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On the Notation of MAXWELLs Field Equations

André Waser* Issued: 28.06.2000 Last revison:

Maxwells equations are the cornerstone in electrodynamics. Despite the fact that this equations are more than hundred years old, they still are subject to changes in content or notation. To get an impression over the historical development of Maxwells equations, the equation systems in different notations are summarized.


The complete set of the equations of James Clerk MAXWELL[15] are known in electrodynamics since 1865. These have been defined for 20 field variables. Later Oliver HEAVISIDE[11] and William GIBBS[23] have transformed this equations into the todays most used notation with vectors. This has not been happened without background noise[3], then at that time many scientists one of them has been MAXWELL himself was convinced, that the correct notation for electrodynamics must be possible with quaternions[5] and not with vectors. A century later EINSTEIN introduced Special Relativity and since then it was common to summarize MAXWELLs equations with four-vectors.

The search at magnetic monopoles has not been coming to an end, since DIRAC[4] introduced a symmetric formulation of MAXWELLs equations without using imaginary fields. But in this case the conclusion from the Special Theory of Relativity, that the magnetic field originates from relative motion only, can not be hold anymore.

The non-symmetry in MAXWELLs equations of the todays vector notation may have disturbed many scientists intuitively, what could be the reason, that they published an extended set of equations, which they sometime introduced for different applications. This essay summarizes the main different notation forms of MAXWELLs equations.

* André Waser, Birchli 35, CH-8840 Einsiedeln; copyright (2000) by AW-Verlag, Page 1

Maxwells Equations

The Original Equations

With the knowledge of fluid mechanics MAXWELL[15] has introduced the following eight equations to the electromagnetic fields (the right equations correspond with the original text, the left equations correspond with todays vector notation):











E: electrical field strength [V / m]

H: magnetic field strength [A / m]

D: electric displacement [As / m2]

B: magnetic Induction [Vs / m2]

j: electric current density [A / m2]

e: electric permeability [As / Vm]

m: magnetic permeability [Vs / Am]

Please note that the MAXWELL equation of today have became subset of the original equations which in turn have got an expansion with the introduction of the magnetic induction (1.31).

Today traditionally not included in MAXWELLs equations are FARADAYs law and sometime also OHMs law. Seldom the continuity equation (1.8) is even mentioned. But this equation defines the conservation of charge:


Real Expansions of Maxwells Equations

The HERTZ-Ansatz

Recently Thomas PHIPPS[20] has shown that Heinrich Rudolf HERTZ has suggested another possibility to adapt Maxwells equations. During Hertz life this was hardly criticized and his proposal was vastly forgotten after his death. Usually the differentials are partial derivative and not total derivatives as shown in the comparison (1.1) to (1.8) between MAXWELLs original equations and the todays vector notation. Now in the equations (1.26) and (1.27) HERTZ has substituted the partial derivatives with the total derivatives d. With this the Maxwell equations become invariant to the GALILEI-transformation:


The HARMUTH-Ansatz

Henning HARMUTH[5] and Konstantin MEYL[17] have gone a step further and suggested new equations, which differ to the DIRAC ansatz only in that point, that no source fields exists anymore. Harmuth has used this proposition to solve the problem of propagation of electromagnetic impulses in lossy media (impulses in media with low OHM dissipation) for the boundary conditions E = 0 and H = 0 for t 0:


FARADAYs law (1.26): (1.60) -(1.59) (1.65)

AMPÈREs law (1.27): (1.60) + (1.59) (1.66)

COULOMBs law (1.28): (1.62) -(1.61) (1.67)

(1.62) + (1.61) (1.68) In this notation the current density J and the charge density r are not understood as electric only but merely as electromagnetic entities. With an analysis of MÚNERA and GUZMÁN it can

be shown, that beneath the electric scalar field also a non-trivial magnetic scalar field should exist.


Imaginary Expansions of Maxwells Equations


The Notation in Minkowski-Space

In electrodynamics the relativistic notation is fully established. Because of the second EINSTEINian postulate[5] about the absolute constancy of the speed of light (therefore its independency of the speed of the light source or light detector) the four-dimensional notation has been developed. But the force field vectors E and H can not be used for four-vectors. But the potentials and the charge densities have been regarded as very optimal to formulate the electrodynamics in a compact form. If we first have an event vector

X = ict + x then it follows in MINKOWSKI-Space the invariance of the four-dimensional length ds

But then the possibility for a compact and easy calculation within the MINKOWSKI-Space comes to an end. To include the electric and magnetic fields, the following definition


additional rules also this notation can be expanded to eight dimensions. This should be presented in another paper.

Closing Remarks

Different notations to the MAXWELL equations are presented. Depending on the application one or another notation can be very useful, but at the end the presented variety is not satisfactory. This variety can be a hint, that the correct final form has not been found until now.

Many discussions have been presented about the existence of magnetic monopoles. But either the electric field is only a subjective measuring caused by the relative motion between charges -as it is said by the Special Theory of Relativity or the magnetic force field can be derived from a scalar potential field. In the first case magnetic monopoles can not exist, in the second case they can exist. Despite of extensive experiments no magnetic monopoles have been found until now. So we can conclude, that no magnetic potential fields must be postulated and that the non symmetry in MAXWELLs equations still are correct.

Proposals to enhance the symmetry with imaginary numbers are interesting but covers the danger, that with the simple mathematical tool i a symmetric formulation can be reached vastly, but that the physical models do become nebulous.


[1] AHARONOV Yakir & David BOHM, Significance of Electromagnetic Potentials in the Quantum Theory, Physical Review 115 /3 (01 August 1959)

[2] BARRETT Terence W., Comments on the HARMUTH ansatz: Use of a magnetic current density in the calculation of the propagation velocity of signals by amended Maxwell theory, IEEE Trans. Electromagn. Compatibility EMC30 (1988) 419420

[3] BORK Alfred M., Vectors Versus Quaternions The Letters of Nature, American Journal of Physics 34 (1966) 202-211

[4] DIRAC Paul André Morice, Quantised Singularities in the Electromagnetic Field, Proceedings of the London Royal Society A 133 (1931) 60-72

[5] EINSTEIN Albert, Zur Elektrodynamik bewegter Körper, Annalen der Physik und Chemie 17

(30. Juni 1905) 891-921

[6] GOUGH W., Quaternions and spherical harmonics, European Journal of Physics 5 (1984) 163171

[7] HAMILTON William Rowan, On a new Species of Imaginary Quantities connected with a theory of Quaternions, Proceedings of the Royal Irish Academy 2 (13 November 1843) 424-434

[8] HARMUTH Henning F. Corrections of Maxwells equations for signals I,, IEEE Transactions of Electromagnetic Compatibility EMC-28 (1986) 250-258

[9] HARMUTH Henning F. Corrections of Maxwells equations for signals II, IEEE Transactions of Electromagnetic Compatibility EMC-28 (1986) 259-266

[10] HARMUTH Henning F. Reply to T.W. Barretts Comments on the Harmuth ansatz: Use of a magnetic current density in the calculation of the propagation velocity of signals by amended Maxwell theory,IEEE Transactions of Electromagnetic Compatibility EMC-30 (1988) 420421

[11] HEAVISIDE Oliver, On the Forces, Stresses and Fluxes of Energy in the Electromagnetic Field, Philosophical Transactions of the Royal Society 183A (1892) 423

[12] HONIG William M., Quaternionic Electromagnetic Wave Equation and a Dual Charge-Filled Space, Lettere al Nuovo Cimento, Ser. 2 19 /4 (28 Maggio 1977) 137-140

