ridiculously brief history of E_M

A Ridiculously Brief History of Electricity and Magnetism Mostly from E. T. Whittaker's A History of the Theories of Aether and Electricity... 900 BC - Magnus, a Greek shepherd, walks across a eld of black stones which pull the iron nails out of his sandals and the iron tip from his shepherd's sta (authenticity not guaranteed). This region becomes known as Magnesia. 600 BC - Thales of Miletos rubs amber ( elektron in Greek) with cat fur and picks up bits of feathers. 1269 - 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. 1600 - 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 discusses static electricity and invents an electric uid which is liberated by rubbing. ca. 1620 - Niccolo Cabeo discovers that electricity can be repulsive as well as attractive. 1630 - Vincenzo Cascariolo, a Bolognese shoemaker, discovers uorescence. 1638 - 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 uid that make it possible to calculate the re ection and refraction of light. The \modern" notion of the aether is born. 1638 - Galileo attempts to measure the speed of light by a lantern relay between distant hilltops. He gets a very large answer. 1644 - Rene Descartes theorizes that the magnetic poles are on the central axis of a spinning vortex of one of his uids. 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. 1657 - Pierre de Fermat shows that the principle of least time is capable of explaining refraction and re ection of light. Fighting with the Cartesians begins. (This principle for re ected light had been anticipated anciently by Hero of Alexandria.) 1665 - Francesco Maria Grimaldi, in a posthumous report, discovers and gives the name of diraction to the bending of light around opaque bodies. 1667 - 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 rst 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 re ection 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. 1671 - 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." 1675 - Olaf Roemer repeats Galileo's experiment using the moons of Jupiter as the distant hilltop. He measuresc= 2:3108m/s. 1678 - 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. 1 1717 - 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 right; longitudinal waves can't be polarized.) 1728 - 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 nite speed of travel. 1729 - 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 electried objects resides on their surfaces. 1733 - Charles Francois du Fay discovers that electricity comes in two kinds which he called resinous (-) and vitreous (+). 1742 - Thomas Le Seur and Francis Jacquier, in a note to the edition of Newton's Principia that they publish, show that the force law between two magnets is inverse cube. 1749 - Abbe0Jean-Antoine Nollet invents the two- uid theory electricity. 1745 - Pieter van Musschenbroek invents the Leyden jar, or capacitor, and nearly kills his friend Cunaeus. 1747 - Benjamin Franklin invents the theory of one- uid electricity in which one of Nollet's uids exists and the other is just the absence of the rst. He proposes the principle of conservation of charge and calls the uid that exists and ows \positive". This educated guess ensures that undergraduates will always be confused about the direction of current ow. He also discovers that electricity can act at a distance in situations where uid ow makes no sense. 1748 - Sir William Watson uses an electrostatic machine and a vacuum pump to make the rst glow discharge. His glass vessel is three feet long and three inches in diameter: the rst uorescent light bulb. 1750 - 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. 1752 - Johann Sulzer puts lead and silver together in his mouth, performing the rst recorded \tongue test" of a battery. 1759 - Francis Ulrich Theodore Aepinus shows that electrical eects are a combination of uid ow conned to matter and action at a distance. He also discovers charging by induction. 1762 - Canton reports that a red hot poker placed close to a small electried body destroys its electri- cation. 1764 - Joseph Louis Lagrange discovers the divergence theorem in connection with the study of gravi- tation. It later becomes known as Gauss's law. (See 1813). 1766 - 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. ca 1775 - 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 indierent to fame he is content to wait for his work to be published by Lord Kelvin in 1879. 