History of electromagnetic theory

For a chronological guide to this subject, seeTimeline of electromagnetic theory.

The history of electromagnetic theory begins with an-cient measures to deal with atmospheric electricity, inparticular lightning.*[1] People then had little under-standing of electricity, and were unable to scienticallyexplain the phenomena.*[2] In the 19th century therewas a unication of the history of electric theory withthe history of magnetic theory. It became clear thatelectricity should be treated jointly with magnetism, be-cause wherever electricity is in motion, magnetism is alsopresent.*[3] Magnetism was not fully explained until theidea of magnetic induction was developed.*[4] Electric-ity was not fully explained until the idea of electric chargewas developed.

1 Ancient and classical historyThe knowledge of static electricity dates back to the ear-liest civilizations, but for millennia it remained merelyan interesting and mystifying phenomenon, without atheory to explain its behavior and often confused withmagnetism. The ancients were acquainted with rathercurious properties possessed by two minerals, amber(Greek: , electron) and magnetic iron ore(Greek: , Magnes lithos, the Magne-sian stone, lodestone). Amber, when rubbed, attractslight bodies; magnetic iron ore has the power of attractingiron.*[5]Based on his nd of an Olmec hematite artifact in CentralAmerica, the American astronomer John Carlson hassuggested thatthe Olmec may have discovered and usedthe geomagnetic lodestone compass earlier than 1000BC. If true, thispredates the Chinese discovery ofthe geomagnetic lodestone compass by more than a mil-lennium.*[6]*[7] Carlson speculates that the Olmecsmay have used similar artifacts as a directional device forastrological or geomantic purposes, or to orient their tem-ples, the dwellings of the living or the interments of thedead. The earliest Chinese literature reference to mag-netism lies in a 4th-century BC book called Book of theDevil Valley Master (): The lodestone makesiron come or it attracts it.*[8]Long before any knowledge of electromagnetism existed,people were aware of the eects of electricity. Lightningand other manifestations of electricity such as St. Elmo'sre were known in ancient times, but it was not under-

The discovery of the property of magnets.Magnets were rst found in a natural state; certain iron ox-ides were discovered in various parts of the world, notably inMagnesia in AsiaMinor, that had the property of attracting smallpieces of iron, which is shown here.

Electric catsh are found in tropical Africa and the Nile River.

stood that these phenomena had a common origin.*[9]Ancient Egyptians were aware of shocks when interactingwith electric sh (such as the electric catsh) or other ani-mals (such as electric eels).*[10] The shocks from animalswere apparent to observers since pre-history by a varietyof peoples that came into contact with them. Texts from2750 BC by the ancient Egyptians referred to these sh

1

2 2 MIDDLE AGES AND THE RENAISSANCE

asthunderer of the Nile" and saw them as theprotec-torsof all the other sh.*[5] Another possible approachto the discovery of the identity of lightning and electric-ity from any other source, is to be attributed to the Arabs,who before the 15th century used the same Arabic wordfor lightning (barq) and the electric ray.*[9]Thales of Miletus, writing at around 600 BC, noted thatrubbing fur on various substances such as amber wouldcause them to attract specks of dust and other light ob-jects.*[11] Thales wrote on the eect now known as staticelectricity. The Greeks noted that if they rubbed the am-ber for long enough they could even get an electric sparkto jump.The electrostatic phenomena was again reported mil-lennia later by Roman and Arabic naturalists andphysicians.*[12] Several ancient writers, such as Pliny theElder and Scribonius Largus, attested to the numbing ef-fect of electric shocks delivered by catsh and torpedorays. Pliny in his books writes: The ancient Tuscansby their learning hold that there are nine gods that sendforth lightning and those of eleven sorts.This was ingeneral the early pagan idea of lightning.*[9] The ancientsheld some concept that shocks could travel along conduct-ing objects.*[13] Patients suering from ailments such asgout or headache were directed to touch electric sh in thehope that the powerful jolt might cure them.*[14]A number of objects found in Iraq in 1938 dated to theearly centuries AD (Sassanid Mesopotamia), called theBaghdad Battery, resembles a galvanic cell and is believedby some to have been used for electroplating.*[15] Theclaims are controversial because of supporting evidenceand theories for the uses of the artifacts,*[16]*[17] physi-cal evidence on the objects conducive for electrical func-tions,*[18] and if they were electrical in nature. As a re-sult the nature of these objects is based on speculation,and the function of these artifacts remains in doubt.*[19]

2 MiddleAges and theRenaissanceMagnetic attraction was once accounted by Aristotle andThales for as the working of a soul in the stone.*[20]In the 11th century, the Chinese scientist Shen Kuo(10311095) was the rst person to write of the mag-netic needle compass and that it improved the accuracyof navigation by employing the astronomical concept oftrue north (Dream Pool Essays, AD 1088 ), and by the12th century the Chinese were known to use the lodestonecompass for navigation. In 1187, Alexander Neckam wasthe rst in Europe to describe the compass and its use fornavigation.Magnetism was one of the few sciences which progressedin medieval Europe; for in the thirteenth century PeterPeregrinus, a native of Maricourt in Picardy, made adiscovery of fundamental importance.*[21] The French13th century scholar conducted experiments on mag-

Shen Kua wrote Dream Pool Essays (); Shen also rstdescribed the magnetic needle.

netism and wrote the rst extant treatise describing theproperties of magnets and pivoting compass needles.*[5]The dry compass was invented around 1300 by Italian in-ventor Flavio Gioja.*[22]Archbishop Eustathius of Thessalonica, Greek scholarand writer of the 12th century, records thatWoliver, kingof the Goths, was able to draw sparks from his body.The same writer states that a certain philosopher was ablewhile dressing to draw sparks from his clothes, a resultseemingly akin to that obtained by Robert Symmer in hissilk stocking experiments, a careful account of whichmaybe found in the 'Philosophical Transactions,' 1759.*[9]Italian physician Gerolamo Cardano wrote about electric-ity in De Subtilitate (1550) distinguishing, perhaps for therst time, between electrical and magnetic forces.Toward the late 16th century, a physician of Queen Eliz-abeth's time, Dr. William Gilbert, in De Magnete, ex-panded on Cardano's work and invented the New Latinword electricus from (elektron), the Greekword foramber. Gilbert, a native of Colchester, Fel-low of St John's College, Cambridge, and sometime Pres-ident of the College of Physicians, was one of the earliestand most distinguished English men of sciencea manwhose work Galileo thought enviably great. He was ap-pointed Court physician, and a pension was settled on himto set him free to continue his researches in Physics andChemistry.*[23]Gilbert undertook a number of careful electrical exper-iments, in the course of which he discovered that manysubstances other than amber, such as sulphur, wax, glass,etc.,*[24] were capable of manifesting electrical proper-ties. Gilbert also discovered that a heated body lost itselectricity and that moisture prevented the electricationof all bodies, due to the now well-known fact that mois-ture impaired the insulation of such bodies. He also no-ticed that electried substances attracted all other sub-

3stances indiscriminately, whereas a magnet only attractediron. The many discoveries of this nature earned forGilbert the title of founder of the electrical science.*[9]By investigating the forces on a light metallic needle, bal-anced on a point, he extended the list of electric bodies,and found also that many substances, including metalsand natural magnets, showed no attractive forces whenrubbed. He noticed that dry weather with north or eastwind was the most favourable atmospheric condition forexhibiting electric phenomenaan observation liable tomisconception until the dierence between conductorand insulator was understood.*[23]

Robert Boyle.