[13] INOMATA Shiuji, Paradigm of New Science Principa for the 21st Century, Gijutsu Shuppan Pub. Co. Ltd. Tokyo (1987)

[14] KÜHNE Rainer W., A Model of Magnetic Monopoles, Modern Physics Letters A 12 /40 (1997) 3153-3159

[15] MAXWELL James Clerk, A Dynamical Theory of the Electromagnetic Field, Royal Society Transactions 155 (1865) 459512

[16] MAXWELL James Clerk, A Treatise on Electricity & Magnetism, (1873) Dover Publications, New York ISBN 0-486-60636-8 (Vol. 1) & 0-486-60637-6 (Vol. 2)

[17] MEYL Konstantin, Potentialwirbel, Indel Verlag, Villingen-Schwenningen Band 1 ISBN 39802542-1-6 (1990)

[18] MEYL Konstantin, Potentialwirbel, Indel Verlag, Villingen-Schwenningen Band 2 ISBN 39802542-2-4 (1992)

[19] MÚNERA Héctor A. and Octavio GUZMÁ, A Symmetric Formulation of MAXWELLs Equations, Modern Physics Letters A 12 No.28 (1997) 2089-2101

[20] PHIPPS Thomas E. Jr, On Hertzs Invariant Form of Maxwells Equations, Physics Essays 6 /2 (1993) 249-256

[21] RAUSCHER Elizabeth A., Electromagnetic Phenomena in Complex Geometries and Nonlinear Phenomena, Non-HERTZian Waves and Magnetic Monopoles, Tesla Book Company, Chula Vista CA-91912

[22] TAIT Peter Guthrie, An elementary Treatise on Quaternions, Oxford University Press 1st Edition (1875)

[23] WILSON E. B., Vector Analysis of Josiah Willard Gibbs The History of a Great Mind, Charles Scribners Sons New York (1901)











VIII A Dynamical Theory of the Electromagnetic Field. By J. Clerk Maxwell, F.R.S


Received October 27, -- Read December 8, 1864


(note: Maxwell's 1873 work see*)-sm.jpg       )


(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)




(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.


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.


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.


This theory, as developed by MM. W. WEBER and C. NEUMANN, 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.


* electrodynamiache Maassbestimmung. Leipzic Trans, vol. i. 1840, and TAYLOR'S Scientific Memorirs, vol. v. art. xiv.


"Explicare tentaur quomodo fiat ut lucia planum polarizationis per vires electricas vol magneticas declinetur." -- Halis Saxonum, 1838.


MDCCCLXC                                3R




(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.


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.


(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.


(4) The electromagnetic field is that part of space which contains and surrounds bodies in electric or magnetic conditions.


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.



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.


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 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.


(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.


(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.


Hence the parts of this medium must be so connected that the motion of one part


* "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.




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.


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.


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.


(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.


(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. VERDEThas 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.


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.


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.


(9)  We may now consider another phenomenon observed in the electromagnetic


* Experimental Researches, Series 19.

Comptes Rendux (1856, second half year, p. 329, and 1857, first half year, p. 1209).

Procedings of the Royal Society, June 1856 and June 1861.




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.


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.


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.


(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.


(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*.


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


* FARADAY, Exp. Res. Series XL; Mossotti, Mem. della Soc. Italiann (Modena), vol. xxiv. part 2. p. 49.


PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.                                             463


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.


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.


(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.


(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.


(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 .


(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.  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. 


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


*  Faraday, Exp. Res. 1233-1250

  Reports of British Association, 1859, p. 248; and Report of Committee of Board of Traded on Submarine Cables, pp. 136 & 464

  As, for instance, the composition of glue, treacle, &c., of which small plastic figures are made, which after being distorted gradually recover their shape.
464                                          PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.


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.


(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.


(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.


(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.


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.


(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.


*  "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.





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





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.




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 driving-points 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 dw dt òz=X òx+Y òy


[C*(dw/dt)* òz=X òx+Y òy]


where X and Y are the forces acting at A and B.



        dw dt=p  du dt + q  dv dt










Substituting, and remembering that òx, and òy are independent,




X = d dt  (Cp2u+Cpqv),   

                      ............... (1)                

Y = d dt  (Cpqu+Cq2v),


[X =(d/dt)*(C*p^2*u+C*p*q*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=(Cp2), M= (Cpq), and N= (Cq2)


[L=*(C*p^2), M= *(C*p*q), and N= *(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 = d dt  (Lu+Mv),   

                      ............... (2)                

Y = d dt  (Mu+Nv),


[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 x  and  h are the forces on A and B


x = X+Ru = Ru + d dt    (Lu + Mv)

                      ............... (3)                

h = Y + Sv = Sv + d dt    (Mu +Nv)


[x = X+Ru = Ru +d/dt*(Lu + Mv)]

[h = Y + Sv = Sv +d/dt*(Mu +Nv)]

If the velocity of A be increased at the rate du dt , then in order to prevent B from moving a force, h=+ d dt (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 x be the electromotive force acting on A, c the strength of the current, and


*  Experimental Researches, Series I., IX.    Annales de Chimie, sér. 3. xxxiv (1852) p. 64



PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.                                             469


R the resistance, then Rc will be the resisting force.  In steady currents the electromotive force just balances the resisting force, but in variable currents the resultant force x = Rc 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




and that belong to B,




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 c in A will be


x = Rc  + d dt    (Lc + My), ............... (4)

h = Sy +  d dt    (Mc +Ny),      ............... (5)                


[x = R*c+d/dt* (L*c + M*y)]

[h = S*y+d/dt* (M*c+ N*y)]


where x and h are the electromotive forces, c 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 c, then


Sy+d dt (Mx+Ny)=0






Y = t   o     ydt = -  m 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'-M    x;

[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


xx+hy = Rx2+Sy2 + x  d dt (Lx +My) +y  d dt (Mx+Ny)


[xx+hy= Rx^2+Sy^2 +x*d/d*t*(L*x +M*y) +y*d/d*t* (M*x+N*y)]


Here xx is the work done in unit of time by the electromotive force x acting on the current x and maintaining it, and hy is the work done by the electromotive force h.  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


d dt  (Lx2+2Mxy+ny2) + dL dt  x2 + dM  dt  xy + dN  dt y2


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


Lx2 + Mxy + 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.





Mechanical Action between Conductors.


(34)  The remaining terms,


                 dL dt x2 + dM  dt  xy + dN  dt y2 = 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


dM  ds xy

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 self-induction is L, acted on by an external electromotive force  x, is


             x-Rx = d dt Lx .   .   .   .   .   .   .   .   .   .   .    (13)

[x-R*x =d/dt*L*x]


When x is constant, the solution is of the form


x=b+(a-b)e -R L -t

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 o  xdt = bt +(a-b) L R  .   .   .   .   .   .   .   .   .   .   .   (14)


the integral of x2 with respect to the time is


t o  x2 dt = b2 t +(a-b) L R(3b+a 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= x R , and the final current is b = x / R+r.


The current of induction is then 1/2 x (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/x 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) = x

                                                                                                 .   .   .   .   .   .   .   .   .   .   .   .   .   (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 {xt+L(xo- x1) +M(yo- y1)},

                                                             .   .   .   .   .   .   .   .   .   .   .   .   .   (14*)

Y = 1/S{M(xo- x1) +N(yo- y1)},


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 = x/R,   yo = 0,   y= 0 ;




                                    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 x 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,


            ∫x^2dt = c^2{t-3/2*L/R-1/2*(m^2/RN+LS)},                          .   .   .   .   .   .   .   .   .   .   .   .   .   (19)

            ∫y^2dt = c^2*1/2(M^2*R/(S(RN+LS)) )                                  .   .   .   .   .   .   .   .   .   .   .   .   .   (20)


The effects of the current in the secondary coil on the galvanometer and dynamometer are the same as those of a uniform current




for a time


2*(L/R+N/S).  (note: original shows L/R as L R)


(40)  The equation between work and energy may be easily verified.  The work done by the electromotive force is


xxdt = c^2*(Rt - L).