1777 - Joseph Louis Lagrange invents the concept of the scalar potential for gravitational elds. 1780 - Luigi Galvani causes dead frog legs to twitch with static electricity, then also discovers that the same twitching can be caused by contact with dissimilar metals. His followers invent another invisible uid, that of \animal electricity", to describe this eect. 1782 - Pierre Simon Laplace shows that Lagrange's potential satises r2V= 0. 1785 - Charles Augustin Coulomb uses a torsion balance to verify that the electric force law is inverse square. He also proposes a combined uid/action-at-a-distance theory like that of Aepinus but with two conducting uids instead of one. Fighting breaks out between single and double uid partisans. He also 2 discovers that the electric force near a conductor is proportional to its surface charge density and makes contributions to the two- uid theory of magnetism. 1793 - Alessandro Volta makes the rst batteries and argues that animal electricity is just ordinary electricity owing through the frog legs under the impetus of the force produced by the contact of dissimilar metals. He discovers the importance of \completing the circuit." In 1800 he discovers the Voltaic pile (dissimilar metals separated by wet cardboard) which greatly increases the magnitude of the eect. 1800 - William Nicholson and Anthony Carlisle discover that water may be separated into hydrogen and oxygen by the action of Volta's pile. 1801 - 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 re ection between a less dense and more dense medium, then uses essence of sassafras (whose index of refraction is intermediate between those of crown and int glass) to get a light spot at the center. 1803 - 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 diculty: light is thought of as a longitudinal wave, which makes it dicult to explain double refraction eects in certain crystals. 1807 - Humphrey Davy shows that the essential element of Volta's pile is chemical action since pure water gives no eect. He argues that chemical eects are electrical in nature. 1808 - Laplace gives an explanation of double refraction using the particle theory, which Young attacks as improbable. 1808 - 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 conrm it by experiment." He discovers that light re ected 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 dierent polarizations. 1811 - Arago shows that some crystals alter the polarization of light passing through them. 1812 - Biot shows that Arago's crystals rotate the plane of polarization about the propagation direction. 1812 - Simeon Denis Poisson further develops the two- uid 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 satises the equation r2V==o 1812 - Michael Faraday, a bookbinders apprentice, writes to Sir Humphrey Davy asking for a job as a scientic assistant. Davy interviews Faraday and nds that he has educated himself by reading the books he was supposed to be binding. He gets the job. ca. 1813 - Laplace shows that at the surface of a conductor the electric force is perpendicular to the surface and that E==o. 1813 - Karl Friedrich Gauss rediscovers the divergence theorem of Lagrange. It will later become known as Gauss's law. 1815 - David Brewster establishes his law of complete polarization upon re ection 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. 1816 - David Brewster invents the kaleidoscope. 1816 - 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 diering polarizations 3 cannot interfere. Re ecting later on this curious eect Young sees that it can be explained if light is transverse instead of longitudinal. This idea is communicated to Fresnel in 1818 and he immediately sees how it clears up many of the remaining diculties of the wave theory. Six years later the particle theory is dead. 1817 - Augustin Fresnel annoys the French Academy. The Academy, hoping to destroy the wave theory once and for all, proposes diraction 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 diraction 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." 1820 - Hans Christian Oersted discovers that electric current in a wire causes a compass needle to orient itself perpendicular to the wire. 1820 - Andre Marie Ampere, one week after hearing of Oersted's discovery, shows that parallel currents attract each other and that opposite currents attract. 1820 - Jean-Baptiste Biot and Felix Savart show that the magnetic force exerted on a magnetic pole by a wire falls o like 1 =rand is oriented perpendicular to the wire. Whittaker then says that \This result was soon further analyzed," to obtain dB/Idsr r3 1820 - John Herschel shows that quartz samples that rotate the plane of polarization of light in opposite directions have dierent crystalline forms. This dierence is helical in nature. 1821 - Faraday begins electrical work by repeating Oersted's experiments. 1821 - Humphrey Davy shows that direct current is carried throughout the volume of a conductor and establishes that R/` A for long wires. He also discovers that resistance is increased as the temperature rises. 1822 - Thomas Johann Seebeck discovers the thermoelectric eect by showing that a current will ow in a circuit made of dissimilar metals if there is a temperature dierence between the metals. 1823 - Ampere invents the coaxial cable. 1824 - Poisson invents the concept of the magnetic scalar potential and of surface and volume pole densities described by the formulas V=Z M