Gilbert's work was followed up by Robert Boyle (16271691), the famous natural philosopher who was oncedescribed asfather of Chemistry, and uncle of the Earlof Cork.Boyle was one of the founders of the RoyalSociety when it met privately in Oxford, and became amember of the Council after the Society was incorpo-rated by Charles II. in 1663. He worked frequently at thenew science of electricity, and added several substancesto Gilbert's list of electrics. He left a detailed account ofhis researches under the title of Experiments on the Ori-gin of Electricity.*[23] Boyle, in 1675, stated that electricattraction and repulsion can act across a vacuum. Oneof his important discoveries was that electried bodies ina vacuum would attract light substances, this indicatingthat the electrical eect did not depend upon the air as amedium. He also added resin to the then known list ofelectrics.*[9]*[25]*[26]*[27]This was followed in 1660 by Otto von Guericke, who in-vented an early electrostatic generator. By the end of the17th Century, researchers had developed practical means

of generating electricity by friction with an electrostaticgenerator, but the development of electrostatic machinesdid not begin in earnest until the 18th century, when theybecame fundamental instruments in the studies about thenew science of electricity.The rst usage of the word electricity is ascribed to SirThomas Browne in his 1646 work, Pseudodoxia Epidem-ica.The rst appearance of the term electromagnetism onthe other hand comes from an earlier date: 1641.Magnes,*[28] by the Jesuit luminary Athanasius Kircher,carries on page 640 the provocative chapter-heading:"Elektro-magnetismos i.e. On the Magnetism of am-ber, or electrical attractions and their causes(- id est sive De Magnetismo electri, seu elec-tricis attractionibus earumque causis).

3 18th century

3.1 Improving the electric machine

Main article: electrostatic machineThe electric machine was subsequently improved by

Generator built by Francis Hauksbee.*[29]

Francis Hauksbee, Litzendorf, and by Prof. Georg

4 3 18TH CENTURY

Matthias Bose, about 1750. Litzendorf, researching forChristian August Hausen, substituted a glass ball for thesulphur ball of Guericke. Bose was the rst to employ theprime conductorin such machines, this consisting ofan iron rod held in the hand of a person whose body wasinsulated by standing on a block of resin. Ingenhousz,during 1746, invented electric machines made of plateglass.*[30] Experiments with the electric machine werelargely aided by the discovery of the property of a glassplate, when coated on both sides with tinfoil, of accu-mulating a charge of electricity when connected with asource of electromotive force. The electric machine wassoon further improved by Andrew Gordon, a Scotsman,Professor at Erfurt, who substituted a glass cylinder inplace of a glass globe; and by Giessing of Leipzig whoadded a rubberconsisting of a cushion of woollenmaterial. The collector, consisting of a series of metalpoints, was added to the machine by Benjamin Wilsonabout 1746, and in 1762, John Canton of England (alsothe inventor of the rst pith-ball electroscope) improvedthe eciency of electric machines by sprinkling an amal-gam of tin over the surface of the rubber.*[9]

3.2 Electrics and non-electrics

In 1729, Stephen Gray conducted a series of experi-ments that demonstrated the dierence between conduc-tors and non-conductors (insulators), showing amongstother things that a metal wire and even pack thread con-ducted electricity, whereas silk did not. In one of his ex-periments he sent an electric current through 800 feet ofhempen thread which was suspended at intervals by loopsof silk thread. When he tried to conduct the same ex-periment substituting the silk for nely spun brass wire,he found that the electric current was no longer carriedthroughout the hemp cord, but instead seemed to vanishinto the brass wire. From this experiment he classiedsubstances into two categories: electricslike glass,resin and silk andnon-electricslike metal and water.Electricsconducted charges whilenon-electricsheldthe charge.*[9]*[31]

3.3 Vitreous and resinous

Intrigued by Gray's results, in 1732, C. F. du Fay beganto conduct several experiments. In his rst experiment,Du Fay concluded that all objects except metals, animals,and liquids could be electried by rubbing and thatmetals,animals and liquids could be electried by means of anelectric machine, thus discrediting Gray'selectricsandnon-electricsclassication of substances.In 1737 Du Fay and Hauksbee independently discoveredwhat they believed to be two kinds of frictional electricity;one generated from rubbing glass, the other from rubbingresin. From this, Du Fay theorized that electricity consistsof two electrical uids,vitreousandresinous, that

are separated by friction and that neutralize each otherwhen combined.*[32] This two-uid theory would latergive rise to the concept of positive and negative electricalcharges devised by Benjamin Franklin.*[9]

3.4 Leyden jar

Pieter van Musschenbroek.

The Leyden jar, a type of capacitor for electrical en-ergy in large quantities, was invented independently byEwaldGeorg vonKleist on 11October 1744 and by Pietervan Musschenbroek in 17451746 at Leiden Univer-sity (the latter location giving the device its name).*[33]William Watson, when experimenting with the Leydenjar, discovered in 1747 that a discharge of static elec-tricity was equivalent to an electric current. Capacitancewas rst observed by Von Kleist of Leyden in 1754.*[34]Von Kleist happened to hold, near his electric machine,a small bottle, in the neck of which there was an ironnail. Touching the iron nail accidentally with his otherhand he received a severe electric shock. In much thesame way Musschenbroeck assisted by Cunaens receiveda more severe shock from a somewhat similar glass bot-tle. Sir WilliamWatson of England greatly improved thisdevice, by covering the bottle, or jar, outside and in withtinfoil. This piece of electrical apparatus will be easilyrecognized as the well-known Leyden jar, so called bythe Abbot Nollet of Paris, after the place of its discov-ery.*[9]In 1741, John Ellicottproposed to measure the strengthof electrication by its power to raise a weight in one scaleof a balance while the other was held over the electri-ed body and pulled to it by its attractive power. TheSirWilliamWatson alreadymentioned conducted numer-

3.5 Late 18th century 5

ous experiments, about 1749, to ascertain the velocity ofelectricity in a wire. These experiments, although per-haps not so intended, also demonstrated the possibility oftransmitting signals to a distance by electricity. In theseexperiments, the signal appeared to travel the 12,276-footlength of the insulated wire instantaneously. Le Monnierin France had previously made somewhat similar exper-iments, sending shocks through an iron wire 1,319 feetlong.*[9]About 1750, rst experiments in electrotherapeutics weremade. Various experimenters made tests to ascertainthe physiological and therapeutical eects of electricity.Demainbray in Edinburgh examined the eects of elec-tricity upon plants and concluded that the growth of twomyrtle trees was quickened by electrication. These myr-tles were electried during the whole month of Oc-tober, 1746, and they put forth branches and blossomssooner than other shrubs of the same kind not electri-ed..*[35] Abb Mnon in France tried the eects ofa continued application of electricity upon men and birdsand found that the subjects experimented on lost weight,thus apparently showing that electricity quickened theexcretions.*[36]*[37] The ecacy of electric shocks incases of paralysis was tested in the county hospital atShrewsbury, England, with rather poor success.*[38]

3.5 Late 18th century

Benjamin Franklin.

