, I wish to be understood literally. All energy is the same as mechanical energy, whether it exists in the form of motion or in that of elasticity, or in any other form. The energy in electromagnetic phenomena is mechanical energy. The only question is, Where does it reside? On the old theories
|
||
* "On Physical Lines of Force," Philosophical Magazine, 1861-62
|
||
|
||
488
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
it resides in the electrified bodies, conducting circuits, and magnets, in the form of an unknown quality called potential energy, or the power of producing certain effects at a distance. On our theory it resides in the electromagnetic field, in the space surrounding the electrified and magnetic bodies, as well as in those bodies themselves, and is in two different forms, which may be described without hypothesis as magnetic polarization and electric polarization, or, according to a very probably hypothesis, as the motion and strain of one and the same medium.
|
||
|
||
(75) The conclusions arrived at in the present paper are independent of this hypothesis, being deduced from experimental facts of three kinds: --
|
||
1. The induction of electric currents by the increase or diminuation of neighbouring currents according to the changes in the lines of force passing through the circuit.
|
||
2. The distribution of magnetic intensity according to the variations of a magnetic potential.
|
||
3. The induction (or influence) of statical electricity through dielectrics.
|
||
|
||
PART IV.-- MECHANICAL ACTIONS IN THE FIELD.
|
||
|
||
Mechanical Force on a Moveable Conductor.
|
||
|
||
(76) We have shown (§§ 34 & 35) that the work done by the electromagnetic forces in aiding the motion of a conductor is equal to the product of the current in the conductor multiplied by the increment of the electromagnetic momentum due to the motion.
|
||
|
||
Let a short straight conductor of length a move parallel to itself in the direction of x, with its extremities on two parallel conductors. Then the increment of the electromagnetic momentum due to the motion of a will be
|
||
|
||
ɲ (dF/dx dx/ds + dG/dx dy/ds + dH/dx dz/ds ) ɷʖ
|
||
|
||
That due to the lengthening of the circuit by increasing the length of the parallel conductors will be -ɲ (dF/dx dx/dx + dF/dydy/ds + dF/dzdz/ds ) ɷʖ The total increment is ɲɷʖ{dy/dx - dF/dy} - dz/ds (dF/dz - dH/dx)}, which is by the equations of Magnetic Force (B), p. 482, ɲɷʖ (dy/dx ʘɶ - dz/ds ʅɴ) . Let X be the force acting along the direction of x per unit of length of the conductor, then the work done is Xɲɷʖ.
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
489
|
||
|
||
Let C be the current in the conductor, and let p', q', r' be its components, then
|
||
|
||
Xaɷ = Caɷxx(dy/ds ʅɶ - dz/ds ʅɴ) ,
|
||
|
||
or Similarly,
|
||
|
||
X = ʅɶq' - ʅɴr' Y = ʅɲ r' - ʅɶp' Z = ʅɴp' - ʅɲq'
|
||
|
||
. . . . . . . (J)
|
||
|
||
These are the equations which determine the mechanical force acting on a conductor carrying a current. The force is perpendicular to the current and to the lines of force, and is measured by the area of the parallelogram formed by lines parallel to the current and lines of force, and proportional to their intensities.
|
||
|
||
Mechanical Force on a Magnet.
|
||
|
||
(77) In any part of the field not traversed by electric currents the distribution of magnetic intensity may be represented by the differential coefficients of a function which may be called the magnetic potential. When there are no currents in the field, this quantity has a single value for each point, but its differential coefficients have only one value namely,
|
||
dʔ/dx =ɲ , dʔ/dy=ɴ, dʔ/dz=ɶ
|
||
Substituting these values of ɲ, ɴ, ɶ in the expression (equation 38) for the intrinsic energy of the field, and integrating by parts, it becomes
|
||
|
||
The expression
|
||
|
||
- ɇ [{ ʔ 1/8ʋ (dʅɲ/dx + dʅɴ/dy +dʅɶ/dz)}d V. ɇ (dʅɲ/dx + dʅɴ/dy +dʅɶ/dz)dV = ɇmdV . . . . . . . . . (39)
|
||
|
||
indicates the number of lines of magnetic force which have their origin within the space V. Now a magnetic pole is known to us only as the origin or termination of lines of magnetic force, and a unit pole is one which has 4ʋ lines belonging to it, since it produces unit of magnetic intensity at unit of distance over a sphere whose surface is 4ʋ.