Work done in overcoming resistance and producing heat,


R∫x^2*dt+S∫y^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 ∫y^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 x=E cos pt 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 and a, 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      Q(x-z) = D - C

                                    Ry = A - E       S(y+z) = E - C             .   .   .    .   .   .    .   .   .    .   .   .    .   .   .  (21)

                                    Gz = D - E       B(x+y) = - A+C+F.


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-1/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

being p, between P and Q, h, and so on.

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 = ∫Adt-∫Ddt,

                                    Q(X-Z)+(h+q)x+(m+n)y = ∫Ddt-∫Cdt,

                                    RY        +(k+m)x+(r+o)y = ∫Adt-∫Edt,           .   .   .   .   .   .   .   (24)

                                    S(Y+Z)+(l+n)x+(o+s)y = ∫Edt-∫Cdt,

                                    GZ = ∫Dtd - ∫Edt


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, instead of unity, be .

Also let the time of vibration of the galvanometer needle from rest to rest be T. 




PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.                                             477


Then calling the quantity


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) = t, (26)


we find


Z/z = (2sin1/2a/tan ) T/ = t/1 -


(this letter looks more like, but it is not listed under Greek in any font style.)


In determining 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 from 1 to 1.01 can be accurately measured.


We observe () the greatest deflection produced by the permanent current when the resistance of one of the arms is increased in the ration of 1 to , 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 till  = 2 nearly; then


=T* 1/(1-)((2 sin 1/2* )/(tan 1/2* ))  .  .  .  .  .  .  .  .  .  (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 will be reduced to p/P .  The electric balance therefore affords the means of measuring the self-induction 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 is the same in whatever direction the lines of force pass through the substance, but in crystallized, strained, and organized solids the value of may depend on the direction of the lines of force with respect to the axes of crystallization, strain, or growth.


In all bodies 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 = 4 , where 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 has 4 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.




(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 of an element dx, dy, dz cut from the body be, 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 cal p', q', r', so that


p' = p +  df/dt ,

q' = q + dg/dt ,                   . . . . . . .         (A)             

r' =  r + dh/dt ,


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








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.




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



(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 dy dz, then its electromagnetic momentum is


(dH/dy - dG/dz) dy dz ,


and this is the number of lines of magnetic force which pass through the area dy dz.


Magnetic Force (, , ).

(59)  Let , ,   represent the force acting on a unit magnetic pole placed at the given point resolved in the direction of x, y and z.




Coefficient of Magnetic Induction ().


(60)  Let 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 will be    (  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.


  = (dH/dy - dG/dz) ,

                         = (dF/dz - dH/dx) ,            . . . . . . . . . . . . .  (B)

                          = (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,         


dx +   dydz  =  d   . . . . . . . . . . . . . . .   (31)


a complete differential of  , the magnetic potential.


The quantity 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 corresponding to a passage completely round a current of strength c being 4c.


Hence if there is no electric current,


dy - d / dz  = 0;


but if there is a current of p',                  d/dy - d/dz  = 4p'.


Similarly,                                              d /dz - d /dx  = 4q' ,


                                                            d /dx - d /dy  = 4r'.


We may call these the Equations of Currents.




Electromotive Force in a Circuit.


(63)  Let be the electromotive force acting round the circuit A, then


=(P(ds/dx)+Q(dy/ds)+R(dz/ds))ds , .   .   .   .   .   .   .   .   .   .   .   . (32)


where ds 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


(F(dx/ds)+G(dy/ds)+H(dz/ds))ds= Lu + Mv ,  .   .   .   .   .   .   .   .   .   .   (33)


where u and v are the currents in A and B and


                                                                        = - d/dt(Lu+Mv)  .   .   .   .   .   .   .   .   .   .   .   .   (34)


Hence, if there is no motion of the circuit A,

                                                P  = - dF/dt - d/dx,

                                                Q = - dG/dt  - d/dy,                           .   .   .   .   .   .   .   .   .   .    . (35)

                                                R = - dH/dt  - d/dz,


where 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 can always, however, be determined in any particular case when we know the actual conditions of the question.  The physical interpretation of 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


         (F(dx/ds)+G(dy/ds)+H(dz/ds) )ds


MDCCCLXV.            3U




will be increased by the following increments,


a (dF/dx dv/dt + dF/dy dy/dt + dF/dz dz/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 ( dy/dt -   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 = dy/dt- 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  =  ( dy/dt -    dz/dt) - dF/dt - d /dx ,

                        Q =  ( dz/dt  -  dx/dt)  - dG/dt - d /dy ,               .   .   .   .   .   .   .   .    (D)

                        R =  ( dx/dt - dy/dt)  -  dH/dt - d /dz .


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 .  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,            . . . . . . . . . . . . . . . . .  (E)

                                                                        R = kh.


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   is the specific resistance referred to unit of volume, we may write the


Equations of Electric Resistance,


                                                P  = -   p,

                                                Q = - q,        . . . . . . . . . . . . . . .  (F)

                                                R = - r.


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




quantities, namely


            For       Electromagnetic Momentum      . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  F  G  H

            "           Magnetic Intensity  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     

            "           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   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

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 (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


            1/8 *  {F(d/dy - d /dz) +G(d/dz - d /dx) + H(d /dx - d /dy)} dV . .   .   .   .   .   (38)



PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.                                             487


where , ,    are the components of magnetic intensity or the force on a unit magnetic pole, and  , ,     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 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


( / )  * 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




from P = 0 to the given value of P.


Since P = kf, equation (E), this quantity becomes 




Hence the intrinsic energy of any part of the field, as existing in the form of electric displacement, is


                        1/2{1/8*(++ )+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.




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


(dF/dx dx/ds + dG/dx dy/ds + dH/dx dz/ds )


That due to the lengthening of the circuit by increasing the length of the parallel conductors will be


- (dF/dx  dx/dx + dF/dy  dy/ds + dF/dz  dz/ds )


The total increment is


{ dy/dx - dF/dy} - dz/ds (dF/dz - dH/dx)},


which is by the equations of Magnetic Force (B), p. 482,


(dy/dx    -   dz/ds ) .


Let X be the force acting along the direction of x per unit of length of the conductor, then the work done is X.





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


                                                Xa = Caxx(dy/ds   -   dz/ds ) ,


                        or                     X =  q' -  r'

                        Similarly,          Y = r' -  p'                     .   .   .   .   .   .   .  (J)

                                                Z = p' - q'


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,


d/dx = ,   d/dy=d/dz=


Substituting these values of , , in the expression (equation 38) for the intrinsic energy of the field, and integrating by parts, it becomes


                                                - [{ 1/8 (d/dx + d/dy +d/dz)}d V.

The expression

                                                (d/dx + d/dy +d/dz)dV = mdV    .   .   .   .   .   .   .   .   .   (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 4 lines belonging to it, since it produces unit of magnetic intensity at unit of distance over a sphere whose surface is  4.


                                                            E = -(1/2 m)dV   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   . (40)


If there are two magnetic poles m1 and m2 producing potentials of 1 and 2 in 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(1+2)(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 1 is determined by m1 , and 2 by  m2 , the quantities  1m1 and 2 m2  will remain constant.


It can be shown also, as Green has proved (Essay, p. 10), that


                                                                        m12 = m2 1 ,

so that we get


                                                                        Xdx = d(m2 1),


                                                                        X=m2  d1/dx = m21 ,


where 1 represents the magnetic intensity due to m1 .


