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작성일 2007.03.25댓글 1건
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A Brief History of The Development of Classical Electrodynamics

Magnus, a Greek shepherd, walks across a field of black stones which pull the iron nails out of his sandals and the iron tip from his shepherd's staff (authenticity not guaranteed). This region becomes known as Magnesia.
Thales of Miletos(Greece) discovered that by rubbing an "elektron" (a hard, fossilized resin that today is known as amber) against a fur cloth, it would attract particles of straw and feathers. This strange effect remained a mystery for over 2000 years.
Petrus Peregrinus of Picardy, Italy, discovers that natural spherical magnets(lodestones) align needles with lines of longitude pointing between two pole positions on the stone.
Dr. William Gilbert(court physician to Queen Elizabeth) discovers that the earth is a giant magnet just like one of the stones of Peregrinus, explaining how compasses work. He also investigates static electricity and invents an electric fluid which is liberated by rubbing, and is credited with the first recorded use of the word 'Electric' in a report on the theory of magnetism. Gilbert's experiments subsequently led to a number of investigations by many pioneers in the development of electricity technology over the next 350 years.
Niccolo Cabeo discovers that electricity can be repulsive as well as attractive.
Vincenzo Cascariolo, a Bolognese shoemaker, discovers fluorescence.
Rene Descartes theorizes that light is a pressure wave through the second of his three types of matter of which the universe is made. He invents properties of this fluid that make it possible to calculate the reflection and refraction of light. The `modern' notion of the aether is born.
Galileo attempts to measure the speed of light by a lantern relay between distant hilltops. He gets a very large answer.
Rene Descartes theorizes that the magnetic poles are on the central axis of a spinning vortex of one of his fluids. This vortex theory remains popular for a long time, enabling Leonhard Euler and two of the Bernoullis to share a prize of the French Academy as late as 1743.
Pierre de Fermat shows that the principle of least time (using variational calculus) is capable of explaining refraction and reflection of light. Fighting with the Cartesians begins. (n.b. This principle for reflected light had been anticipated anciently by Hero of Alexandria.)
Francesco Maria Grimaldi, in a posthumous report, discovers and gives the name of diffraction to the bending of light around opaque bodies.
Robert Hooke reports in his Micrographia the discovery of the rings of light formed by a layer of air between two glass plates. These were actually first observed by Robert Boyle, which explains why they are now called Newton's rings. In the same work he gives the matching-wave-front derivation of reflection and refraction that is still found in most introductory physics texts. These waves travel through the aether. He also develops a theory of color in which white light is a simple disturbance and colors are complex distortions of the basic simple white form.
Isaac Newton destroys Hooke's theory of color by experimenting with prisms to show that white light is a mixture of all the colors and that once a pure color is obtained it can never be changed into another color. Newton argues against light being a vibration of the ether, preferring that it be something else that is capable of traveling through the aether. He doesn't insist that this something else consist of particles, but allows that it may be some other kind of emanation or impulse. In Newton's own words, "...let every man here take his fancy."
Olaf Roemer repeats Galileo's experiment using the moons of Jupiter as the distant hilltop. He measures c=m/s.
Christiaan Huygens introduces his famous construction and principle, thinks about translating his manuscript into Latin, then publishes it in the original French in 1690. He uses his theory to discuss the double refraction of Iceland Spar. His is a theory of pulses, however, not of periodic waves.
Isaac Newton shows that the "two-ness" of double refraction clearly rules out light being aether waves. (All aether wave theories were sound-like, so Newton was correct: longitudinal waves cannot be polarized.)
James Bradley shows that the orbital motion of the earth changes the apparent motions of the stars in a way that is consistent with light having a finite speed of travel.
Stephen Gray shows that electricity doesn't have to be made in place by rubbing, but can also be transferred from place to place with conducting wires. He also shows that the charge on electrified objects resides on their surfaces.
Charles Francois du Fay discovers that electricity comes in two kinds, which he called resinous (−.) and vitreous (+).
Thomas Le Seur and Francis Jacquier, in a note to the edition of Newton's Principia that theypublish, show that the force law between two magnets varies as the inverse cube of their separation.
Pieter van Musschenbroek invents the Leyden jar, or capacitor, and nearly kills his friend Cunaeus.