rr0) jrr0j3dV0=Zr0
M jrr0jdV0ZM
n0 jrr0jdA0 He also nds the magnetic eld inside a spherical cavity within magnetized material. 1825 - Ampere publishes his collected results on magnetism. His expression for the magnetic eld produced by a small segment of current is dierent 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 itH B
ds=0I. 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." 4 1825 - Fresnel shows that combinations of waves of opposite circular polarization traveling at dierent speeds can account for the rotation of the plane of polarization. 1826 - Georg Simon Ohm establishes the result now known as Ohm's law. V=IRseems 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 ow and electricity. In his scheme temperature and voltage correspond as do heat ow and electrical current. It was not until some years later that Ohm's electroscopic force (Vin his law) and Poisson's electrostatic potential were shown to be identical. 1827 - Augustin Fresnel publishes a decade of research in the wave theory of light. Included in these collected papers are explanations of diraction eects, polarization eects, double refraction, and Fresnel's sine and tangent laws for re ection at the interface between two transparent media. 1827 - 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. 1827 - 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. 1828 - 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. 1828 - 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 dierent elastic properties. 1828 - 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. 1831 - Faraday shows that changing currents in one circuit induce currents in a neighboring circuit. Over the next several years he performs hundreds of experments and shows that they can all be explained by the idea of changing magnetic ux. No mathematics is involved, just picture thinking using his eld-lines. 1831 - Ostrogradsky rediscovers the divergence theorem of Lagrange, Gauss, and Green. 1832 - Joseph Henry independently discovers induced currents. 1833 - 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." 1834 - Faraday discovers self inductance. 1834 - Jean Charles Peltier discovers the ip side of Seebeck's thermoelectric eect. He nds that current driven in a circuit made of dissimilar metals causes the dierent metals to be at dierent temperatures. 1834 - Emil Lenz formulates his rule for determining the direction of Faraday's induced currents. 1835 - James MacCullagh and Franz Neumann extend Cauchy's theory to crystalline media 1837 - Faraday discovers the idea of the dielectric constant. 1837 - George Green attacks the elastic aether problem from a new angle. Instead of deriving boundary conditions between dierent media by nding 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 t with light. 5 1838 - Faraday shows that the eects 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 P,D, and. 1838 - Faraday discovers Faraday's dark space , a dark region in a glow discharge near the negative electrode. 1839 - 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 satised by Eand Bin Maxwell's theory. 1839 - William Thomson (Lord Kelvin) removes some of the objections to MacCullagh's rotation theory by inventing a mechanical model which satises MacCullagh's energy of rotation hypothesis. It has spheres, rigid bars, sliding contacts, and ywheels. 1839 - Cauchy and Green present more rened elastic aether theories, Cauchy's removing the longitudinal waves by postulating a negative compressibility, and Green's using an involved description of crystalline solids. 1841 - Michael Faraday is completely exhausted by his eorts of the previous 2 decades, so he rests for 4 years. 1841 - James Prescott Joule shows that energy is conserved in electrical circuits involving current ow, thermal heating, and chemical transformations. 1842 - F. Neumann and Matthew O'Brien suggest that optical properties in materials arise from dierences in the amount of force that the particles of matter exert on the aether as it ows around and between them. 1842 - Julius Robert Mayer asserts that heat and work are equivalent. His paper is rejected by Annalen der Physik . 