Benjamin Franklin is frequently confused as the key lu-minary behind electricity; William Watson and Ben-

jamin Franklin share the discovery of electrical poten-tials . Benjamin Franklin promoted his investigations ofelectricity and theories through the famous, though ex-tremely dangerous, experiment of having his son y akite through a storm-threatened sky. A key attached tothe kite string sparked and charged a Leyden jar, thus es-tablishing the link between lightning and electricity.*[39]Following these experiments, he invented a lightning rod.It is either Franklin (more frequently) or Ebenezer Kin-nersley of Philadelphia (less frequently) who is consid-ered to have established the convention of positive andnegative electricity.Theories regarding the nature of electricity were quitevague at this period, and those prevalent were more orless conicting. Franklin considered that electricity wasan imponderable uid pervading everything, and which,in its normal condition, was uniformly distributed in allsubstances. He assumed that the electrical manifestationsobtained by rubbing glass were due to the production ofan excess of the electric uid in that substance and thatthe manifestations produced by rubbing wax were due toa decit of the uid. This theory was opposed by RobertSymmer's Two-uidtheory in 1759. By Symmer'stheory, the vitreous and resinous electricities were re-garded as imponderable uids, each uid being composedof mutually repellent particles while the particles of theopposite electricities are mutually attractive. When thetwo uids unite as a result of their attraction for one an-other, their eect upon external objects is neutralized.The act of rubbing a body decomposes the uids, one ofwhich remains in excess on the body and manifests itselfas vitreous or resinous electricity.*[9]Up to the time of Franklin's historic kite experi-ment,*[40] the identity of the electricity developed byrubbing and by electrostatic machines (frictional electric-ity) with lightning had not been generally established.Dr. Wall,*[41] Abbot Nollet, Hauksbee,*[42] StephenGray*[43] and John HenryWinkler*[44] had indeed sug-gested the resemblance between the phenomena ofelec-tricityandlightning, Gray having intimated that theyonly diered in degree. It was doubtless Franklin, how-ever, who rst proposed tests to determine the samenessof the phenomena. In a letter to Peter Comlinson of Lon-don, on 19 October 1752, Franklin, referring to his kiteexperiment, wrote,

At this key the phial (Leyden jar) maybe charged; and from the electric re thus ob-tained spirits may be kindled, and all the otherelectric experiments be formed which are usu-ally done by the help of a rubbed glass globeor tube, and thereby the sameness of the elec-tric matter with that of lightning be completelydemonstrated.*[45]

On 10 May 1742 Thomas-Franois Dalibard, at Mar-ley (near Paris), using a vertical iron rod 40 feet long,

6 3 18TH CENTURY

obtained results corresponding to those recorded byFranklin and somewhat prior to the date of Franklin'sexperiment. Franklin's important demonstration of thesameness of frictional electricity and lightning doubtlessadded zest to the eorts of the many experimenters inthis eld in the last half of the 18th century, to advancethe progress of the science.*[9]Franklin's observations aided later scientists such asMichael Faraday, Luigi Galvani, Alessandro Volta,Andr-Marie Ampre and Georg Simon Ohm, whosecollective work provided the basis for modern electri-cal technology and for whom fundamental units of elec-trical measurement are named. Others who would ad-vance the eld of knowledge included William Watson,Boze, Smeaton, Louis Guillaume Le Monnier, Jacquesde Romas, Jean Jallabert, Giovanni Battista Beccaria,Tiberius Cavallo, John Canton, Robert Symmer, AbbotNollet, John Henry Winkler, Richman, Dr. Wilson,Kinnersley, Joseph Priestley, Franz Aepinus, EdwardHussey Dlavai, Henry Cavendish and Charles-Augustinde Coulomb. Descriptions of many of the experimentsand discoveries of these early electrical scientists may befound in the scientic publications of the time, notablythe Philosophical Transactions, Philosophical Magazine,Cambridge Mathematical Journal, Young's Natural Phi-losophy, Priestley's History of Electricity, Franklin's Ex-periments and Observations on Electricity, Cavalli's Trea-tise on Electricity and De la Rive's Treatise on Electric-ity.*[9]Henry Elles was one of the rst people to suggest linksbetween electricity and magnetism. In 1757 he claimedthat he had written to the Royal Society in 1755 aboutthe links between electricity and magnetism, assertingthatthere are some things in the power of magnetismvery similar to those of electricitybut he didnot byany means think them the same. In 1760 he simi-larly claimed that in 1750 he had been the rstto thinkhow the electric re may be the cause of thunder.*[46]Among the more important of the electrical research andexperiments during this period were those of Franz Aepi-nus, a noted German scholar (17241802) and HenryCavendish of London, England.*[9]Franz Aepinus is credited as the rst to conceive of theview of the reciprocal relationship of electricity and mag-netism. In his work Tentamen Theoria Electricitatis etMagnetism,*[47] published in Saint Petersburg in 1759,he gives the following amplication of Franklin's theory,which in some of its features is measurably in accord withpresent-day views:The particles of the electric uid re-pel each other, attract and are attracted by the particles ofall bodies with a force that decreases in proportion as thedistance increases; the electric uid exists in the poresof bodies; it moves unobstructedly through non-electric(conductors), but moves with diculty in insulators; themanifestations of electricity are due to the unequal distri-bution of the uid in a body, or to the approach of bodiesunequally charged with the uid.Aepinus formulated a

corresponding theory of magnetism excepting that, in thecase of magnetic phenomena, the uids only acted on theparticles of iron. He also made numerous electrical ex-periments apparently showing that, in order to manifestelectrical eects, tourmaline must be heated to between37.5 and 100 C. In fact, tourmaline remains unelectri-ed when its temperature is uniform, but manifests elec-trical properties when its temperature is rising or falling.Crystals that manifest electrical properties in this way aretermed pyroelectric; along with tourmaline, these includesulphate of quinine and quartz.*[9]Henry Cavendish independently conceived a theory ofelectricity nearly akin to that of Aepinus.*[48] In 1784,he was perhaps the rst to utilize an electric spark to pro-duce an explosion of hydrogen and oxygen in the properproportions that would create pure water. Cavendish alsodiscovered the inductive capacity of dielectrics (insula-tors), and, as early as 1778, measured the specic induc-tive capacity for beeswax and other substances by com-parison with an air condenser.

Drawing of Coulomb's torsion balance. From Plate 13 of his1785 memoir.

Around 1784 C. A. Coulomb devised the torsion balance,discovering what is now known as Coulomb's law: theforce exerted between two small electried bodies variesinversely as the square of the distance, not as Aepinus inhis theory of electricity had assumed, merely inverselyas the distance. According to the theory advanced byCavendish, the particles attract and are attracted in-versely as some less power of the distance than the cube.*[9] A large part of the domain of electricity became virtu-ally annexed by Coulomb's discovery of the law of inversesquares.

7Through the experiments of William Watson and othersproving that electricity could be transmitted to a distance,the idea of making practical use of this phenomenon be-gan, around 1753, to engross theminds of inquisitive peo-ple. To this end, suggestions as to the employment ofelectricity in the transmission of intelligence were made.The rst of the methods devised for this purpose wasprobably that of Georges Lesage in 1774.*[49]*[50]*[51]This method consisted of 24 wires, insulated from one an-other and each having had a pith ball connected to its dis-tant end. Each wire represented a letter of the alphabet.To send a message, a desired wire was charged momen-tarily with electricity from an electric machine, where-upon the pith ball connected to that wire would y out.Other methods of telegraphing in which frictional elec-tricity was employed were also tried, some of which aredescribed in the history on the telegraph.*[9]The era of galvanic or voltaic electricity represented arevolutionary break from the historical focus on frictionalelectricity. Alessandro Volta discovered that chemical re-actions could be used to create positively charged anodesand negatively charged cathodes. When a conductor wasattached between these, the dierence in the electricalpotential (also known as voltage) drove a current betweenthem through the conductor. The potential dierence be-tween two points is measured in units of volts in recogni-tion of Volta's work.*[9]The rst mention of voltaic electricity, although not rec-ognized as such at the time, was probably made by JohannGeorg Sulzer in 1767, who, upon placing a small disc ofzinc under his tongue and a small disc of copper overit, observed a peculiar taste when the respective metalstouched at their edges. Sulzer assumed that when themet-als came together they were set into vibration, acting uponthe nerves of the tongue to produce the eects noticed.In 1790, Prof. Luigi Alyisio Galvani of Bologna, whileconducting experiments on "animal electricity", noticedthe twitching of a frog's legs in the presence of an electricmachine. He observed that a frog's muscle, suspended onan iron balustrade by a copper hook passing through itsdorsal column, underwent lively convulsions without anyextraneous cause, the electric machine being at this timeabsent.*[9]To account for this phenomenon, Galvani assumed thatelectricity of opposite kinds existed in the nerves andmuscles of the frog, the muscles and nerves constitutingthe charged coatings of a Leyden jar. Galvani publishedthe results of his discoveries, together with his hypothe-sis, which engrossed the attention of the physicists of thattime. The most prominent of these was Volta, professorof physics at Pavia, who contended that the results ob-served by Galvani were the result of the two metals, cop-per and iron, acting as electromotors, and that the mus-cles of the frog played the part of a conductor, completingthe circuit. This precipitated a long discussion betweenthe adherents of the conicting views. One group agreedwith Volta that the electric current was the result of an