|
||
|
||
E = -ɇ(1/2ʔ m)dV . . . . . . . . . . . . . . . . (40)
|
||
|
||
If there are two magnetic poles m1 and m2 producing potentials of ʔ1 and ʔ2in the field, then if m2, is moved a distance of dx, and is urged in that direction by a force X, then the work done is Xdx, and the decrease of energy in the field is
|
||
|
||
d(1/2(ʔ1+ʔ2)(m1 + m2)),
|
||
|
||
and these must be equal by the principle of Conservation of Energy.
|
||
|
||
490
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
Since the distribution ʔ1is determined by m1 , and ʔ2 by m2 , the quantities ʔ1m1 and ʔ2 m2 will remain constant.
|
||
|
||
It can be shown also, as Green has proved (Essay, p. 10), that
|
||
|
||
so that we get
|
||
|
||
m1ʔ2 = m2ʔ1 ,
|
||
|
||
Xdx = d(m2ʔ1), or
|
||
X=m2 dʔ1/dx = m2ɲ1 ,
|
||
|
||
whereɲ1 represents the magnetic intensity due to m1 .
|
||
|
||
Similarly,
|
||
|
||
Y = M2ɴ1 , Z = M2ɶ1 .
|
||
|
||
So that a magnetic pole is urged in the direction of the lines of magnetic force with a force equal to the product of the strength of the pole and the magnetic intensity.
|
||
(78) If a single magnetic pole, that is one pole of a very long magnet, be placed in the field, the only solution of ʔ is
|
||
ʔ= - m1ʅ 1/ ʏ , . . . . . . . . . . . . . . . . (41)
|
||
|
||
wherem1 is the strength of the pole and ʏ the distance from it.
|
||
|
||
The repulsion between two poles of strength m1 and m2 is
|
||
|
||
ʅʏ2x . . . . . . . . . . (42)
|
||
|
||
m2d ʔ1 /dr = m1m2 /
|
||
|
||
In air or any medium in which =1 this is simply m1m2 / ʏ2, but in other media the force acting
|
||
between two given magnetic poles is inversely proportional to the coefficient of magnetic induction for the medium. This may be explained by the magnetization of the medium induced by the action of the poles.
|
||
|
||
Mechanical Force on an Electrified Body.
|
||
|
||
(79) If there is no motion or charge of strength of currents or magnets in the field, the electromotive force is entirely due to variation of electric potential, and we shall have (§ 65)
|
||
|
||
P = - dʗ/dx,Q = -dʗ/dy, R-= dʗ/dz .
|
||
|
||
Integrating by parts the expression (I) for the energy due to electric displacement, and remembeing that P, Q, R vanish at an infinite distance, it becomes
|
||
|
||
1/2 ɇ { ʗ(df/dx + dg/dy + dh/dz)}dV ,
|
||
|
||
or by the equation of Free Electriciy (G), p. 485 - 1/2 ɇ (ʗe)dV .
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
491
|
||
|
||
By the same demonstration as was used in the case of the mechanical action on a magnet, it may be shown that the mechanical force on a small body containing a quantity e2 of free electricity placed in a field whose potential arising from other electrified bodies is Ɏ1 , has for components
|
||
|
||
X = e2 dɎ1/dx = - P1e2, Y = e2 dɎ1/dy = - Q1e2 , Z = e2 dɎ1/dz = - R1e2.
|
||
|
||
. . . . . . . (D)
|
||
|
||
So that an electrified body is urged in the direction of the electromotive force with a force equal to the product of the quantity of free electricity and the electromotive force. If the electrification of the field arises from the presence of a small electrified body containing e1 of free electricity, the only solution of Ɏ1 , is
|
||
|
||
Ɏ1 = k/4ʋ e1/r , . . . . . . . . . (43)
|
||
|
||
wherer is the distance from the electrified body. The repulsion between two electrified bodies e1 ,e2 is therefore
|
||
e2 dɎ1/dr = k/4ʋ e1e2/r2 . . . . . . . . . (43)
|
||
|
||
Measurement of Electrical Phenomena by Electrostatic Effects.