Similarly,                                                          Y = M21 ,

                                                                        Z = M2 1 .


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 is

                                                                        = -  m1   1/ ,   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   . (41)

where m1 is the strength of the pole and   the distance from it.


The repulsion between two poles of strength m1 and m2  is


                                                                         m2 d 1 /dr = m1 m2 / 2x .   .   .   .   .   .   .   .   .   . (42)


In air or any medium in which  =1 this is simply m1 m2 / 2, 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 = - d/dx,  Q = -d/dy,  R-= d/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  { (df/dx + dg/dy + dh/dz)}dV ,


or by the equation of Free Electriciy (G), p. 485             - 1/2 (e)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 1 , has for components

                                                X = e2 d1/dx = - P1e2  ,

                                                Y = e2 d1/dy = - Q1e2 ,          .    .    .    .    .    .    .   (D)

                                                Z = e2 d1/dz = - R1e.


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 1 , is


                                    1 = k/4  e1/r ,   .    .    .    .    .    .    .    .    .   (43)


where r is the distance from the electrified body.

The repulsion between two electrified bodies e1 , e2 is therefore


                        e2 d1/dr  = k/4  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   1 , 2  of electricity is


1 2 /r2


where r is the distance between them.


Let the relation of the two systems be such that one electromagnetic unit of electricity contains v electrostatic units;  then 1=ve1 and 2=ve2 , and this repulsion becomes


                                                v2 e1e2/r2 = k/4 e1e2/r2 by equation  (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 = 4v2   .   .   .   .   .   .   .   .   .  .   .   .   .   .  .   .   .   .   .   .   .   .   .   .   (46)


MDCCCLXV                                      3 X




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

X   =  E ,                                         X'   =  E' ;                                      


now X and  are equal in numerical value, but of opposite signs; so that


                                    E = -E'



                                    E = C - E'


                                    =  C - (1/8 )(  ^2 + ^2 +^2)dV



PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.                                             493


where  , , 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', and ^2+ ^2+ ^2 = R^2 = R'^2


E = C - (1/8) 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


1/8 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.




Capacity of a Condenser.


(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.


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 1 and on the other side  1+1, and within its substance



                                                dx = 1/a =Kf .    .    .    .    .    .    ..    .    .    .     .    .    .    .     .    .    .    .    (48)


Since d/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.


                                                            3  X  2





Specific Capacity of Electric Induction (D).


(84) If the dielectric of the condenser be air, then its capacity in electrostatic measure is S/4a (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/4a.


Hence                                                              D = k0 /k  ,  .   .   .   .   .    .    .    .    .    .    .    .    .    .    .  (49)


where k0 is the value of k in air, which is taken for unity.


Electric Absorption.


(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."


(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.


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


                                                            a1 k2 + a2 k2  + &c. = ak,    .    .    .    .    .    .    .    .    .    .    .    .    . (50)


where k is the "electric elasticity" of air, and a is the thickness of an equivalent condenser of air.


Let the resistances of the layers be respectively r1 ,  r2 , &c, and let r1  + r + &c. = r be the resistance of the whole condenser, to a steady current through it per unit of surface.


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 1 , and the electricity per unit of surface e1 .

Let the corresponding quantities at the boundary of the first and second surface be 2 , and e2 , and so on.  Then by equations (G) and (H),


                        e1  = -f1 ,          de1 / dt = - p1 ,

                        e2  = -f1  - f2 ,    de2 /dt = - p2 ,               .    .    .    .    .    .    .    .    .    .     (51)

                             &c.                   &c.






But by equations (E) and (F),

                        12 = a1 k1 f1  = -r1 p1 ,

                        23 = a2 k2 f2  = -r2 p2 ,          .    .    .    .    .    .    .    .     (52)

                            &c.              &c.          &c.



After the electromotive force has been kept up for a sufficient time the current becomes the same in each layer, and


p1 = p1 = &c. = p = / r ,


where is the total difference of potentials between the extreme layers.  We have then


                        f1  = - /r r1 /a1 k1  ,    f2  = - /r r2 /a2 k2, &c.


                                                                                                                        .   .   .   .   .   (53)

                        e1  =/r r1 /a1 k1 ,            e2  =/r (r2 /a2 k2 - r/ak1 ), &c.


These are the quantities of electricity on the different surfaces.


(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



                        1 =  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,


                                                e'1 /r  r1/a1k1 - /ak  =  e1 - /ak  .    .    .    .    .    .    .    .    .  (55)


The instantaneous discharge is therefore /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.


(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 1 be the difference of potential of the extreme surfaces at any time t; then


                                       1 = a1k1f1 + a2k2f2 + &c.  ;    .    .    .    .    .    .    .    .    .    .    .    .    (56)

but                                           a1k1f1 =  - r1  df/dt  ,

                                                a2k2f2 = - r2 df/dt








496                                          PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.


Hence  f1 = A1 e^((-a1k1/r1)t)   , F2= A2e^((-a2k2/r2)t)   , &c.;  and by referring to the values of e'1 e2 , &c., we find


                                                            A1  =  /  r1 /a1k1  - /ak ,

                                                            A2  =  / r2 /a2k -  /ak ,      .   .   .   .   .   .   .   .   (57)

                                                            &c. ;


so that we may find for the differences of extreme potentials at any time,


' = {( r1/r - a1k1/ak)e^((-a1k1/r1)*t) + (r2/r-a2k2/ak)e^((-a2k2/r2)*t)+ &c. . . . (58)


(89)  It appears from this result that if all the layers are made of the same substance, 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 ' can never change sign, but must start from zero, become positive, and finally disappear.


(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,


                                                            ' = p1r1 + p2r2 + &c. = pR   .   .   .   .   .   .   .   .   .   .   .   . (59)


Integrating with respect to the time, and calling q1, q2, q the quantities of electricity which traverse the different conductors,


                                                            q1r1 + q2r2 + &c. = qR          .   .   .   .   .   .   .   .   .   .   .   . (60)

The quantities of electricity on the several surfaces will be


                                                e'1  - q   - q1 ,

                                                e   + q1 - q2 ,

                                                &c. ;

and since at last all these quantities vanish, we find


q1 = e'1 -q1 ,

q2 = e'1 + e2 - q;


whence             qR =   /r(r1^2/a1k1 + r2^2/a2k2 + &c.) -  r/ak,


or                     q =  /akrR{a1k1a2k2(r1/a1k1 - r/a2k2) ^2+ a2k2a3k3(r2/a2k2- r3/a3k3)^2 + &c.} , .   .   .   .   .   (61)







a quantity essentially positive; so that, when the primary electrification is in one direction, the secondary discharge is always in the same direction as the primary discharge*.




(91)  At the commencement of this paper we made use of the optical hypothesis of an elastic medium through which the vibrations of light are propagated, in order to show that we have warrantable grounds for seeking, in the same medium, the cause of other phenomena as well as those of light.  We then examined electromagnetic phenomena, seeking for their explanation in the properties of the field which surrounds the electrified or magnetic bodies.  In this way we arrived at certain equations expressing certain properties of the electromagnetic field.  We now proceed to investigate whether these properties of that which constitutes the electromagnetic field, deduced from electromagnetic phenomena alone, are sufficient to explain the propagation of light through the same substance. 


(92)  Let us suppose that a plane wave whose direction cosines are l, m, n is propagated through the field with with a velocity V.  Then all the electromagnetic functions will be functions of


                                                w = lx + my + nz - Vt


The equations of Magnetic Force (B), p. 482, will become


                                               =  m dH/dw -n dG/dw ,

                                               =  n  dF/dw - l dH/dw ,

                                               =  l  dG/dw - m dF/dw .