1842 - 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 principle discharge in one direction, and then several re ex actions backward and forward, each more feeble than the preceding, until equilibrium is restored." 1842 - Christian Doppler gives the theory of the Doppler eect. 1845 - 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). 1845 - 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 nds a potential function from which the induced electric eld can be obtained, namely the vector potential A(in the Coulomb gauge), thus discovering the result which Maxwell wrote as E=r@A=@t. 1846 - George Airy modies MacCullagh's elastic aether theory to account for Faraday rotation. 1846 - Faraday, inspired by his discovery of the magnetic rotation of light, writes a short paper specu- lating that light might electro-magnetic in nature. He thinks it might be transverse vibrations of his beloved eld lines. 1846 - Faraday discovers diamagnetism. He sees the eect in heavy glass, bismuth, and other materials. 1846 - 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 parti- cles. This theory has a velocity-dependent potential energy and is wrong, but it stimulates much work on 6 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. 1846 - William Thomson shows that Neumann's electromagnetic potential Ais in fact the vector potential from which Bmay be obtained via B=rA. 1847 - Weber proposes that diamagnetism is just Faraday's law acting on molecular circuits. In answering the objection that this would mean that everything should be diamagnetic he correctly guesses that diamagnetism is masked in paramagnetic and ferromagnetic materials because they have relatively strong permanent molecular currents. This work rids the world of magnetic uids. 1847 - Hermann von Helmholtz writes a memoir \On the Conservation of Force" which emphatically states the principle of conservation of energy: \Conservation of energy is a universal principle of nature. Kinetic and potential energy of dynamical systems may be converted into heat according to denite quan- titative laws as taught by Rumford, Mayer, and Joule. Any of these forms of energy may be converted into chemical, electrostatic, voltaic, and magnetic forms." He reads it before the Physical Society of Berlin whose older members regard it as too speculative and reject it for publication in Annalen der Physik . 1848-9 - Gustav Kircho extends Ohm's work to conduction in three dimensions, gives his laws for circuit networks, and nally shows that Ohm's \electroscopic force" which drives current through resistors and the old electrostatic potential of Lagrange, Laplace, and Poisson are the same. He also shows that in steady state electrical currents distribute themselves so as to minimize the amount of Joule heating. 1849 - A. Fizeau repeats Galileo's hilltop experiment (9 km separation distance) with a rapidly rotating toothed wheel and measures c= 3:15108m/s. 1849 - George Gabriel Stokes studies diraction around opaque bodies both theoretically and experimen- tally and shows that the vibration of aether particles are executed at right angles to the plane of polarization. Three years later he comes to the same conclusion by applying aether theory to light scattered from the sky. This result is, however, inconsistent with optics in crystals. ca. 1850 - Stokes overcomes some of the diculties with crystals by turning Cauchy's hypothesis around and letting the elastic properties of the aether be the same in all materials, but allowing the inertia to dier. This gives rise to the conceptual diculty of having the inertia be dierent in dierent directions (in anisotropic crystals). ca. 1850 - Jean Foucault improves on Fizeau's measurement and uses his apparatus to show that the speed of light is less in water than in air. 1850 - Stokes law is stated without proof by Lord Kelvin (William Thomson). Later Stokes assigns the proof of this theorem as part of the examination for the Smith's Prize. Presumably, he knows how to do the problem. Maxwell, who was a candidate for this prize, later remembers this problem, traces it back to Stokes and calls it Stokes theorem. 1850 - William Thomson (Lord Kelvin) invents the idea of magnetic permeability and susceptibility, along with the separate concepts of B,M, and H. 1851 - Thomson gives a general theory of thermoelectric phenomena, describing the eects seen by Seebeck and Peltier. 