|
||
(80) The quantities with which we have had to do have been hitherto expressed in terms of the Electromagnetic System of measurement is founded on the mechanical action between electrified bodies, and is independent of, and incompatible with, the electromagnetic system; so that the units of
|
||
|
||
the different kinds of quantity have different values according to the system we adopt, and to pass from the one system to the other, a reduction of all the quantities is required.
|
||
|
||
According to the electrostatic system, the repulsion between two small bodies charged with quantities ɻ1 ,ɻ2 of electricity is
|
||
ɻ1ɻ2 /r2
|
||
|
||
wherer is the distance between them.
|
||
|
||
Let the relation of the two systems be such that one electromagnetic unit of electricity contains v electrostatic units; thenɻ1=ve1 and ɻ2=ve2 , and this repulsion becomes
|
||
v2 e1e2/r2 = k/4ʋ e1e2/r2byequation (44), . . . . . . . . . . (45)
|
||
|
||
whence k, the coefficient of "electric elasticity" in the medium in which the experiments are made, i.e. common air, is related to v, the number of electrostatic units in one electromagnetic unit, by the equation
|
||
k = 4ʋv2 . . . . . . . . . . . . . . . . . . . . . . . . (46)
|
||
|
||
MDCCCLXV
|
||
|
||
3 X
|
||
|
||
492
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
The quantity v may be determined by experiment in several ways. According to the experiments of MM. WEBER and KOHLRAUSCH,
|
||
|
||
v = 310,740,000 meters per second.
|
||
|
||
(81) It appears from this investigation, that if we assume that the medium which constitutes the electromagnetic field is, when dielectric, capable of receiving in every part of it an electric polarization, in which the opposite side of every element into which we may conceive the medium divided are oppositely electrified, and if we also assume that this polarization or electric displacement is proportional to the electromotive force which produces or maintains it, then we can show that electrified bodies in a dielectric medium will act on one another with forces obeying the same laws as are established by experiment.
|
||
|
||
The energy, by the expenditure of which electrical attractions and repulsions are produced, we suppose to be stored up in the dielectric medium which surrounds the electrified bodies, and not on the surface of those bodies themselves, which on our theory are merely the bounding surfaces of the air or other dielectric in which the true springs of action are to be sought.
|
||
|
||
Note on the Attraction of Gravitation.
|
||
|
||
(82) After tracing to the action of the surrounding medium both the magnetic and the electric attractions and repulsions, and finding them to depend on the inverse square of the distance, we are naturally led to inquire whether the attraction of gravitation, which follows the same law of the distance, is not also traceable to the action of a surrounding medium.
|
||
|
||
Gravitation differs from magnetism and electricity in this; that the bodies concerned are all of the same kind, instead of being of opposite signs, like magnetic poles and electrified bodies, and that the force between these bodies is an attraction and not a repulsion, as in the case between like electric and magnetic bodies.
|
||
|
||
The lines of gravitating force near two dense bodies are exactly of the same form as the lines of magnetic force near two poles of the same name; but whereas the poles are repelled, the bodies are attracted. Let E be the intrinsic energy of the field surrounding two gravitating bodies M1, M2, and let E' be the intrinsic energy of the field surrounding two magnetic poles m1, m2, equal in numerical value to M1, M2, and let X be the gravitating force acting during the displacement
|
||
|
||
Xɷʖ= ɷE ,
|
||
|
||
X'ɷʖ = ɷE' ;
|
||
|
||
now X and are equal in numerical value, but of opposite signs; so that
|
||
|
||
ɷE = -ɷE' or
|
||
|
||
E = C - E'
|
||
|
||
= C -ɇ (1/8ʋ )(ɲ ^2 + ɴ^2 +ɶ^2)dV
|
||
|
||
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
|
||
|
||
493
|
||
|
||
where ɲ, ,ɴ ɶ are the components of magnetic intensity. If R be the resultant gravitating force, and R' the resultant magnetic force at a corresponding part of the field,
|
||
|
||
R = -R', andɲ^2+ ɴ^2+ ɶ^2 = R^2 = R'^2
|
||
|
||
E = C - ɇ (1/8ʋ) R^2dV
|
||
|
||
The intrinsic energy of the field of gravitation must therefore be less wherever there is a resultant gravitating force.