electromotive force of contact at the twometals; the otheradopted a modication of Galvani's view and assertedthat the current was the result of a chemical anity be-tween the metals and the acids in the pile. Michael Fara-day wrote in the preface to his Experimental Researches,relative to the question of whether metallic contact is pro-ductive of a part of the electricity of the voltaic pile: Isee no reason as yet to alter the opinion I have given; ...but the point itself is of such great importance that I in-tend at the rst opportunity renewing the inquiry, and, ifI can, rendering the proofs either on the one side or theother, undeniable to all.*[9]Even Faraday himself, however, did not settle the con-troversy, and while the views of the advocates on bothsides of the question have undergone modications, assubsequent investigations and discoveries demanded, upto 1918 diversity of opinion on these points continuedto crop out. Volta made numerous experiments in sup-port of his theory and ultimately developed the pile orbattery,*[52] which was the precursor of all subsequentchemical batteries, and possessed the distinguishingmeritof being the rst means by which a prolonged continu-ous current of electricity was obtainable. Volta commu-nicated a description of his pile to the Royal Society ofLondon and shortly thereafter Nicholson and Cavendish(1780) produced the decomposition of water by meansof the electric current, using Volta's pile as the source ofelectromotive force.*[9]

4 19th century

4.1 Early 19th century

Alessandro Volta.

In 1800 Alessandro Volta constructed the rst deviceto produce a large electric current, later known as theelectric battery. Napoleon, informed of his works, sum-moned him in 1801 for a command performance of his

8 4 19TH CENTURY

experiments. He received many medals and decorations,including the Lgion d'honneur.Davy in 1806, employing a voltaic pile of approximately250 cells, or couples, decomposed potash and soda, show-ing that these substances were respectively the oxidesof potassium and sodium, which metals previously hadbeen unknown. These experiments were the beginningof electrochemistry, the investigation of which Faradaytook up, and concerning which in 1833 he announced hisimportant law of electrochemical equivalents, viz.: "Thesame quantity of electricitythat is, the same electric cur-rentdecomposes chemically equivalent quantities of allthe bodies which it traverses; hence the weights of elementsseparated in these electrolytes are to each other as theirchemical equivalents.Employing a battery of 2,000 el-ements of a voltaic pile Humphry Davy in 1809 gave therst public demonstration of the electric arc light, usingfor the purpose charcoal enclosed in a vacuum.*[9]Somewhat important to note, it was not until many yearsafter the discovery of the voltaic pile that the samenessof annual and frictional electricity with voltaic electricitywas clearly recognized and demonstrated. Thus as lateas January 1833 we nd Faraday writing*[53] in a paperon the electricity of the electric ray. "After an examina-tion of the experiments of Walsh,*[54]*[55] Ingenhousz,Henry Cavendish, Sir H. Davy, and Dr. Davy, no doubtremains on my mind as to the identity of the electricity ofthe torpedo with common (frictional) and voltaic electric-ity; and I presume that so little will remain on the mind ofothers as to justify my refraining from entering at lengthinto the philosophical proof of that identity. The doubtsraised by Sir Humphry Davy have been removed by hisbrother, Dr. Davy; the results of the latter being the re-verse of those of the former. ... The general conclusionwhich must, I think, be drawn from this collection of facts(a table showing the similarity, of properties of the di-versely named electricities) is, that electricity, whatevermay be its source, is identical in its nature.*[9]It is proper to state, however, that prior to Faraday's timethe similarity of electricity derived from dierent sourceswas more than suspected. Thus, William Hyde Wol-laston,*[56] wrote in 1801:*[57] "This similarity in themeans by which both electricity and galvanism (voltaicelectricity) appear to be excited in addition to the resem-blance that has been traced between their eects showsthat they are both essentially the same and conrm anopinion that has already been advanced by others, thatall the dierences discoverable in the eects of the lat-ter may be owing to its being less intense, but producedin much larger quantity.In the same paper Wollastondescribes certain experiments in which he uses very newire in a solution of sulphate of copper through which hepassed electric currents from an electric machine. This isinteresting in connection with the later day use of almostsimilarly arranged ne wires in electrolytic receivers inwireless, or radio-telegraphy.*[9]

Hans Christian rsted.

In the rst half of the 19th century many very importantadditions were made to the world's knowledge concerningelectricity and magnetism. For example, in 1819 HansChristian rsted of Copenhagen discovered the deect-ing eect of an electric current traversing a wire upon- asuspended magnetic needle.*[9]This discovery gave a clue to the subsequently provedintimate relationship between electricity and magnetismwhich was promptly followed up by Ampre who shortlythereafter (1821) announced his celebrated theory ofelectrodynamics, relating to the force that one currentexerts upon another, by its electro-magnetic eects,namely*[9]

1. Two parallel portions of a circuit attract one anotherif the currents in them are owing in the same di-rection, and repel one another if the currents ow inthe opposite direction.

2. Two portions of circuits crossing one anotherobliquely attract one another if both the currentsow either towards or from the point of crossing,and repel one another if one ows to and the otherfrom that point.

3. When an element of a circuit exerts a force on an-other element of a circuit, that force always tends tourge the second one in a direction at right angles toits own direction.

Ampere brought a multitude of phenomena into theoryby his investigations of the mechanical forces betweenconductors supporting currents and magnets.

4.1 Early 19th century 9

The German physicist Seebeck discovered in 1821 thatwhen heat is applied to the junction of twometals that hadbeen soldered together an electric current is set up. This istermed Thermo-Electricity. Seebeck's device consists ofa strip of copper bent at each end and soldered to a plateof bismuth. A magnetic needle is placed parallel with thecopper strip. When the heat of a lamp is applied to thejunction of the copper and bismuth an electric current isset up which deects the needle.*[9]Around this time, Simon Denis Poisson attacked the dif-cult problem of induced magnetization, and his results,though dierently expressed, are still the theory, as amost important rst approximation. It was in the applica-tion of mathematics to physics that his services to sciencewere performed. Perhaps the most original, and certainlythe most permanent in their inuence, were his memoirson the theory of electricity and magnetism, which virtu-ally created a new branch of mathematical physics.GeorgeGreen wroteAn Essay on the Application ofMath-ematical Analysis to the Theories of Electricity and Mag-netism in 1828. The essay introduced several importantconcepts, among them a theorem similar to the modernGreen's theorem, the idea of potential functions as cur-rently used in physics, and the concept of what are nowcalled Green's functions. George Green was the rst per-son to create a mathematical theory of electricity andmagnetism and his theory formed the foundation for thework of other scientists such as James Clerk Maxwell,William Thomson, and others.Peltier in 1834 discovered an eect opposite to Thermo-Electricity, namely, that when a current is passed througha couple of dissimilar metals the temperature is loweredor raised at the junction of the metals, depending on thedirection of the current. This is termed the Peltieref-fect. The variations of temperature are found to beproportional to the strength of the current and not to thesquare of the strength of the current as in the case of heatdue to the ordinary resistance of a conductor. This sec-ond law is the C^2R law,*[58] discovered experimentallyin 1841 by the English physicist Joule. In other words,this important law is that the heat generated in any partof an electric circuit is directly proportional to the prod-uct of the resistance of this part of the circuit and to thesquare of the strength of current owing in the circuit.*[9]In 1822 Johann Schweigger devised the rstgalvanometer. This instrument was subsequentlymuch improved by Wilhelm Weber (1833). In 1825William Sturgeon of Woolwich, England, invented thehorseshoe and straight bar electromagnet, receivingtherefor the silver medal of the Society of Arts.*[59]In 1837 Carl Friedrich Gauss and Weber (both notedworkers of this period) jointly invented a reectinggalvanometer for telegraph purposes. This was the fore-runner of the Thomson reecting and other exceedinglysensitive galvanometers once used in submarine signalingand still widely employed in electrical measurements.