1853 - Thomson uses Poisson's magnetic theory to derive the correct formula for magnetic energy: U=R H2dV=2. He also gives the formula U=LI2=2 and gives the world the powerful, but confusing, analysis where the forces on circuits are obtained by taking either the positive or negative gradient of the magnetic energy. Knowing which sign to use is, of course, the confusing part. 1853 - Thomson gives the theory of the RLC circuit providing a mathematical description for the observations of Henry and Savery. 1854 - Faraday clears up the problem of disagreements in the measured speeds of signals along trans- mission lines by showing that it is crucial to include the eect of capacitance. 7 1854 - Thomson, in a letter to Stokes, gives the equation of telegraphy ignoring the inductance: @2V=@x2=RC @V=@t , whereRis the cable resistance and where Cis the capacitance per unit length. Since this is the diusion equation, the signal does not travel at a denite speed. 1855 - Faraday retires, living quietly in a house provided by the Queen until his death in 1867. 1855 - James Clerk Maxwell writes a memoir in which he attempts to marry Faraday's intuitive eld line ideas with Thomson's mathematical analogies. In this memoir the physical importance of the divergence and curl operators for electromagnetism rst become evident. The equations r
(E) = 4,rA=B, andrH= 4Jappear in this memoir. 1857 - Gustav Kircho derives the equation of telegraphy for an aerial coaxial cable where the inductance is important and derives the full telegraphy equation: : @2V=@x2=LC@2V=@t2+RC @V=@t , whereLand Care the inductance per unit length and the capacitance per unit length. He recognizes that when the resistance is small, this is the wave equation with propagation speed v=p LC, which for a coaxial cable turns out to be very close to the speed of light. Kircho notices the coincidence, and is thus the rst to discover that electromagnetic signals can travel at the speed of light. 1861 - Bernhard Riemann develops a variant of Weber's electromagnetic theory which is also wrong. 1861 - Maxwell publishes a mechanical model of the electromagnetic eld. Magnetic elds correspond to rotating vortices with idle wheels between them and electric elds correspond to elastic displacements, hence displacement currents. The equation for Hnow becomesrH= 4Jtot, where Jtotis the total current, conduction plus displacement, and is conserved: r
Jtot= 0. This addition completes Maxwell's equations and it is now easy for him to derive the wave equation exactly as done in our textbooks on electromagnetism and to note that the speed of wave propagation was close to the measured speed of light. Maxwell writes, \We can scarcely avoid the inference that light in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena." Thomson, on the other hand, says of the displacement current, \(it is a) curious and ingenious, but not wholly tenable hypothesis." 1864 - Maxwell reads a memoir before the Royal Society in which the mechanical model is stripped away and just the equations remain. He also discusses the vector and scalar potentials, using the Coulomb gauge. He attributes physical signicance to both of these potentials. He wants to present the predictions of his theory on the subjects of re ection and refraction, but the requirements of his mechanical model keep him from nding the correct boundary conditions, so he never does this calculation. 1867 - Stokes performs experiments that kill his own anisotropic inertia theory. 1867 - Joseph Boussinesq suggests that instead of aether being dierent in dierent media, perhaps the aether is the same everywhere, but it interacts dierently with dierent materials, similar to the modern electromagnetic wave theory. 1867 - Riemann proposes a simple electric theory of light in which Poisson's equation is replaced by r2V(1=c2)@2V=@t2=4. 1867 - Ludwig Lorenz develops an electromagnetic theory of light in which the scalar and vector potentials, in retarded form, are the starting point. He shows that these retarded potentials each satisfy the wave equation and that Maxwell's equations for the elds EandHcan be derived from his potentials. His vector potential does not obey the Coulomb gauge, however, but another relation now known as the Lorenz gauge. Although he is able to derive Maxwell's equations from his retarded potentials, he does not subscribe to Maxwell's view that light involves electromagnetic waves in the aether. He feels, rather, that the fundamental basis of all luminous vibrations is electric currents, arguing that space has enough matter in it to support the necessary currents. 1868 - Maxwell decides that giving physical signicance to the scalar and vector potentials is a bad idea and bases his further work on light on EandH. 