If we multiply these equations respectively by l, m, n and add, we find


                                             l +m +n = 0 ,    .    .    .    .    .    .    .    .    .    .    (62)


which shows that the direction of the magnetization must be in the plane of the wave.


(93)  If we combine the equations of Magnetic Force (B) with those of Electric Currents (C), and put for brevity


dF/dx + dG/dy+dH/dz = J, and d^2/dx^2 +d^2/dy^2 + d^2/dz^2 = ^2 ,    .    .    .    .    .   (63)


                                             4 p'= dJ/dx -^2F,

                                             4 q'= dJ/dy -^2G,      .   .   .   .   .   .   .   (64)

                                             4 r'= dJ/dz -^2H.

* Since this paper was communicated to the Royal Society,  I have seen a pape by M. GAUGAIN in the Annales de Chimie for 1864, in which he has deduced the phenomena of electric absorption and secondary discharges from the theory of compound condensers.




If the medium in the field is a perfect dielectric there is no true conduction, and the currents p', q', r' are only variations in the electric displacement, or, by the equations of Total Currents (A),


                                                p' = df/dt,          q'=dg/dt ,        r'= dh/dt  .     .    .    .    .    .    .   (65)


But these electric displacements are caused by electromotive forces, and by the equations of Electric Elasticity (E) ,


                                                P = kf ,       Q = kg ,       R = kh .      .     .    .    .    .    .    .    .   (66)


These electromotive forces are due to the variations either of the electromagnetic or the electrostatic functions, as there is no motion of conductors in the field;  so that the equations of electromotive force (D) are


                                    P   =  - dF/dt  -  d /dx ,

                                    Q  =  - dG/dt  -  d /dy ,         .    .    .    .    .     .    .    .    .    .   (67)

                                    R  =  - dH/dt  -  d /dz .


(94)  Combining these equations, we obtain the following :  -


                        k(dJ/dx - ^2F) + 4 (dF/d^2t^2+ d^2 /dxdt) = 0

                        k(dJ/dy - ^2G) + 4 (dG/d^2t^2+ d^2 /dydt) = 0           .    .    .    .    .   (68)

                        k(dJ/dz - ^2H) + 4 (dH/d^2t^2+ d^2 /dzdt) = 0

If we differentiate the third of these equations with respect to y, and the second with respect to z, and subtract, J and disappear, and by remembering the equations (B) of magnetic force, the results may be written

                                                k^2 = 4 d^2/dt^2

                                                k^2 = 4 d^2/dt^2      .    .    .    .    .    .   (69)

                                                k^2 = 4 d^2/dt^2

(95)  If we assume that are functions of lx+my+nz - Vt = w, the first equation becomes


                                                ^2/dw^2 = 4^2V^2 d^2 /dw^2   .     .     .     .     .     .     .  (70)


                                                V= - (k/4)^.5   .     .     .     .     .     .     .     .     .     .     .     .  (71)


The other equations give the same value for V, so that the wave is propagated in either direction with a velocity V.





PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.                                             499


This wave consists entirely of magnetic disturbances, the direction of magnetization being in the plane of the wave.  No magnetic disturbance whose direction of magnetization is not in the plane of the wave can be propagated as a plane wave at all. 


Hence magnetic disturbances propagated through the electromagnetic field agree with light in this, that the disturbance at any point is transverse to the direction of propagation, and such waves may have all the properties of polarized light.


(96)  The only medium in which experiments have been made to determine the value of k is air, i which = l, and therefore, by equation (46),

                                                V=v.  .   .    .    .    .    .    .    .    .    .    .    .    .    .    .  (72)

By the electromagnetic experiments of MM. WEBER and KOHLRAUSCH*,

                                                v = 3120,740,000 meters per second

is the number of electrostatic units in one electromagnetic unit of electricity, and this, according to our result, should be equal to the velocity of light in air or vacuum.

The velocity of light in air, by M. FIZEAU'S experiments, is



according to the more accurate experiments of M. FOUCAULT


                                                V = 308,000,000.


The velocity of light in the space surrounding the earth, deduced from the coefficient of aberration and the received value of the radius of the earth's orbit, is




(97)  Hence the velocity of light deduced from the experiment agrees sufficiently well with the value of v deduced from the only set of experiments we as yet possess.  The value of v was determined by measuring the electromotive force with which a condenser of know capacity was charged, and then discharging the condenser through a galvanometer, so as to measure the quantity of electricity in it in electromagnetic measure.  The only use made of light in the experiment was to see the instruments.  The value of V found by M. FOUCAULT was obtained by determining the angle through which a revolving mirror turned, while the light reflected from it went and returned along a measured course.  No use whatever was made of electricity or magnetism.


The agreement of the results seems to show that light and magnetism are affections of the same substance, and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws.


(98)  Let us now go back upon the equations in (94), in which the quantities J and occur, to see whether any other kind of disturbance can be propagated through the medium depending on these quantities which disappeared from the final equations.


* Leipzig Transactions, vol. v. (1857), p. 260, or Poggendorff's 'Annalen,' Aug. 1856, p. 10.

Comptes Rendus, vol. xxix. (1849), p. 90.        Ibid, vol. lv. (1862), pp. 501, 702.


MDCCCLXV.                         3 Y


If we determine x from the equation


                        ^2 = d^2 /dx^2 + d^2 /dy^2+d^2 /dz^2 = J ,     .    .    .    .    .    .    .    .    .    .    (73)

(transcriber note here there may be confusion between   (Greek) and x English)


and F', G', H' from the equations


                        F' = F - d/dx,  G'=G-d/dy,   H'=H-d/dz,       .    .    .    .    .    .    .    .    .    .    .    .    (74)


                        dF'/dx + dG'/dy + dH'/dz  =  0 .    .    .    .    .    .    .    .    .    .    .    .    .    .    .    (75)


and the equations in (94) become of the form


                        k^2F' = 4  (a^2F'/dt^2 + d/dxdt ( + d/dt))    .    .    .    .    .    .    .    .    .    .    (76)

Differentiating the three equations with respect to x, y, and z, and adding, we find that


                                                = - d/dt + (x,y,z)  .    .    .    .    .    .      .    .    .    .    .    .    .    .    (77)


and that


                                                k^2F' = 4  (a^2F'/dt^2 ,

                                                k^2G' = 4  (a^2G'/dt^2 ,  .    .    .    .    .    .    .    (78)

                                                k^2H' = 4  (a^2H'/dt^2 .


Hence the disturbances indicated by F', G', H' are propagated with the velocity  V= (k/4 )^.5 through the field; and since

                                                dF'/dx + dG'/dy + dH'/dx = 0 ,


the resultant of these disturbances is in the plane of the wave.


(99)  The remaining part of the total disturbances  F, G, H being the part depending on , is subject to no condition except that expressed in the equation


                                                            d / dt + d^2   /dt^2 = 0.

If we perform the operation ^2 on this equation, it becomes


                                    ke = dJ/dt - k^2 (x,y,z)  .   .   .   .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   . (79)

Since the medium is a perfect insulator, e, the free electricity, is immoveable, and therefore dJ/dt is a function of x, y, z, and the value of J is either constant or zero, or uniformly increasing or diminishing with the time; so that no disturbance depending on J can be propagated as a wave.


(100)  The equations of the electromagnetic field, deduced from purely experimental evidence, show that transversal vibrations only can be propagated.  If we were to go beyond our experimental knowledge and to assign a definite density to a substance which


we should call the electric fluid, and select either vitreous or resinous electricity as the representative of that fluid, then we might have normal vibrations propagated with a velocity depending on this density.  We have, however, no evidence as to the density of electricity, as we do not even know whether to consider vitreous electricity as a substance or as the absence of a substance.