Benjamin Franklin invents the one-fluid theory of electricity, in which one of Nollet's fluids exists and the other is just the absence of the first.
Franklin also proposes the principle of conservation of charge and calls the fluid that exists and flows "positive". {This educated guess ensures that undergraduates will always be confused about the direction of current flow.} He also discovers that electricity can act at a distance in situations where fluid flow makes no sense.
Sir William Watson uses an electrostatic machine and a vacuum pump to make the first glow discharge. His glass vessel is three feet long and three inches in diameter : the first fluorescent light bulb.
Abbe Jean-Antoine Nollet invents the two-fluid theory electricity.
John Michell discovers that the two poles of a magnet are equal in strength and that the force law for individual poles is inverse square.
Johann Sulzer puts lead and silver together in his mouth, performing the first recorded 'tongue test' of a battery.
Benjamin Franklin proved that lightning and the spark from amber were one and the same thing. The story of this famous milestone is a familiar one, in which Franklin fastened an iron spike to a silken kite, which he flew during a thunderstorm, while holding the end of the kite string by an iron key. When lightning flashed, a tiny spark jumped from the key to his wrist. The experiment proved Franklin's theory, but was extremely dangerous - he could easily have been killed.
Francis Ulrich Theodore Aepinus shows that electrical effects are a combination of fluid flow confined to matter and action at a distance. He also discovers charging by induction.
Canton reports that a red hot poker placed close to a small electrified body destroys its electrification.
Joseph Louis Lagrange discovers the divergence theorem in connection with the study of gravitation. It later becomes known as Gauss's law.
Joseph Priestly, acting on a suggestion in a letter from Benjamin Franklin, shows that hollow charged vessels contain no charge on the inside and based on his knowledge that hollow shells of mass have no gravity inside correctly deduces that the electric force law is inverse square.
Henry Cavendish invents the idea of capacitance and resistance (the latter without any way of measuring current other than the level of personal discomfort). But being indifferent to fame he is content to wait for his work to be published by Lord Kelvin in 1879.
Joseph Louis Lagrange invents the concept of the scalar potential for gravitational fields.
Luigi Galvani (an Italian professor of medicine) discovered that when leg of a dead frog was touched with a metal knife, it twitched violently. Galvani deduced that the muscles of a frog must contain electricity. The phenomenon of galvanism is thus named in honor of Galvani. His followers invent another invisible fluid, that of "animal electricity", to describe this effect.
Pierre Simon Laplace shows that Lagrange's potential satisfies .
Charles Augustin Coulomb uses a torsion balance to verify that the electric force law is inverse square. He also proposes a combined fluid/action-at-a-distance theory like that of Aepinus but with two conducting fluids instead of one. Fighting breaks out between single and double fluid partisans. He also discovers that the electric force near a conductor is proportional to its surface charge density and makes contributions to the two-fluid theory of magnetism.
Alessandro Volta, disagreed with Galvani's claims: Volta realized that the main factors in Galvani's discovery were the two different metals - the steel knife and the tin plate - upon which the frog was lying. Volta showed that when moisture comes between two different/dissimilar metals, electricity is created. By 1800 this new understanding had enabled him to invent the first electric battery, the voltaic pile, which he made from thin sheets of copper and zinc separated by moist pasteboard.
In this way, a new kind of electricity was discovered, electricity that flowed steadily like a current of water instead of discharging itself in a single spark or shock. Volta showed that electricity could be made to travel from one place to another by wire, thereby making an important contribution to the science of electricity. The unit of electrical potential, the Volt, is named after Volta.
William Nicholson and Anthony Carlisle discover that water may be separated into hydrogen and oxygen by the action of Volta's pile.
Thomas Young gives a theory of Newton's rings based on constructive and destructive interference of waves. He explains the dark spot in the middle by proposing that there is a phase shift on reflection between a less dense and more dense medium, then uses essence of sassafras (whose index of refraction is intermediate between those of crown and flint glass) to get a light spot at the center.
Thomas Young explains the fringes at the edges of shadows by means of the wave theory of light. The wave theory begins its ascendance, but has one important difficulty: light is thought of as a longitudinal wave, which makes it difficult to explain double refraction effects in certain crystals.