Arago in 1824 made the important discovery that when acopper disc is rotated in its own plane, and if a magneticneedle be freely suspended on a pivot over the disc, theneedle will rotate with the disc. If on the other hand theneedle is xed it will tend to retard the motion of the disc.This eect was termed Arago's rotations.*[9]*[60]*[61]

Georg Simon Ohm.

Futile attempts were made by Charles Babbage, PeterBarlow, John Herschel and others to explain this phe-nomenon. The true explanation was reserved for Fara-day, namely, that electric currents are induced in the cop-per disc by the cutting of the magnetic lines of forceof the needle, which currents in turn react on the nee-dle. Georg Simon Ohm did his work on resistance in theyears 1825 and 1826, and published his results in 1827as the book Die galvanische Kette, mathematisch bear-beitet.*[62]*[63] He drew considerable inspiration fromFourier's work on heat conduction in the theoretical ex-planation of his work. For experiments, he initially usedvoltaic piles, but later used a thermocouple as this pro-vided a more stable voltage source in terms of internalresistance and constant potential dierence. He used agalvanometer to measure current, and knew that the volt-age between the thermocouple terminals was proportionalto the junction temperature. He then added test wires ofvarying length, diameter, and material to complete thecircuit. He found that his data could be modeled througha simple equation with variable composed of the read-ing from a galvanometer, the length of the test conduc-tor, thermocouple junction temperature, and a constantof the entire setup. From this, Ohm determined his lawof proportionality and published his results. In 1827, heannounced the now famous law that bears his name, that

10 4 19TH CENTURY

is:

Electromotive force = Current Resistance*[64]

Ohm brought into order a host of puzzling facts connect-ing electromotive force and electric current in conduc-tors, which all previous electricians had only succeeded inloosely binding together qualitatively under some rathervague statements. Ohm found that the results could besummed up in such a simple law and by Ohm's discoverya large part of the domain of electricity became annexedto theory.

4.2 Faraday and Henry

Joseph Henry.

The discovery of electromagnetic induction was made al-most simultaneously, although independently, byMichaelFaraday, who was rst to make the discovery in 1831,and Joseph Henry in 1832.*[65]*[66] Henry's discoveryof self-induction and his work on spiral conductors usinga copper coil were made public in 1835, just before thoseof Faraday.*[67]*[68]*[69]In 1831 began the epoch-making researches of MichaelFaraday, the famous pupil and successor of HumphryDavy at the head of the Royal Institution, London, re-lating to electric and electromagnetic induction. The re-markable researches of Faraday, the prince of experimen-talists, on electrostatics and electrodynamics and the in-duction of currents. These were rather long in beingbrought from the crude experimental state to a compactsystem, expressing the real essence. Faraday was not

a competent mathematician,*[70]*[71]*[72] but had hebeen one, he would have been greatly assisted in his re-searches, have saved himself much useless speculation,and would have anticipated much later work. He would,for instance, knowing Ampere's theory, by his own re-sults have readily been led to Neumann's theory, and theconnected work of Helmholtz and Thomson. Faraday'sstudies and researches extended from 1831 to 1855 anda detailed description of his experiments, deductions andspeculations are to be found in his compiled papers, en-titled Experimental Researches in Electricity.' Faradaywas by profession a chemist. He was not in the remotestdegree a mathematician in the ordinary senseindeed itis a question if in all his writings there is a single mathe-matical formula.*[9]

Michael Faraday.

The experiment which led Faraday to the discovery ofelectric induction was made as follows: He constructedwhat is now and was then termed an induction coil, theprimary and secondary wires of which were wound on awooden bobbin, side by side, and insulated from one an-other. In the circuit of the primary wire he placed a bat-tery of approximately 100 cells. In the secondary wire heinserted a galvanometer. On making his rst test he ob-served no results, the galvanometer remaining quiescent,but on increasing the length of the wires he noticed a de-ection of the galvanometer in the secondary wire whenthe circuit of the primary wire was made and broken.This was the rst observed instance of the developmentof electromotive force by electromagnetic induction.*[9]He also discovered that induced currents are establishedin a second closed circuit when the current strength is var-

4.3 Middle 19th century 11

ied in the rst wire, and that the direction of the currentin the secondary circuit is opposite to that in the rst cir-cuit. Also that a current is induced in a secondary cir-cuit when another circuit carrying a current is moved toand from the rst circuit, and that the approach or with-drawal of a magnet to or from a closed circuit inducesmomentary currents in the latter. In short, within thespace of a few months Faraday discovered by experi-ment virtually all the laws and facts now known concern-ing electro-magnetic induction and magneto-electric in-duction. Upon these discoveries, with scarcely an ex-ception, depends the operation of the telephone, thedynamo machine, and incidental to the dynamo electricmachine practically all the gigantic electrical industriesof the world, including electric lighting, electric traction,the operation of electric motors for power purposes, andelectro-plating, electrotyping, etc.*[9]In his investigations of the peculiar manner in which ironlings arrange themselves on a cardboard or glass in prox-imity to the poles of a magnet, Faraday conceived the ideaof magnetic "lines of force" extending from pole to poleof the magnet and along which the lings tend to placethemselves. On the discovery being made that magneticeects accompany the passage of an electric current in awire, it was also assumed that similar magnetic lines offorce whirled around the wire. For convenience and toaccount for induced electricity it was then assumed thatwhen these lines of force are "cut" by a wire in passingacross them or when the lines of force in rising and fallingcut the wire, a current of electricity is developed, or to bemore exact, an electromotive force is developed in thewire that sets up a current in a closed circuit. Faradayadvanced what has been termed the molecular theory ofelectricity*[73] which assumes that electricity is the man-ifestation of a peculiar condition of the molecule of thebody rubbed or the ether surrounding the body. Fara-day also, by experiment, discovered paramagnetism anddiamagnetism, namely, that all solids and liquids are ei-ther attracted or repelled by a magnet. For example, iron,nickel, cobalt, manganese, chromium, etc., are paramag-netic (attracted by magnetism), whilst other substances,such as bismuth, phosphorus, antimony, zinc, etc., arerepelled by magnetism or are diamagnetic.*[9]*[74]Brugans of Leyden in 1778 and Le Baillif and Becquerelin 1827*[75] had previously discovered diamagnetism inthe case of bismuth and antimony. Faraday also rediscov-ered specic inductive capacity in 1837, the results of theexperiments by Cavendish not having been published atthat time. He also predicted*[76] the retardation of sig-nals on long submarine cables due to the inductive eectof the insulation of the cable, in other words, the staticcapacity of the cable.*[9]The 25 years immediately following Faraday's discover-ies of electric induction were fruitful in the promulgationof laws and facts relating to induced currents and to mag-netism. In 1834 Heinrich Lenz and Moritz von Jacobi in-dependently demonstrated the now familiar fact that the

currents induced in a coil are proportional to the numberof turns in the coil. Lenz also announced at that time hisimportant law that, in all cases of electromagnetic induc-tion the induced currents have such a direction that theirreaction tends to stop the motion that produces them, alaw that was perhaps deducible from Faraday's explana-tion of Arago's rotations.*[9]*[77]The induction coil was rst designed by Nicholas Callanin 1836. In 1845 Joseph Henry, the American physicist,published an account of his valuable and interesting ex-periments with induced currents of a high order, showingthat currents could be induced from the secondary of aninduction coil to the primary of a second coil, thence toits secondary wire, and so on to the primary of a thirdcoil, etc.*[78] Heinrich Daniel Ruhmkor further devel-oped the induction coil, the Ruhmkor coil was patentedin 1851,*[79] and he utilized long windings of copperwire to achieve a spark of approximately 2 inches (50mm) in length. In 1857, after examining a greatly im-proved version made by an American inventor, EdwardSamuel Ritchie,*[80]*[81] Ruhmkor improved his de-sign (as did other engineers), using glass insulation andother innovations to allow the production of sparks morethan 300 millimetres (12 in) long.*[82]