1869 - Maxwell presents the rst calculation in which a dispersive medium is made up of atoms with natural frequencies. This makes possible detailed modeling of dispersion with refractive indices having resonant denominators. 8 1869 - Hittorf nds that cathode rays can cast a shadow. 1870 - Helmholtz derives the correct laws of re ection and refraction from Maxwell's equations by using the following boundary conditions: Dn,Et, and Bare continuous. Once these boundary conditions are taken Maxwell's theory is just a repeat of MacCullagh's theory. The details were not given by Helmholtz himself, but appear rather in the inaugural dissertation of H. A. Lorentz. 1870-1900 - The hunt is on for physical models of the aether which are natural and from which Maxwell's equations can be derived. The physicists who work on this problem include Maxwell, Thomson, Kircho, Bjerknes, Leahy, Fitz Gerald, Helmholtz, and Hicks. 1872 - E. Mascart looks for the motion of the earth through the aether by measuring the rotation of the plane of polarization of light propagated along the axis of a quartz crystal. No motion is found with a sensitivity of v=c105. 1873 - Maxwell publishes his Treatise on Electricity and Magnetism , which discusses everything known at the time about electromagnetism from the viewpoint of Faraday. His own theory is not very thoroughly discussed, but he does introduce his electromagnetic stress tensor in this work, including the accompanying idea of electromagnetic momentum. 1875 - John Kerr shows that ordinary dielectrics subjected to strong electric elds become double refracting, showing directly that electric elds and light are closely related. 1876 - Henry Rowland performs an experiment inspired by Helmholtz which shows for the rst time that moving electric charge is the same thing as an electric current. 1876 - A. Bartoli infers the necessity of light pressure from thermal arguments, thus beginnning the exploration of the connection between electromagnetism and thermodynamics. 1879 - J. Stefan discovers the Stefan-Boltzmann law, i.e., that radiant emission is proportional to T4. 1879 - Edwin Hall performs an experiment that had been suggested by Henry Rowland and discovers the Hall eect, including its theoretical description by means of the Hall term in Ohm's law. 1879 - Sir William Crookes invents the radiometer and studies the interaction of beams of cathode ray particles in vacuum tubes. 1879 - Ludwig Boltzmann uses Hall's result to estimate the speed of charge carriers (assuming that charge carriers are only of one sign.) 1880 - Rowland shows that Faraday rotation can be obtained by combining Maxwell's equations and the Hall term in Ohm's law, assuming that displacement currents are aected in the same way as conduction currents. 1881 - J. J. Thomson attempts to verify the existence of the displacement current by looking for magnetic eects produced by the changing electric eld made by a moving charged sphere. 1881 - George Fitz Gerald points out that J. J. Thomson's analysis is incorrect because he left out the eects of the conduction current of the moving sphere. Including both currents makes the separate eect of the displacement current disappear. 1881 - Helmholtz, in a lecture in London, points out that the idea of charged particles in atoms can be consistent with Maxwell's and Faraday's ideas, helping to pave the way for our modern picture of particles and elds interacting instead of thinking about everything as a disturbance of the aether, as was popular after Maxwell. 1881 - Albert Michelson and Edwin Morley attempt to measure the motion of the earth through the aether by using interferometry. They nd no relative velocity. Michelson interprets this result as supporting Stokes hypothesis in which the aether in the neighborhood of the earth moves at the earth's velocity. 1883 - Fitz Gerald proposes testing Maxwell's theory by using oscillating currents in what we would now call a magnetic dipole antenna (loop of wire). He performs the analysis and discovers that very high 9 frequencies are required to make the test. Later that year he proposes obtaining the required high frequencies by discharging a capacitor into a circuit. 1883-5 - Horace Lamb and Oliver Heaviside analyze the interaction of oscillating electromagnetic elds with conductors and discover the eect of skin depth. 