|
||
|
||
As energy is essentially positive, it is impossible for any part of space to have negative intrinsic energy. Hence those parts of space in which there is no resultant force, such as the points of equilibrium in the space between the different bodies of a system, and within the substance of each body, must have an intrinsic energy per unit of volume greater than
|
||
|
||
1/8ʋ R^2 ,
|
||
|
||
where R is the greatest possible value of the intensity of gravitating force in any part of the universe.
|
||
|
||
The assumption, therefore, that gravitation arises from the action of the surrounding medium in the way pointed out, leads to the conclusion that every part of this medium possesses, when undistrubed, an enormous intrinsic energy, and that the presence of dense bodies influences the medium so as to diminish this energy wherever there is a resultant attraction.
|
||
|
||
As I am unable to understand in what way a medium can possess such properties, I cannot go any further in this direction in searching for the cause of gravitation.
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PART V. --- THEORY OF CONDENSERS.
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Capacity of a Condenser.
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(83) The simplest form of condenser consists of a uniform layer of insulating matter bounded by two conducting surfaces, and its capacity is measured by the quantity of electricity on either surface when the difference of potentials is unity.
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Let S be the area of either surface, a the thickness of the dielectric, and k its coefficient of electric elasticity; then on one side of the condenser the potential is ʗ1 and on the other side ʗ1+1, and within its substance
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dʗ dx = 1/a =Kf . . . . . . .. . . . . . . . . . . . (48)
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Since dʗ/dx and therefore f is zero outside the condenser, the quantity of electricity on its first surface = -Sf, and on the second + Sf. The capacity of the condenser is therefore Sf = S/ak in electromagnetic measure.
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3 X 2
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494
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PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
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Specific Capacity of Electric Induction (D).
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(84) If the dielectric of the condenser be air, then its capacity in electrostatic measure is S/4ʋa (neglecting corrections arising from the conditions to be fulfilled at the edges). If the dielectric have a capacity whose ratio to that of air is D, then the capacity of the condenser will be DS/4ʋa.
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Hence
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D = k0 /k , . . . . . . . . . . . . . . . (49)
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where k0 is the value of k in air, which is taken for unity.
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Electric Absorption.
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(85) When the dielectric of which the condenser is formed is not a perfect insulator, the phenomena of conduction are combined with those of electric displacement. The condenser, when left charged, gradually loses its charge, and in some cases, after being discharged completely, it gradually acquires a new charge of the same sign as the original charge, and this finally disappears. These phenomena have been described by Professor FARADAY (Experimental Researches, Series XL) and by Mr. F. JENKIN (Report of Committee of Board of Trade on Submarine Cables), and may be classed under the name of, "Electric Absorption."
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(86) We shall take the case of a condenser composed of any number of parallel layers of different materials. If a constant difference of potentials between its extreme surfaces if they are insulated, or , if they are connected by a conductor, a certain quantity of electricity may be urged through the conductor during the reestablishment of equilibrium.
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Let the thickness of the several layers of the condenser be a1 , a2 , &c. Let the values of k for these layers be respectively k1, k2, k3, and let
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a1 k2 + a2k2 +&c. = ak, . . . . . . . . . . . . . (50)
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where k is the "electric elasticity" of air, and a is the thickness of an equivalent condenser of air.
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Let the resistances of the layers be respectively r1 , r2 , &c, and let r1 + r2 + &c. = r be the resistance of the whole condenser, to a steady current through it per unit of surface.