4.3 Middle 19th centuryUp to the middle of the 19th century, indeed up to about1870, electrical science was, it may be said, a sealedbook to the majority of electrical workers. Prior tothis time a number of handbooks had been publishedon electricity and magnetism, notably Auguste de LaRive's exhaustive ' Treatise on Electricity,'*[84] in 1851(French) and 1853 (English); August Beer's Einleitungin die Elektrostatik, die Lehre vom Magnetismus und dieElektrodynamik,*[85] Wiedemann's ' Galvanismus,' andReiss'*[86] 'Reibungsal-elektricitat.' But these works con-sisted in the main in details of experiments with elec-tricity and magnetism, and but little with the laws andfacts of those phenomena. Henry d'Abria*[87]*[88]published the results of some researches into the lawsof induced currents, but owing to their complexity ofthe investigation it was not productive of very notableresults.*[89] Around the mid-19th century, FleemingJenkin's work on ' Electricity and Magnetism*[90] ' andClerk Maxwell's ' Treatise on Electricity and Magnetism 'were published.*[9]These books were departures from the beaten path. AsJenkin states in the preface to his work the science ofthe schools was so dissimilar from that of the practicalelectrician that it was quite impossible to give studentssucient, or even approximately sucient, textbooks. Astudent he said might have mastered de la Rive's large andvaluable treatise and yet feel as if in an unknown coun-try and listening to an unknown tongue in the companyof practical men. As another writer has said, with thecoming of Jenkin's and Maxwell's books all impediments

12 4 19TH CENTURY

in the way of electrical students were removed, "the fullmeaning of Ohm's law becomes clear; electromotive force,dierence of potential, resistance, current, capacity, linesof force, magnetization and chemical anity were mea-surable, and could be reasoned about, and calculationscould be made about them with as much certainty as cal-culations in dynamics".*[9]*[91]About 1850, Kirchho published his laws relating tobranched or divided circuits. He also showed mathe-matically that according to the then prevailing electro-dynamic theory, electricity would be propagated alonga perfectly conducting wire with the velocity of light.Helmholtz investigated mathematically the eects of in-duction upon the strength of a current and deducedtherefrom equations, which experiment conrmed, show-ing amongst other important points the retarding eectof self-induction under certain conditions of the cir-cuit.*[9]*[92]

Sir William Thomson.

In 1853, Sir William Thomson (later Lord Kelvin) pre-dicted as a result of mathematical calculations the oscil-latory nature of the electric discharge of a condenser cir-cuit. To Henry, however, belongs the credit of discern-ing as a result of his experiments in 1842 the oscillatorynature of the Leyden jar discharge. He wrote:*[93] Thephenomena require us to admit the existence of a princi-pal discharge in one direction, and then several reex ac-tions backward and forward, each more feeble than thepreceding, until the equilibrium is obtained. These oscil-lations were subsequently observed by B. W. Feddersen(1857)*[94]*[95] who using a rotating concave mirrorprojected an image of the electric spark upon a sensi-tive plate, thereby obtaining a photograph of the spark

which plainly indicated the alternating nature of the dis-charge. Sir William Thomson was also the discoverer ofthe electric convection of heat (theThomsoneect).He designed for electrical measurements of precision hisquadrant and absolute electrometers. The reecting gal-vanometer and siphon recorder, as applied to submarinecable signaling, are also due to him.*[9]About 1876 the American physicist Henry AugustusRowland of Baltimore demonstrated the important factthat a static charge carried around produces the samemagnetic eects as an electric current.*[96]*[97] The Im-portance of this discovery consists in that it may aorda plausible theory of magnetism, namely, that magnetismmay be the result of directed motion of rows of moleculescarrying static charges.*[9]After Faraday's discovery that electric currents could bedeveloped in a wire by causing it to cut across the linesof force of a magnet, it was to be expected that attemptswould be made to construct machines to avail of this factin the development of voltaic currents.*[98] The rst ma-chine of this kind was due to Hippolyte Pixii, 1832. Itconsisted of two bobbins of iron wire, opposite whichthe poles of a horseshoe magnet were caused to rotate.As this produced in the coils of the wire an alternatingcurrent, Pixii arranged a commutating device (commuta-tor) that converted the alternating current of the coils orarmature into a direct current in the external circuit. Thismachine was followed by improved forms of magneto-electric machines due to RItchie, Saxton, Clarke 1834,Stohrer 1843, Nollet 1849, Shepperd 1856, VanMaldern,Siemens, Wilde and others.*[9]A notable advance in the art of dynamo constructionwas made by Mr. S. A. Varley in 1866*[99] and byDr. Charles William Siemens and Mr. Charles Wheat-stone,*[100] who independently discovered that when acoil of wire, or armature, of the dynamo machine is ro-tated between the poles (or in theeld) of an electro-magnet, a weak current is set up in the coil due to resid-ual magnetism in the iron of the electromagnet, and thatif the circuit of the armature be connected with the cir-cuit of the electromagnet, the weak current developed inthe armature increases the magnetism in the eld. Thisfurther increases the magnetic lines of force in which thearmature rotates, which still further increases the currentin the electromagnet, thereby producing a correspondingincrease in the eld magnetism, and so on, until the max-imum electromotive force which the machine is capableof developing is reached. By means of this principle thedynamomachine develops its own magnetic eld, therebymuch increasing its eciency and economical operation.Not by any means, however, was the dynamo electric ma-chine perfected at the time mentioned.*[9]In 1860 an important improvement had been made byDr. Antonio Pacinotti of Pisa who devised the rst elec-tric machine with a ring armature. This machine wasrst used as an electric motor, but afterward as a gener-