Hence electromagnetic science leads to exactly the same conclusions as optical science with respect to the direction of the disturbances which can be propagated through the field; both affirm the propagation of transverse vibrations, and both give the same velocity of propagation.  On the other hand, both sciences are at a loss when called on to affirm or deny the existence of normal vibrations.


Relation between the Index of Refraction and the Electromagnetic Character of the substance.


(101)  The velocity of light in a medium, according to the Undulatory Theory, is


                                                                                    1/iV0 ,


where i is the index of refraction and V0 is the velocity in vacuum.  The velocity, according to the Electromagnetic Theory, is


                                                                                    (k/4 ) ^ .5


where, by equations (49) and (71), k = 1D/k0 and k0  =  4(V0)^2 .


(transcriber note: k sub 0 = 4 times Vsub 0 squared)


Hence                                                                          D=i^2/ ,  .   .   .   .   .   .   .   .   .   .   .   .   .   .   . (80)


or the Specific Inductive Capacity is equal to the square of the index of refraction divided by the coefficient of magnetic induction.


Propagation of Electromagnetic Disturbances in a Crystallized Medium.


(102)  Let us now calculate the conditions of propagation of a plane wave in a medium for which the values of k and are different in different directions.  As we do not propose to give a complete investigation of the question in the present imperfect state of the theory as extended to disturbances of short period, we shall assume that the axes of magnetic induction coincide in direction with those of electric elasticity.


(103)  Let the values of the magnetic coefficient for the three axes by ,  then the equations of magnetic force (B) become


                                                            = dH/dy - dG/dz ,


                                                            = dF/dz - dH/dx               .    .   .   .   .   .   .   .   . (81)


                                                            = dG/dx - dF/dy




502                                          PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.


The equations of electric currents (C) remain as before.

The equations of electric elasticity (E) will be


                                                            P = 4a^2f ,

                                                            Q =4b^2 g,         .   .   .    .   .   .   .   .   .   .   .   .   .   .   .   . (82)

                                                            R=4 c^2h,

where 4a^2, 4b^2, and 4c^2 are the values of k for the axes of x, y, z.


Combining these equations with (A) and (D), we get equations of the form


1/ ( d^2F/dx^2 +d^2F/dy^2 + d^2F/dz^2) - 1/ d/dx(dF/dx + dG/dy + dH/dz) =

1/a^2(d^2F/dt^2 + d^2 /dxdt) .   .   .   . (83)


(104) If l, m, n are the direction-cosines of the wave, and V its velocity, and if


                                                                        lx+my+nz-Vt = ,  .   .   .   .    .   .   .   .   .   .   .   .   .   .   . (84)


then F, G, H, and  will be functions of ; and if we put F', G', H',  ' for the second differentials of these quantities with respect to , the equations will be


            (V^2-a^2(m^2/ + n^2/))  F' + a^2lm/   G' + a^2ln/   H' -lV' = 0 ,

            (V^2-b^2(n^2/   + l^2/))   G' + b^2mn/ H' + b^2ln/   F'  -mV' = 0 ,        .    .    .    .    .    (85)

            (V^2-c^2(l^2/    + m^2/)) H' + c^2nl/   F' + c^2ln/    G' -nV' = 0 .


If we now put


V^4 - V^2 1/{l^2(b^2 +c^2) + m^2 (c^2+a^2) + n^2 (a^2+b^2  )}

                                                                                                                                        .  .   .   .   .   (86)

            + a^2b^2c^2/ (l^2/a^2+m^2/b^2+n^2/c^2) (l^2 +m^2+n^2) = U,


                                                F'V^2U - l'VU = 0,  .   .   .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   . (87)


with two similar equations for G' and H'.  Hence either


                                                                        V = 0 ,    .   .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   . (88)

                                                                        U = 0 ,    .   .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   . (89)

                        or     VF' = l' , VG' =m' ,  and VH' = n'  .   .   .   .   .   .   .   .   .   .   .   .   .   .   . (90)


The third supposition indicates that the resultant of F', G', H', is in the direction normal to the plane of the wave; but the equations do not indicate that such a disturbance, if possible, could be propagated, as we have no other relation between and F', G', H'.

The solution V=0 refers to a case in which there is no propagation.

The solution U = 0 gives two values for V^2 corresponding to values of F', G', H', which




are given by the equations


l/a^2 F' + m/b^2G' + n/c^2H' = 0


a^2l/F' (b^2 -c^2 ) +b^2m /G'(c^2 -a^2)+c^2nv/H'(a^2-b^2 ) = 0 , .   .   .   .   .   .   .   .   (92)


(105)  The velocities along the axes are as follows: ---




Direction of propagation          




Direction of the electric displacements



Now we know that in each principal plane of a crystal the ray polarized in that plane obeys the ordinary law of refraction, and therefore its velocity is the same in whatever direction in that plane it is propagated.


If polarized light consists of electromagnetic disturbances in which the electric displacement is in the plane of polarization, then


                                    a^2=b^2=c^2 .   .   .   .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   . (93)


If, on the contrary, the electric displacements are perpendicular to the plane of polarization,


                                      = = .   .   .   .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   (94)


we know, from the manetic experiments of FARADAY, PLUCKER, &c., that in many crystals

  , ,    are unequal.

The experiments of KNOBLAUCH* on electric induction through crystals seem to show that a, b and c, may be different.

The inequality, however, of   , ,  is so small that great magnetic forces are required to indicate their difference, and the differences do not seem of sufficient magnitudes to account for the double refraction of the crystals.

On the other hand, experiments on electric induction are liable to error on account of minute flaws, or portions of conducting matter in the crystal.

Further experiments on the magnetic and dielectric properties of crystals are required before we can decide whether the relation of these bodies to magnetic and electric forces is the same, when these forces are permanent as when they are alternating with the rapidity of the vibrations of light


* Philosophical Magazine, 1852.



504                                          PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.


Relation between Electric Resistance and Transparency.


(106)   If the medium, instead of being a perfect insulator, is a conductor whose resistance per unit of volume is  , then there will be not only electric displacements, but true currents of conduction in which electrical energy is transformed into heat, and the undulation is thereby weakened.  To determine the coefficient of absorption, let us investigate the propagation along the axis of x of the transverse disturbance G.

By the former equations

d^2G/dx^2 = - 4 (q')


= - 4 (df/dt + q) by (A),


d^2G/dx^2 = + 4 (1/k  d^2G/dt^2 - 1/ dG/dt) by (E) and  (F).     .   .   .   .   .   .   .   .   . (95)  


If G is of the form


                                G=e^(-px) cos(qx + nt),    .   .   .   .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   (96)  


we find that


                                p = 2/ n/q = 2/ V/t^2   .   .   .   .    .   .   .   .   .   .   .   .   .   .   .   .   . (97)  


where V is the velocity of light in air, and i is the index of refraction.  The proportion of incident light transmitted through the thickness x is


                                                        e^(-2px)         .   .   .   .    .   .   .   .   .   .   .   .   .   .   .   .   . (98)


Let R be the resistance in electromagnetic measure of a plate of the substance whose thickness is x, breadth b, and length l, then


                                                                    R = lq/bx,


                                                        2px = 4 V/i  l/bR     .   .   .   .    .   .   .   .   .   .   .   .   .   .   .   .   . (99)   


(107)  Most transparent solid bodies are good insulators, whereas all good conductors are very opaque.