1884 - John Poynting shows that Maxwell's equations predict that energy ows through empty space with the energy ux given by EB=4. He also investigates energy ow in Faraday fashion by assigning energy to moving tubes of electric and magnetic ux. 1884 - Heinrich Hertz asserts that Emade by charges and Emade by a changing magnetic eld are identical. Working from dynamical ideas based on this assumption and some of Maxwell's equations, Hertz is able to derive the rest of them. 1887 - Svante Arrhenius deduces that in dilute solutions electrolytes are completely dissociated into positive and negative ions. 1887 - Hertz nds that ultraviolet light falling on the negative electrode in a spark gap facilitates conduction by the gas in the gap. 1888 - R. T. Glazebrook revives one of Cauchy's wave theories and combines it with Stokes anisotropic aether inertia theory to get agreement with the experiments of Stokes in 1867. 1888 - Hertz discovers that oscillating sparks can be produced in an open secondary circuit if the frequency of the primary is resonant with the secondary. He uses this radiator to show that electrical signals are propagated along wires and through the air at about the same speed, both about the speed of light. He also shows that his electric radiations, when passed through a slit in a screen, exhibit diraction eects. Polarization eects using a grating of parallel metal wires are also observed. 1888 - Roentgen shows that when an uncharged dielectric is moved at right angles to Ea magnetic eld is produced. 1889 - Hertz gives the theory of radiation from his oscillating spark gap. 1889 - Oliver Heaviside nds the correct form for the electric and magnetic elds of a moving charged particle, valid for all speeds v<c . 1889 - J. J. Thomson shows that Canton's eect (1762) in which a red hot poker can neutralize the electrication of a small charged body is due to electron emission causing the air between the poker and the body to become conducting. 1890 - Fitz Gerald uses the retarded potentials of L. Lorenz to calculate electric dipole radiation from Hertz's radiator. 1892 - Oliver Lodge performs experiments on the propagation of light near rapidly moving steel disks to test Stokes hypothesis that moving matter drags the aether with it. No such eect is observed. 1892 - Hendrik Anton Lorentz presents his electron theory of electried matter and the aether. This theory combines Maxwell's equations, with the source terms andJ, with the Lorentz force law for the acceleration of charged particles: ma=qE+qvB: Lorentz's aether is simply space endowed with certain dynamical properties. Lorentz gives the modern theory of dielectrics involving DandP, and also includes the eect of magnetized matter. He also gives what we now call the Drude-Lorentz harmonic oscillator model of the index of refraction. But Lorentz's theory has a \stationary aether", which con icts with the negative Michelson-Morley result. 1892 - George Fitz Gerald proposes length contraction as a way to reconcile Lorentz's theory and the null results on the motion of the earth through the aether. At the end of this year Lorentz endorses this idea. 1894 - J. J. Thomson measures the speed of cathode rays and shows that they travel much more slowly than the speed of light. The aether model of cathode rays begins to die. 10 1894 - Philip Lenard studies the penetration of cathode rays through matter. 1895 - Pierre Curie experimentally discovers Curie's law for paramagnetism and also shows that there is no temperature eect for diamagnetism. 1895 - Lorentz, in his \Search for a theory of electrical and optical eects in moving bodies" gives the Lorentz transformation to rst order in v=c. The transformed time variable he calls \local time". 1895 - Wilhelm Roentgen discovers X-rays produced by bremsstrahlung in cathode ray tubes. 1896 - Arthur Shuster, Emil Wiechert, and George Stokes propose that X-rays are aether waves of exceedingly small wavelength. 1896 - J. J. Thomson discovers that materials through which X-rays pass are rendered conducting. 1896 - Henri Becquerel discovers that some sort of natural radiation from uranium salts can expose a photographic plate wrapped in thick black paper. 1896 - P. Zeeman discovers the splitting of atomic line spectra by a magnetic eld. 1896 - Lorentz gives an electron theory of the Zeeman eect. 1897 - J. J. Thomson argues that cathode rays must be charged particles smaller in size than atoms (Emil Wiechert made the same suggestion independently in this same year). In response Fitz Gerald suggests that \we are dealing with free electrons in these cathode rays." 1897 - W. Wien discovers that positively-charged moving particles can also be made (the so-called canal rays of E. Goldstein) and that they have a much smaller q=m ratio than cathode rays. 1897 - J. J. Thomson de ects cathode rays by crossed electric and magnetic elds and measures e=m. 1898 - Marie and Pierre Curie separate from pitchblende two highly radioactive elements which they name polonium and radium. 