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Let the electric displacement in each layer be f1 , f2&c. Let the electric current in each layer be p1 , p2&c. Let the potential on the first surface be ʗ1 , and the electricity per unit of surface e1 . Let the corresponding quantities at the boundary of the first and second surface be ʗ2 , and e2 , and so on. Then by equations (G) and (H),
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e1 = -f1 ,
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de1 / dt = - p1 ,
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e2 = -f1 - f2 , de2 /dt = - p2 ,
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&c.
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&c.
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. . . . . . . . . . (51)
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But by equations (E) and (F),
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ʗ1- ʗ2 = a1 k1 f1 = -r1 p1 ,
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ʗ2- ʗ3 = a2 k2 f2 = -r2 p2 ,
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&c.
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&c.&c.
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. . . . . . . . (52)
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After the electromotive force has been kept up for a sufficient time the current becomes the same in each layer, and
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p1 = p1 = &c. = p = ʗ / r ,
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whereʗ is the total difference of potentials between the extreme layers. We have then
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f1 = - ʗ/r r1 /a1 k1 , f2 = - ʗ/r r2 /a2 k2, &c. and
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e1 =ʗ/r r1 /a1 k1 ,
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e2 =ʗ/r (r2 /a2 k2 - r/ak1 ), &c.
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. . . . . (53)
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These are the quantities of electricity on the different surfaces.
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(87) Now let the condenser be discharged by connecting the extreme surfaces through a perfect conductor so that their potentials are instantly rendered equal, then the electricity on the extreme surfaces will be altered, but that on the internal surfaces will not have time to escape. The total difference of potentials is now
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ʗ1= a1 k1 e'1 + a2k2 (e'1 + e2) + a3 k3 (e'1 +e2 +e2), &c. = 0 , . . . . . (54) whence if e'1 is what e1 becomes at the instant of discharge,
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e'1 = ʗ/r r1/a1k1 - ʗ/ak = e1 - ʗ/ak . . . . . . . . . (55)
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The instantaneous discharge is therefore ʗ/ak, or the quantity which would be discharged by a condenser of air or of the equivalent thickness a, and it is unaffected by the want of perfect insulation.
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(88) Now let us suppose the connexion between the extreme surfaces broken, and the condenser left to itself, and let us consider the gradual dissipation of the internal charges. Let ʗ1 be the difference of potential of the extreme surfaces at any time t; then
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ʗ1 = a1k1f1 + a2k2f2 + &c. ; . . . . . . . . . . . . (56)
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but
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a1k1f1 = - r1df/dt ,
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||
a2k2f2 = - r2df/dt
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496
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PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.
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Hence f1 = A1 e^((-a1k1/r1)t) , F2= A2e^((-a2k2/r2)t) , &c.; and by referring to
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the values of e'1 e2 , &c., we find
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A1= ʗ / r1 /a1k1 - ʗ/ak ,
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A2= ʗ / r2 /a2k2 - ʗ/ak ,
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. . . . . . . .
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(57)
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|
||
&c. ;
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||
so that we may find for the differences of extreme potentials at any time,
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||
|
||
ʗ' = ʗ{( r1/r - a1k1/ak)e^((-a1k1/r1)*t) + (r2/r-a2k2/ak)e^((-a2k2/r2)*t)+ &c. . . . (58)
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(89) It appears from this result that if all the layers are made of the same substance, ʗ will be zero always. If they are of different substances, the order in which they are placed is indifferent, and the effect will be the same whether each substance consists of one layer, or is divided into any number of thin layers and arranged in any order among thin layers of the other substances. Any substance, therefore, the parts of which are not mathematically homogenous, though they may be apparently so, may exhibit phenomena of absorption. Also, since the order of magnitude of the coefficients is the same as that of the indices, the value of ʗ' can never change sign, but must start from zero, become positive, and finally disappear.
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(90) Let us next consider the total amount of electricity which would pass from the first surface to the second, if the condenser, after being thoroughly saturated by the current and then discharged, has its extreme surfaces connected by a conductor of resistance R. Let p be the current in this conductor; then, during the discharge,
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||
ʗ' = p1r1 + p2r2 + &c. = pR . . . . . . . . . . . . (59)
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