Electrolytes allow a current to pass easily and yet are often very transparent.  We may suppose, however, that in the rapidly alternating vibrations of light, the electromotive forces act for so short a time that they are unable to effect a complete separation between the particles in combination, so that when the force is reversed the particles oscillate into their former position without loss of energy.

Gold, silver and platinum are good conductors, and yet when reduced to sufficiently thin plates they allow light to pass through them.  If the resistance of gold is the same for electromotive forces of short period as those with which we make experiments,  the amount of light which passes through a piece of gold-leaf, of which the resistance was determined by Mr. C. HOCKIN, would be only 10^-50 of the incident light, a totally imperceptible quantity.  I find that between 1/500 and 1/1000 of green light gets through



PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.                                             505


such gold-leaf.  Much of this is transmitted through holes and cracks;  there is enough, however, transmitted through the gold itself to give a strong green hue to the transmitted light.  This result cannot be reconciled with the electromagnetic theory of light, unless we suppose that there is less loss of energy when the electromotive forces are reversed with the rapidity of the vibrations of light than when they act for sensible times, as in our experiments.


Absolute values of the Electromotive and Magnetic Forces called into play in the Propagation of Light.


(108)  If the equation of propagation of light is


                                                            F = A cos 2 / (z-Vt),

the electromotive force will be

                                                            P = -A 2 / V sin 2 /(z-Vt);

and the energy per unit of volume will be

                                                            P^2/S V^2 ,

where P represents the greatest value of the electromotive force.  Half of this consists of magnetic and half of electric energy.

The energy passing through a unit of area is


                                                            W=P^2/8 V ;

so that


                                                            P = (8 VW)^.5 ,


where V is the velocity of light, and W is the energy communicated to unit of area by the light in a second.

According to POUILLET'S data as calculated by Professor W. THOMSON*, the mechanical value of direct sunlight at the Earth is

                        83.4 foot pounds per second per square foot.

This gives the maximum value of P in direct sunlight at the Earth's distance from the Sun,

                                                            P = 60,000,000 ,

or about 600 DANIELL'S cells per metre,

At the Sun's surface the value of P would be about

                                                            13,000 DANIELL'S cells per metre.

At the Earth the maximum magnetic force would be .193.

At the Sun it would be 4.13

These electromotive and magnetic forces must be conceived to be reversed twice in every vibration of light;  that is, more than a thousand million million times a second.


* Transactions of the Royal Society of Edinburg, 1854 ("Mechanical Energies of the Solar System").

  The horizontal magnetic force at Kew is about 1.70 in metrical units.


506                                          PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.




General Methods


(109)  The electromagnetic relations between two conducting circuits, A and B, depend upon a function M of their form and relative position, as has been already shown.

M may be calculated in several different ways, which must of course all lead to the same result.

First Method.  M is the electromagnetic momentum of the circuit B when A carries a unit current, or


M = ∫(F dx/ds' + G dy/ds' + H dz/ds') ds' ,


where F, G, H are the components of electromagnetic momentum due to a unit current in A, and ds' is an element of length of B, and the integration is performed round the circuit of B.

To find F, G, H, we observe that by (B) and C)


d^2F/dx^2 + d^2F/dy^2 + d^2F/dz^2 = -4 p' ,


with corresponding equations for G and H, p', q' and r' being the components of the current in A.

Now if we consider only a single element ds of A, we shall have


                        p' = dx/ds ds ,              q' = dy/ds ds ,              r' = dz/ds ds ,


and the solution of the equation gives


F =  / dx/ds ds ,                    G= / dy/ds ds ,                     H =  / dz/ds ds ,


where is the distance of any point from ds.  Hence


                                                M = ∫∫ /(dx/ds dx/ds' + dy/ds dy/ds' + dz/ds dz/ds')dsds'


                                                = ∫∫ / cos dsds' ,


where is the angle between the directions of the two elements ds, ds' and is the distance between them, and the integration is performed round both circuits.

In this method we confine our attention during integration to the two linear circuits alone.


(110)  Second Method.  M is the number of lines of magnetic force which pass through the circuit B where A carries a unit current, or


M = ( l +  m + n)dS' ,


where ,  ,   are the components of magnetic induction due to unit current in A,




PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.                                             507


S' is a surface bounded by the current B, and l, m, n are the direction-cosines of the normal to the surface, the integration being extended over the surface.

We may express this in the form

                                                            M =   1/^2 sin sin ' dS'ds ,


where dS' is an element of the surface bounded by B, ds is an element of the circuit A,   is the distance between them,   and  ' are the angles between and ds and between and the normal to dS' respectively, and

is the angle between the planes in which and  ' are measured.  The integration is performed round the circuit A and over the surface bounded by B. 

This method is most convenient in the case of circuits lying in one plane, in which case sin = 1, and sin = 1.


(111)  Third Method.  M is that part of the intrinsic magnetic energy of the whole field which depends on the product of the currents in the two circuits, each current being unity.

Let ,  ,      be the components of magnetic intensity at any point due to the first circuit, ' ,  ' ,  '   the same for the second circuit;  then the intrinsic energy of the element of volume dV of the field is


                                    /8 (( + ')^2 +( +')^2 +( +' )^2)dV.


The part which depends on the product of the currents is


            `                       /4 ( ' + ) + ' )dV.



Hence if we know the magnetic intensities I and I' due to unit current in each circuit, we may obtain M by integrating


over all space, where is the angle between the directions of I and I'.


Application to a Coil.


(112)  To find the coefficient (M) of mutual induction between two circular linear conductors in parallel planes, the distance between the curves being everywhere the same, and small compared with the radius of either.


If r be the distance between the curves, and a the radius of either, then when r is very small compared with a, we find by the second method, as a first approximation,


                                                            M = 4a (loge  8a/r-2) .


To approximate more closely to the value of M, let a and a1 be the radii of the circles, and b the distance between their planes; then


                                                            r^2 = (a-a1 ) + b^2


MDCCCLXV                                                                          3 Z




We obtain M by considering the following conditions: --

1st. M must fulfill the differential equation


                                                d^2M/da^2 + d^2M/db^2 + 1/a dM/da = 0 .


This equation being true for any magnetic field symmetrical with respect to the common axis of the circles, cannot of itself lead to the determination of M as a function of a, a1 , and b.  We therefore make use of other conditions.

2ndly.  The value of M must remain the same when a and a1 are exchanged.  3rdly.  The first two terms of M must be the same as those given above.

M may thus be expanded in the following series: ---


M =4a log 8a/r{1 + 1/2 a-a1 /a + 1/16 3b^2-(a1-a)^2/a^2 - 1/32 (3b^2+ (a1-a)^2(a-a1))/a^3 + &c.}


                 -4a{2+1/2 (a-a1 /a) +1/16 3b^2(a-a1^2/a^2) - 1/48 ((6b^2+ (a-a1)^2)(a-a1))/a^3 + &c.}


(113)  We may apply this result to find the coefficient of self-induction (L) of a circular coil of wire whose section is small compared with the radius of the circle.

Let the section of the coil be a rectangle, the breadth in the plane of the circle being c, and the depth perpendicular to the plane of the circle being b.


Let the mean radius of the coil be a, and the number of windings n; then we find, by integrating,


L = n^2/(b^2 c^2) ∫∫∫∫M(xy x'y')dx dy dx' dy' ,


where M(xy x'y') means the value of M for the two windings whose coordinates are xy and x'y' respectively; and the integration is performed first with respect to x and y over the rectangular section, and then with respect to x' and y' over the same space.