4.4 Maxwell 13

ator of electricity. The discovery of the principle of thereversibility of the dynamo electric machine (variouslyattributed to Walenn 1860; Pacinotti 1864 ; Fontaine,Gramme 1873; Deprez 1881, and others) whereby it maybe used as an electric motor or as a generator of electric-ity has been termed one of the greatest discoveries of the19th century.*[9]In 1872 the drum armature was devised by Hefner-Alteneck. This machine in a modied form was subse-quently known as the Siemens dynamo. These machineswere presently followed by the Schuckert, Gulcher,*[101]Fein,*[102]*[103] Brush, Hochhausen, Edison and thedynamo machines of numerous other inventors. In theearly days of dynamo machine construction the machineswere mainly arranged as direct current generators, andperhaps the most important application of such machinesat that time was in electro-plating, for which purpose ma-chines of low voltage and large current strength were em-ployed.*[9]*[104]Beginning about 1887 alternating current generatorscame into extensive operation and the commercial devel-opment of the transformer, by means of which currentsof low voltage and high current strength are transformedto currents of high voltage and low current strength, andvice versa, in time revolutionized the transmission ofelectric power to long distances. Likewise the intro-duction of the rotary converter (in connection with thestep-downtransformer) which converts alternating cur-rents into direct currents (and vice versa) has eectedlarge economies in the operation of electric power sys-tems.*[9]*[105]Before the introduction of dynamo electric machines,voltaic, or primary, batteries were extensively used forelectro-plating and in telegraphy. There are two distincttypes of voltaic cells, namely, theopenand theclosed, or constant, type. The open type in brief is thattype which operated on closed circuit becomes, after ashort time, polarized; that is, gases are liberated in thecell which settle on the negative plate and establish a re-sistance that reduces the current strength. After a briefinterval of open circuit these gases are eliminated or ab-sorbed and the cell is again ready for operation. Closedcircuit cells are those in which the gases in the cells areabsorbed as quickly as liberated and hence the output ofthe cell is practically uniform. The Leclanch and Daniellcells, respectively, are familiar examples of theopenand closedtype of voltaic cell. The opencellsare used very extensively at present, especially in the drycell form, and in annunciator and other open circuit sig-nal systems. Batteries of the Daniell orgravitytypewere employed almost generally in the United States andCanada as the source of electromotive force in telegra-phy before the dynamo machine became available, andstill are largely used for this service or aslocalcells.Batteries of thegravityand the Edison-Lalande typesare still much used inclosed circuitsystems.*[9]

In the late 19th century, the term luminiferous aether,meaning light-bearing aether, was a conjectured mediumfor the propagation of light.*[106] The word aether stemsvia Latin from the Greek , from a root meaning tokindle, burn, or shine. It signies the substance whichwas thought in ancient times to ll the upper regions ofspace, beyond the clouds.

4.4 Maxwell

James Clerk Maxwell.

In 1864 James Clerk Maxwell of Edinburgh announcedhis electromagnetic theory of light, which was perhapsthe greatest single step in the world's knowledge of elec-tricity.*[107] Maxwell had studied and commented onthe eld of electricity and magnetism as early as 1855/6when On Faraday's lines of force*[108] was read to theCambridge Philosophical Society. The paper presenteda simplied model of Faraday's work, and how the twophenomena were related. He reduced all of the cur-rent knowledge into a linked set of dierential equa-tions with 20 equations in 20 variables. This work waslater published as On Physical Lines of Force in March1861.*[109] In order to determine the force which is act-ing on any part of the machine we must nd its momen-tum, and then calculate the rate at which this momentumis being changed. This rate of change will give us theforce. The method of calculation which it is necessary toemploy was rst given by Lagrange, and afterwards devel-oped, with some modications, by Hamilton's equations.It is usually referred to as Hamilton's principle; when theequations in the original form are used they are knownas Lagrange's equations. Now Maxwell logically showedhow these methods of calculation could be applied to the

14 4 19TH CENTURY

electro-magnetic eld.*[110] The energy of a dynamicalsystem is partly kinetic, partly potential. Maxwell sup-poses that the magnetic energy of the eld is kinetic en-ergy, the electric energy potential.*[111]Around 1862, while lecturing at King's College, Maxwellcalculated that the speed of propagation of an electro-magnetic eld is approximately that of the speed of light.He considered this to be more than just a coincidence,and commented "We can scarcely avoid the conclusionthat light consists in the transverse undulations of the samemedium which is the cause of electric and magnetic phe-nomena."*[112]Working on the problem further, Maxwell showed thatthe equations predict the existence of waves of oscillat-ing electric and magnetic elds that travel through emptyspace at a speed that could be predicted from simple elec-trical experiments; using the data available at the time,Maxwell obtained a velocity of 310,740,000 m/s. In his1864 paper A Dynamical Theory of the ElectromagneticField, Maxwell wrote, The agreement of the results seemsto show that light and magnetism are aections of the samesubstance, and that light is an electromagnetic disturbancepropagated through the eld according to electromagneticlaws.*[113]As already noted herein Faraday, and before him, Am-pre and others, had inklings that the luminiferous etherof space was also the medium for electric action. It wasknown by calculation and experiment that the velocity ofelectricity was approximately 186,000 miles per second;that is, equal to the velocity of light, which in itself sug-gests the idea of a relationship between -electricity andlight.A number of the earlier philosophers or mathe-maticians, as Maxwell terms them, of the 19th century,held the view that electromagnetic phenomena were ex-plainable by action at a distance. Maxwell, followingFaraday, contended that the seat of the phenomena wasin the medium. The methods of the mathematicians inarriving at their results were synthetical while Faraday'smethods were analytical. Faraday in his mind's eye sawlines of force traversing all space where the mathemati-cians saw centres of force attracting at a distance. Faradaysought the seat of the phenomena in real actions going onin the medium; they were satised that they had foundit in a power of action at a distance on the electric u-ids.*[114]Both of these methods, as Maxwell points out, had suc-ceeded in explaining the propagation of light as an elec-tromagnetic phenomenon while at the same time the fun-damental conceptions of what the quantities concernedare, radically diered. The mathematicians assumed thatinsulators were barriers to electric currents; that, for in-stance, in a Leyden jar or electric condenser the electric-ity was accumulated at one plate and that by some occultaction at a distance electricity of an opposite kind wasattracted to the other plate.Maxwell, looking further than Faraday, reasoned that if

light is an electromagnetic phenomenon and is transmis-sible through dielectrics such as glass, the phenomenonmust be in the nature of electromagnetic currents in thedielectrics. He therefore contended that in the chargingof a condenser, for instance, the action did not stop atthe insulator, but that somedisplacementcurrents areset up in the insulating medium, which currents continueuntil the resisting force of the medium equals that of thecharging force. In a closed conductor circuit, an electriccurrent is also a displacement of electricity.The conductor oers a certain resistance, akin to friction,to the displacement of electricity, and heat is developedin the conductor, proportional to the square of the cur-rent(as already stated herein), which current ows as longas the impelling electric force continues. This resistancemay be likened to that met with by a ship as it displacesin the water in its progress. The resistance of the dielec-tric is of a dierent nature and has been compared to thecompression of multitudes of springs, which, under com-pression, yield with an increasing back pressure, up to apoint where the total back pressure equals the initial pres-sure. When the initial pressure is withdrawn the energyexpended in compressing the springsis returned tothe circuit, concurrently with the return of the springs totheir original condition, this producing a reaction in theopposite direction. Consequently the current due to thedisplacement of electricity in a conductor may be contin-uous, while the displacement currents in a dielectric aremomentary and, in a circuit or medium which containsbut little resistance compared with capacity or inductancereaction, the currents of discharge are of an oscillatory oralternating nature.*[115]Maxwell extended this view of displacement currents indielectrics to the ether of free space. Assuming light to bethe manifestation of alterations of electric currents in theether, and vibrating at the rate of light vibrations, thesevibrations by induction set up corresponding vibrations inadjoining portions of the ether, and in this way the undu-lations corresponding to those of light are propagated asan electromagnetic eect in the ether. Maxwell's electro-magnetic theory of light obviously involved the existenceof electric waves in free space, and his followers set them-selves the task of experimentally demonstrating the truthof the theory. By 1871, he presented the Remarks on themathematical classication of physical quantities.*[116]

4.5 End of the 19th centuryIn 1887, the German physicist Heinrich Hertz ina series of experiments proved the actual existenceelectromagnetic waves, showing that transverse free spaceelectromagnetic waves can travel over some distance aspredicted by Maxwell and Faraday. Hertz published hiswork in a book titled: Electric waves: being researcheson the propagation of electric action with nite velocitythrough space.*[117] The discovery of electromagneticwaves in space led to the development in the closing years

4.5 End of the 19th century 15

Heinrich Hertz.