Humphrey Davy shows that the essential element of Volta's pile is chemical action since pure water gives no effect. He argues that chemical effects are electrical in nature.
Laplace gives an explanation of double refraction using the particle theory, which Young attacks as improbable.
Etienne Louis Malus, a military engineer, enters a prize competition sponsored by the French Academy "To furnish a mathematical theory of double refraction, and to confirm it by experiment.'' He discovers that light reflected at certain angles from transparent substances as well as the separate rays from a double-refracting crystal have the same property of polarization. In 1810 he receives the prize and emboldens the proponents of the particle theory of light because no one sees how a wave theory can make waves of different polarizations.
Francois Arago shows that some crystals alter the polarization of light passing through them.
Jean-Baptiste Biot shows that Arago's crystals rotate the plane of polarization of light about the propagation direction.
Simeon Denis Poisson further develops the two-fluid theory of electricity, showing that the charge on conductors must reside on their surfaces and be so distributed that the electric force within the conductor vanishes. This surface charge density calculation is carried out in detail for ellipsoids. He also shows that the potential within a distribution of electricity satisfies the equation .
Michael Faraday, a bookbinder’s apprentice, writes to Sir Humphrey Davy asking for a job as a scientific assistant. Davy interviews Faraday and finds that he has educated himself by reading the books he was supposed to be binding. He gets the job.
Laplace shows that at the surface of a conductor the electric force is perpendicular to the surface and that .
Karl Friedrich Gauss rediscovers the divergence theorem of Lagrange. It will later become known as Gauss's law.
David Brewster establishes his law of complete polarization upon reflection at a special angle now known as Brewster's angle. He also discovers that in addition of uniaxial cystals there are also biaxial ones. For uniaxial crystals there is the faint possibility of a wave theory of longitudinal-type, but this appears to be impossible for biaxial ones.
David Brewster invents the kaleidoscope.
Francois Arago, an associate of Augustin Fresnel, visits Thomas Young and describes to him a series of experiments performed by Fresnel and himself, which shows that light of differing polarizations cannot interfere. Reflecting later on this curious effect Young sees that it can be explained if light is a transverse, instead of longitudinal wave. This idea is communicated to Fresnel in 1818 and he immediately sees how it clears up many of the remaining difficulties of the wave theory. Six years later the particle theory is dead.
Augustin Fresnel annoys the French Academy. The Academy, hoping to destroy the wave theory once and for all, proposes diffraction as the prize subject for 1818. To the chagrin of the particle-theory partisans in the Academy, the winning memoir in 1818 is that of Augustin Fresnel, who explains diffraction as the mutual interference of the secondary waves emitted by the unblocked portions of the incident wave, in the style of Huygens. One of the judges from the particle camp of the Academy is Poisson, who points out that if Fresnel's theory were to be indeed correct, then there should be a bright spot at the center of the shadow of a circular disc. This, he suggests to Fresnel, must be tested experimentally. The experiment doesn't go as Poisson hopes, however, and the spot becomes known as "Poisson's spot."
Hans Christian Oersted discovers that electric current in a wire causes a compass needle to orient itself perpendicular to the wire.
Andre Marie Ampere, a French mathematician, one week after hearing of Oersted's discovery, shows that parallel currents attract each other and that opposite currents attract. He was the first to explain the electro-dynamic theory. A permanent memorial to Ampere is the use of his name for the unit of electric current.
Jean-Baptiste Biot and Felix Savart show that the magnetic force exerted on a magnetic pole by a wire falls off like 1/r and is oriented perpendicular to the wire. Whittaker then says that "This result was soon further analyzed," to obtain :
John Herschel shows that quartz samples that rotate the plane of polarization of light in opposite directions have different crystalline forms. This difference is helical in nature.
Michael Faraday begins electrical work by repeating Oersted's experiments.
Humphrey Davy shows that direct current is carried throughout the volume of a conductor and establishes that : for long wires. He also discovers that resistance is increased as the temperature rises.
Thomas Johann Seebeck discovers the thermoelectric effect by showing that a current will flow in a circuit made of dissimilar metals if there is a temperature difference between the metals.
Poisson invents the concept of the magnetic scalar potential and of surface and volume pole densities described by the formulas :
He also finds the magnetic field inside a spherical cavity within magnetized material.