1899 - Ernest Rutherford discovers that the rays from uranium come in two types, which he calls alpha and beta radiation. 1900 - Marie and Pierre Curie show that beta rays and cathode rays are identical. 1900 - Emil Wiechert shows that simply replacing the distributed charge dxdydz from Lorentz's theory with the charge of a moving point particle gives incorrect results. Instead the Lienard-Wiechert retarded potentials must be used. 1900 - Joseph Larmor obtains the second order corrections to the Lorentz Transformation. 1901 - R. Blondlot performs experiments that show that Lorentz's theory in which there is no moving aether gives the correct result in cases where the hypothesis of a moving aether gives the wrong result. 1902 - Lord Rayleigh performs experiments to test whether the Fitz Gerald contraction is capable of causing double refraction in moving transparent substances. No such eect is found. 1903 - The Hagen-Rubens connections between the conductivity of metals and their optical properties are experimentally established. 1903 - Lorentz gives the famous square root formulas for the Lorentz transformation giving the eect to all orders in v=c. 1904 - Lorentz gives his electron-collision theory of electrical conduction 1905 - H. A. Wilson performs experiments similar to those of Blondlot; again, Lorentz's theory is found to give the correct result. 1905 - Albert Einstein completes Lorentz's work on space-time transformations and relativity is born. 11 Index Aepinus, Francis Ulrich Theodore - 1759 Airy, George - 1846 Ampere, Andre Marie - 1820, 1825 Arago, Francois - 1811, 1816 Arrhenius, Svante - 1887 Bartoli, A. - 1876 Becquerel, Henri - 1896 Biot, Jean-Baptiste - 1812, 1820 Blondlot, R. - 1901 Boltzmann, Ludwig - 1879 Boussinesq, Joseph - 1867 Bradley, James - 1728 Brewster, David - 1815, 1816 Cabeo, Niccolo - 1620 Canton - 1762 Carlisle, Anthony - 1800 Cascariolo, Vincenzo - 1630 Cauchy, Augustine Louis - 1828, 1839 Cavendish, Henry - 1775 Coulomb, Charles Augustin - 1785 Crookes, William - 1879 Curie, Marie - 1895, 1900 Curie, Pierre - 1895, 1898, 1900 Davy, Humphrey - 1807, 1821 Descartes, Rene - 1638, 1644 Doppler, Christian - 1842 du Fay, Charles Francois - 1733 Einstein, Albert - 1905 Faraday, Michael - 1812, 1821, 1831, 1833, 1834, 1837, 1838, 1841, 1845, 1846, 1854, 1855 Fermat, Pierre de - 1657 Fitz Gerald, George - 1881, 1883, 1890, 1892 Fizeau, A. - 1849 Foucault, Jean - 1850 Franklin, Benjamin - 1747 Fresnel, Augustin - 1817, 1825, 1827 Galileo - 1638 Galvani, Luigi - 1780 Gauss,Karl Friedrich - 1813 Gilbert, William - 1600 Glazebrook, R. T. - 1888 Gray, Stephen - 1729 Green, George - 1828, 1837, 1839 Grimaldi, Francesco Maria - 1665 Hagen - 1903 Hall, Edwin - 1879 Heaviside, Oliver - 1883, 1889 Helmholtz, Hermann von - 1847, 1870, 1881 Henry, Joseph - 1832, 1842 Herschel, John - 1820 Hertz, Heinrich - 1884, 1887, 1888, 1889 Hittorf - 1869 Hooke, Robert - 1667 12 Huygens, Christiaan - 1678 Jacquier, Francis - 1742 Joule, James Prescott - 1841 Kerr, John - 1875 Kircho, Gustav - 1848, 1857 Lagrange, Joseph Louis - 1764, 1777 Lamb, Horace - 1883 Laplace, Pierre Simon - 1782, 1808, 1813 Larmor, Joseph - 1900 Le Seur, Thomas - 1742 Lenard, Philip - 1894 Lenz,Emil - 1834 Lodge, Oliver - 1892 Lorentz, Hendrik Anton - 1892, 1895, 1896, 1903, 1904 Lorenz, Ludwig - 1867 MacCullagh, James - 1835, 1839 Magnus - 900 BC Malus, Etienne Louis - 1808 Mascart, E. - 1872 Maxwell, James Clerk - 1855, 1861, 1864, 1868, 1869, 1873 Mayer, Julius Robert - 1842 Michell, John - 1750 Michelson, Albert - 1881 Morley, Edwin - 1881 Musschenbroek, Pieter van - 1745 Navier, Claude Louis Marie Henri - 1827 Neumann, Franz - 1835, 1842, 1845 Newton, Isaac - 1671, 1717 Nicholson, William - 1800 Nollet, Abbe0Jean-Antoine - 1749 O'Brien, Matthew - 1842 Oersted, Hans Christian - 1820 Ohm,Georg Simon - 1826 Ostrogradsky - 1831 Peltier, Jean Charles - 1834 Peregrinus, Peytrus - 1269 Poisson, Simeon Denis - 1812, 1824, 1828 Poynting, John - 1884 Priestly, Joseph - 1766 Rayleigh, Lord - 1902 Riemann, Bernhard - 1861, 1867 Roemer, Olaf - 1675 Roentgen, Wilhelm - 1888, 1895 Rowland, Henry - 1876, 1880 Rubens - 1903 Rutherford, Ernest - 1899 Savart, Felix - 1820 Savery, F. - 1827 Seebeck,Thomas Johann - 1822 Shuster, Arthur - 1896 Stefan, J. - 1879 Stokes, George Gabriel - 1825, 1849, 1850, 1867, 1896 Sulzer, Johann - 1752 Thales of Miletos - 600 BC 13 Thomson, J. J. - 1881, 1889, 1894, 1896, 1897 Thomson, William (Lord Kelvin) - 1839, 1846, 1850, 1851, 1853, 1854 Volta, Alessandro - 1793 Watson, William - 1748 Weber, Wilhelm - 1846, 1847 Wiechert, Emil - 1896, 1900 Wien, W. - 1897 Wilson, H. A. - 1905 Young, Thomas - 1801, 1803 Zeeman, Pieter - 1896 14