L = 4n^2a{log, 8a/r + 1/12 -3/4( -/4)cot 2-.3cos 2-1/6 cot^2 log cos -1/6 tan ^2 log sin }

+ n^2 r^2/24a{log 8a/r(2 sin^2 +1) + 3.45 +27.475 cos^2 - 3.2 (/2-)sin^3/cos + 1/5 cos^4/5 sin&2 log cos + 13/3 sin^4/cos^2 log sin } + &c.


   Here a = mean radius of the coil

"         r = diagonal of the rectangular section = (b^2+c^2)^.5

"         = angle between r and the plane of the circle.

"         n = number of windings.


The logarithms are Napierian, and the angles are in circular measure.

In the experiments made by the Committee of the British Association for determining a standard of Electrical Resistance, a double coil was used, consisting of two nearly equal coils of rectangular section, placed parallel to each other, with a small interval between them.






PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.                                             509


The value of L for this coil was found in the following way.

The value of L was calculated by the preceding formula for six different cases, in which the rectangular section considered has always the same breadth, while the depth was


                                                A, B, C            A+B     B+C     A+B+C

and n = 1 in each case.

Calling the results

                                                L(A),    L(B),    L(C), &c.


we calculate the coefficient of mutual induction M(AC) of the two coils thus,




Then if n1 is the number of windings in the coil A and n2 in the coil B, the coefficient of self-induction of the two coils together is


L = n^2L(A)+2n1 n2 L(AC) + n2^2L(B).


(114)  These values of L are calculated on the supposition that the windings of the wire are evenly distributed so as to fill up exactly the whole section.  This, however, is not the case, as the wire is generally circular and covered with insulating material.  Hence the current in the wire is more concentrated that it would have been if it had been distributed uniformly over the section, and the currents in the neighbouring wires do not act on it exactly as such a uniform current would do.


The corrections arising from these considerations may be expressed as numerical quantities, by which we must multiply the length of the wire, and they are the same whatever be the form of the coil.


Let the distance between each wire and the next, on the supposition that they are arranged in square order, be D, and let the diameter of the wire be d, then the correction for diameter of wire is


                                                + 2(log D/d + 4/3 log 2+ /3-11/16).


The correction for the eight nearest wires is   + 0.0236

For the sixteen in the next row  +0.00083


These corrections being multiplied by the length of wire and added to the former result, give the true value of L, considered as the measure of the potential of the coil on itself for unit current in the wire when that current has been established for some time, and is uniformly distributed through the section of the wire.


(115)  But at the commencement of a current and during its variation the current is not uniform throughout the section of the wire, because the inductive action between different portions of the current tends to make the current stronger at one part of the section than at another.  When a uniform electromotive force P arising from any cause


                                                                                    3 z 2




acts on a cylindrical wire of specific resistance , we have


P = P -dP/dt ,


where F is got from the equation


d^2F/dr^2 + 1/r dF/dr = -4 p,


r being the distance from the axis of the cylinder.


Let one term of the value of F be of the form Tr^n, where T is a function of the time, then the term of p which produced it is of the form


                                    - 1/4 n^2Tr^(n-2) .

Hence if we write


F= T + / (-P+dT/dt) r^2 + ^2/ 1/l^22^2 dT^2/dt^2 r^2 + &c.

p = (P+dT/dt) - / d^2T/dt^2 r^2 -^2/ 1/l^2 2^2 d^3T/dt^2 r^4 - &c.


The total counter current of self-induction at any point is


∫(p/-p) dt = 1/T + /^2 dT/dt r^2 +  ^2^2/^3 1/l^22^2 d^2T/dt^2r^4 + &c.


from t = 0 to t = ∞

When  t = 0, p = 0 , (dT/dt)0= P, (d^2T/dt^2)0 = 0, &c. (origional had t=0, p-0, )

When  t = ∞, p =P/ , (dT/dt)= 0, (d^2T/dt^2)= 0, &c.


            2(p/ - p) rdrdt = 1/Tr^2+1/2 ^2/^2 dT/dt r^4 + ^2 ^2/^3l/l^2. 2^2.3 d^2T/dt^2 r^6 + &c.



from t = 0 to =


When t = 0, p=0 throughout the section,  (dT/dt)0= P, (d^2T/dt^2)0 = 0, &c

When t =∞, p = 0 throughout .  .    .    .    (dT/dt)= 0, (d^2T/dt^2)= 0, &c


Also if l be the length of the wire, and R its resistance,


R = l / r^2;


and if C be the current when established in the wire, C = Pl/R .

The total counter current may be written


l/R(T-T0) - 1/2 l/R C = -LC/R by (35).





Now if the current instead of being variable from the centre to the circumference of the section of the wire had been the same throughout, the value of F would have been


                                                            F=T+ (1-r^2/r0^2) ,

where   is the current in the wire at any instant, and the total countercurrent would have been


1/ dF/dt 2rdr = l/R(T∞ - T0)- 3/4  l/R C = L'C/R, say




                                                            L = L' - 1/4 l

or the value of L which must be used in calculating the self-induction of a wire for variable currents is less than that which is deduced from the supposition of the current being constant throughout the section of the wire by 1/4 l, where l is the length of the wire and is the coefficient of magnetic induction for the substance of the wire.


(116)  The dimensions of the coil used by the Committee of the British Association in their experiments at King's College in 1864 were as follows: ---


                                    Mean radius   .   .   .   .    .   .   .   .   .   .   . = a = .158194

                                    Depth of each coil .   .   .   .    .   .   .   .   .  = b = .01608

                                    Breadth of each coil  .   .   .   .    .   .   .   .  = c = .01841

                                    Distance between the coils   .   .   .   .    .        = .02010

                                    Number of windings    .   .   .   .    .   .   .  = n = 313

                                    Diameter of wire  .   .   .   .    .   .   .   .   .        = .00126


The value of L derived from the first term of the expression is 437440 meters.  The correction depending on the radius not being infinitely great compared with the section of the coil as found from the second term is - 7345 meters.


The correction depending on the diameter of the wire is per unit length  .   +.44997

Correction of eight neighbouring wires     .   .   .   .    .   .   .   .   .   .   .   .   .  +.0236

For sixteen wires next to these     .   .   .   .    .   .   .   .   .   .   .   .   .   .   .   .   + .0008

Correction for variation of current in different parts of section     .   .   .   .   -.2500

Total correction per unit of length    .   .   .   .    .   .   .   .   .   .   .   .   .   .   .  +.22473

Length .   .   .   .    .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .    .   .   .   311.236 meters

Sum of corrections of this kind .   .   .   .    .   .   .   .   .   .   .    .   .   .   .   .   .   70               "

Final value of L by calculation .   .   .   .    .   .   .   .   .   .   .   .   .   .   .   . 430165               "


This value of L was employed in reducing the observations, according to the method explained in the Report of the Committee*.  The corrections depending on L varies as the square of the velocity.  The results of sixteen experiments to which this correction has been applied and in which the velocity varied from 100 revolutions in seventeen seconds to 100 in seventy-seven seconds were compared by the method of


* British Association Reports, 1863, p. 169.



512                                          PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.


least squares to determine what further correction depending on the square of the velocity should be applied to make the outstanding errors a minimum.


The result of this examination showed that the calculated value of L should be multiplied by 1.0618 to obtain the value of L, which would give the most consistent results.


We have therefore L by calculation     .   .   .   .    .   .   .   .   .   .   .   .   .   .   .   .   .   .    430165 meters

Probable value of L by method of least squares  .   .   .   .    .   .   .   .   .   .   .   .   .   .    456748     "

Result of rough experiment with the Electric Balance (see 46)   .    .   .   .   .   .   .    410000     "


The value of L calculated from the dimensions of the coil is probably much more accurate than either of the other determinations.


(20070826, pak, Ione, Wa)