Ampere publishes his collected results on magnetism. His expression for the magnetic field produced by a small segment of current is different from that which follows naturally from the Biot-Savart law by an additive term which integrates to zero around closed circuit.
It is unfortunate that electrodynamics and relativity decide in favor of Biot and Savart rather than for the much more sophisticated Ampere, whose memoir contains both mathematical analysis and experimentation, artfully blended together. In this memoir are given some special instances of the result we now call Stokes theorem or as we usually write it .
Maxwell describes this work as "one of the most brilliant achievements in science". The whole, theory and experiment, seems as if it had leaped, full-grown and full-armed, from the brain of the "Newton of electricity". It is perfect in form and unassailable in accuracy ; and it is summed up in a formula from which all the phenomena may be deduced, and which must always remain the cardinal formula of electrodynamics.
Fresnel shows that combinations of waves of opposite circular polarization traveling at different speeds can account for the rotation of the plane of polarization.
Georg Simon Ohm, a German mathematician and physicist, was a college teacher in Cologne. He established the result now known as Ohm's law. seems a pretty simple law to name after someone, but the importance of Ohm's work does not lie in this simple proportionality. What Ohm did was develop the idea of voltage as the driver of electric current. He reasoned by making an analogy between Fourier's theory of heat flow and electricity. In his scheme temperature and voltage correspond as do heat flow and electrical current. It was not until some years later that Ohm's electroscopic force (V in his law) and Poisson's electrostatic potential were shown to be identical. In 1827 he published, "The Galvanic Circuit Investigated Mathematically". His theories were coldly received by German scientists, however his research was recognized in Britain and he was awarded the Copley Medal in 1841. His name has been given to the unit of electrical resistance.
Augustin Fresnel publishes a decade of research in the wave theory of light. Included in these collected papers are explanations of diffraction effects, polarization effects, double refraction, and Fresnel's sine and tangent laws for reflection at the interface between two transparent media.
Claude Louis Marie Henri Navier publishes the correct equations for vibratory motions in one type of elastic solid. This begins the quest for a detailed mathematical theory of the aether based on the equations of continuum mechanics.
F. Savery, after noticing that the current from a Leyden jar magnetizes needles in alternating layers, conjectures that the electric motion during the discharge consists of a series of oscillations.
George Green generalizes and extends the work of Lagrange, Laplace, and Poisson and attaches the name potential to their scalar function. Green's theorems are given, as well as the divergence theorem (Gauss's law), but Green doesn't know of the work of Lagrange and Gauss and only references Priestly's deduction of the inverse square law from Franklin's experimental work on the charging of hollow vessels.
Augustine Louis Cauchy presents a theory similar to Navier's, but based on a direct study of elastic properties rather than using a molecular hypothesis. These equations are more general than Navier's. In Cauchy's theory, and in much of what follows, the aether is supposed to have the same inertia in each medium, but different elastic properties.
Poisson shows that the equations of Navier and Cauchy have wave solutions of two types: transverse and longitudinal. Mathematical physicists spend the next 50 years trying to invent an elastic aether for which the longitudinal waves are absent.
Michael Faraday reasoned: If electricity could produce magnetism, then why couldn't magnetism produce electricity? Faraday found the solution. Electricity could be produced through magnetism by motion. He discovered that when a magnet was moved inside a coil of copper wire, a tiny electric current flowed through the wire. Of course, by today's standards, Faraday's electric dynamo/electric generator was crude, and provided only a small electric current, however he discovered the first method of generating electricity by means of motion in a magnetic field.
Faraday convincingly showed the world that changing currents in one circuit induce currents in a neighboring circuit. Over the next several years he performed hundreds of experiments and showed that his results could all be explained by the idea of changing magnetic flux. No mathematics was involved, just picture-thinking using his concept of magnetic field-lines.
Faraday also investigated many other aspects of electromagnetism, the unit of capacitance (Farad) is named in honor of him.
Ostrogradsky rediscovers the divergence theorem of Lagrange, Gauss, and Green.
Joseph Henry independently discovers induced currents.
Faraday begins work on the relation of electricity to chemistry. In one of his notebooks he concludes after a series of experiments, "...there is a certain absolute quantity of the electric power associated with each atom of matter."
Faraday discovers self inductance.
Jean Charles Peltier discovers the flip side of Seebeck's thermoelectric effect. He finds that current driven in a circuit made of dissimilar metals causes the different metals to be at different temperatures.
Emil Lenz formulates his rule for determining the direction of Faraday's induced currents. In its original form it was a force law rather than an induced emf law: "Induced currents flow in such a direction as to produce magnetic forces that try to keep the magnetic flux the same." So Lenz would predict that if you try to push a conductor into a strong magnetic field, it will be repelled. He would also predict that if you try to pull a conductor out of a strong magnetic field that the magnetic forces on the induced currents will oppose the pull.
James MacCullagh and Franz Neumann extend Cauchy's theory to crystalline media.
Faraday discovers the idea of the dielectric constant.
George Green attacks the elastic aether problem from a new angle. Instead of deriving boundary conditions between different media by finding which ones give agreement with the experimental laws of optics, he derives the correct boundary conditions from general dynamical principles. This advance makes the elastic theories not quite fit with light.
Faraday shows that the effects of induced electricity in insulators are analogous to induced magnetism in magnetic materials. Those more mathematically inclined immediately appropriate Poisson's theory of induced magnetism, inventing , and ε.
Faraday discovers Faraday's dark space, a dark region in a glow discharge near the negative electrode.
James MacCullagh invents an elastic aether in which there are no longitudinal waves. In this aether the potential energy of deformation depends only on the rotation of the volume elements and not on their compression or general distortion. This theory gives the same wave equation as that satisfied by E and B in Maxwell's theory.
William Thomson(Lord Kelvin) removes some of the objections to MacCullagh's rotation theory by inventing a mechanical model which satisfies MacCullagh's energy of rotation hypothesis. It has spheres, rigid bars, sliding contacts, and flywheels.
Cauchy and Green present more refined elastic aether theories, Cauchy's removing the longitudinal waves by postulating a negative compressibility, and Green's using an involved description of crystalline solids.
Michael Faraday is completely exhausted by his efforts of the previous 2 decades, so he rests for 4 years.
James Prescott Joule shows that energy is conserved in electrical circuits involving current flow, thermal heating, and chemical transformations.
F. Neumann and Matthew O'Brien suggest that optical properties in materials arise from differences in the amount of force that the particles of matter exert on the aether as it flows around and between them.
1842: Julius Robert Mayer asserts that heat and work are equivalent. His paper is rejected by Annalen der Physik.
Joseph Henry rediscovers the result of F. Savery about the oscillation of the electric current in a capacitive discharge and states, "The phenomena require us to admit the existence of a principal discharge in one direction, and then several reflex actions backward and forward, each more feeble than the preceding, until equilibrium is restored."
Christian Doppler gives the theory of the Doppler effect.
Faraday quits resting and discovers that the plane of polarization of light is rotated when it travels in glass along the direction of the magnetic lines of force produced by an electromagnet (Faraday rotation).
Franz Neumann uses (i) Lenz's law, (ii) the assumption that the induced emf is proportional to the magnetic force on a current element, and (iii) Ampere's analysis to deduce Faraday's law. In the process he finds a potential function from which the induced electric field can be obtained, namely the vector potential (in the Coulomb gauge), thus discovering the result which Maxwell wrote as .
George Airy modifies MacCullagh's elastic aether theory to account for Faraday rotation.
Faraday, inspired by his discovery of the magnetic rotation of light, writes a short paper speculating that light might electro-magnetic in nature. He thinks it might be transverse vibrations of his beloved field lines.
Faraday discovers diamagnetism. He sees the effect in heavy glass, bismuth, and other materials.
Wilhelm Weber combines Ampere's analysis, Faraday's experiments, and the assumption of Fechner that currents consist of equal amounts of positive and negative electricity moving opposite to each other at the same speed to derive an electromagnetic theory based on forces between moving charged particles. This theory has a velocity-dependent potential energy and is wrong, but it stimulates much work on electromagnetic theory which eventually leads to the work of Maxwell and Lorenz. It also inspires a new look at gravitation by William Thomson to see if a velocity-dependent correction to the gravitational energy could account for the precession of Mercury's perihelion.
William Thomson shows that Neumann's electromagnetic potential is in fact the vector potential from which may be obtained via .


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