of the 19th century of radio.The electron as a unit of charge in electrochemistry wasposited by G. Johnstone Stoney in 1874, who also coinedthe term electron in 1894. Plasma was rst identied in aCrookes tube, and so described by Sir William Crookesin 1879 (he called itradiant matter).*[118] The placeof electricity in leading up to the discovery of those beau-tiful phenomena of the Crookes Tube (due to Sir WilliamCrookes), viz., Cathode rays,*[119] and later to the dis-covery of Roentgen or X-rays, must not be overlooked,since without electricity as the excitant of the tube thediscovery of the rays might have been postponed inde-nitely. It has been noted herein that Dr. William Gilbertwas termed the founder of electrical science. This must,however, be regarded as a comparative statement.*[9]Oliver Heaviside was a self-taught scholar who reformu-lated Maxwell's eld equations in terms of electric andmagnetic forces and energy ux, and independently co-formulated vector analysis. His series of articles con-tinued the work entitled "Electromagnetic Induction andits Propagation", commenced in The Electrician in 1885to nearly 1887 (ed., the latter part of the work deal-ing with the propagation of electromagnetic waves alongwires through the dielectric surrounding them), when thegreat pressure on space and the want of readers appearedto necessitate its abrupt discontinuance.*[120] (A strag-gler piece appeared December 31, 1887.) He wrote aninterpretation of the transcendental formulae of electro-magnetism. Following the real object of true natural-ists*[121] when they employ mathematics to assist them,he wrote to nd out the connections of known phenom-ena, and by deductive reasoning, to obtain a knowledgeof electromagnetic phenomena. Although at odds with

1Oliver Heaviside.

the scientic establishment for most of his life, Heavisidechanged the face of mathematics and science for years tocome.Of the changes in the eld of electromagnetic theory, cer-tain conclusions from Electro-Magnetic Theory*[122] byHeaviside are, if not drawn, at least indicated in this book.Two of them may be stated as follows:

1. That magnetism is a phenomenon of motion and nota statical phenomenon; also that this motion is morelikely to be translational than vortical.

2. That all electric currents are phenomena consequentupon the emission of electro-magnetic wave distur-bances in the aether, and that the proper treatmentof all the phenomena of currents and magnetic uxshould be considered as the consequence, and not asthe cause, of electro-magnetic waves.

The ultimate results of his work are twofold. (1) The rstultimate result is purely mathematical, which is impor-tant only to those who study mathematical physics. Thesystem of vectorial algebra*[123] as developed by Mr.Heaviside was used because of ease for physical inves-tigations to the methods of quaternions. (2) The sec-ond ultimate result is physical. It consists in more closelyuniting the more recondite problems of telegraphy, tele-phony, Teslaic phenomena and Hertzian phenomena withthe fundamental properties of the aether. In elucidat-ing this connection, the merit of the book appears mostprominently as a stepping-stone to the goal in the full viewof all physical analysis, namely, the resolution of all phys-

16 4 19TH CENTURY

ical phenomena to the activities of the aether, and of mat-ter in the aether, under the laws of dynamics.*[124]During the late 1890s a number of physicists proposedthat electricity, as observed in studies of electrical con-duction in conductors, electrolytes, and cathode ray tubes,consisted of discrete units, which were given a variety ofnames, but the reality of these units had not been con-rmed in a compelling way. However, there were alsoindications that the cathode rays had wavelike proper-ties.*[9]Faraday, Weber, Helmholtz, Cliord and others hadglimpses of this view; and the experimental works ofZeeman, Goldstein, Crookes, J. J. Thomson and othershad greatly strengthened this view. Weber predicted thatelectrical phenomena were due to the existence of elec-trical atoms, the inuence of which on one another de-pended on their position and relative accelerations and ve-locities. Helmholtz and others also contended that the ex-istence of electrical atoms followed from Faraday's lawsof electrolysis, and Johnstone Stoney, to whom is due thetermelectron, showed that each chemical ion of thedecomposed electrolyte carries a denite and constantquantity of electricity, and inasmuch as these chargedions are separated on the electrodes as neutral substancesthere must be an instant, however brief, when the chargesmust be capable of existing separately as electrical atoms;while in 1887, Cliord wrote:There is great reason tobelieve that every material atom carries upon it a smallelectric current, if it does not wholly consist of this cur-rent.*[9]

Nikola Tesla, c. 1896.

The Serbian American engineer Nikola Tesla learned of

Hertzexperiments at the Exposition Universelle in 1889and launched into his own experiments in high frequencyand high potential current developinghigh-frequencyalternators (which operated around 15,000 hertz).*[125].He concluded from his observations that Maxwell andHertz were wrong about the existence of airborne electro-magnetic waves (which he attributed it to what he calledelectrostatic thrusts)*[126] but saw great potential inMaxwell's idea that that electricity and light were part ofthe same phenomena, seeing it as a way to create a newtype of wireless electric lighting.*[127] By 1893 he wasgiving lectures on "On Light and Other High FrequencyPhenomena", including a demonstration where he wouldlight a Geissler tubes wirelessly. Tesla worked for manyyears after that trying to develop a wireless power distri-bution system.*[128]

J.J. Thomson.

In 1896, J.J. Thomson performed experiments indicatingthat cathode rays really were particles, found an accuratevalue for their charge-to-mass ratio e/m, and found thate/m was independent of cathode material. He made goodestimates of both the charge e and the mass m, ndingthat cathode ray particles, which he calledcorpuscles,had perhaps one thousandth of the mass of the least mas-sive ion known (hydrogen). He further showed that thenegatively charged particles produced by radioactive ma-terials, by heated materials, and by illuminated materials,were universal. The nature of the Crookes tube "cathoderay" matter was identied by Thomson in 1897.*[129]In the late 19th century, the MichelsonMorley experi-ment was performed by Albert A. Michelson and EdwardW. Morley at what is now Case Western Reserve Univer-sity. It is generally considered to be the evidence against

4.6 Second Industrial Revolution 17

the theory of a luminiferous aether. The experiment hasalso been referred to as the kicking-o point for thetheoretical aspects of the Second Scientic Revolution.*[130] Primarily for this work, Michelson was awardedthe Nobel Prize in 1907. Dayton Miller continued withexperiments, conducting thousands of measurements andeventually developing the most accurate interferometer inthe world at that time. Miller and others, such as Mor-ley, continue observations and experiments dealing withthe concepts.*[131] A range of proposed aether-draggingtheories could explain the null result but these were morecomplex, and tended to use arbitrary-looking coecientsand physical assumptions.*[9]By the end of the 19th century electrical engineers hadbecome a distinct profession, separate from physicistsand inventors. They created companies that investigated,developed and perfected the techniques of electricitytransmission, and gained support from governments allover the world for starting the rst worldwide electricaltelecommunication network, the telegraph network. Pio-neers in this eld included Werner von Siemens, founderof Siemens AG in 1847, and John Pender, founder ofCable & Wireless.The rst public demonstration of aalternator systemtook place in 1886. Large two-phase alternating currentgenerators were built by a British electrician, J.E.H. Gor-don,*[132] in 1882. Lord Kelvin and Sebastian Ferrantialso developed early alternators, producing frequenciesbetween 100 and 300 hertz. After 1891, polyphase alter-nators were introduced to supply currents of multiple dif-fering phases.*[133] Later alternators were designed forvarying alternating-current frequencies between sixteenand about one hundred hertz, for use with arc lighting,incandescent lighting and electric motors.*[134]The possibility of obtaining the electric current in largequantities, and economically, by means of dynamo elec-tric machines gave impetus to the development of incan-descent and arc lighting. Until these machines had at-tained a commercial basis voltaic batteries were the onlyavailable source of current for electric lighting and power.The cost of these batteries, however, and the dicul-ties of maintaining them in reliable operation were pro-hibitory of their use for practical lighting purposes. Thedate of the employment of arc and incandescent lampsmay be set at about 1877.*[9]Even in 1880, however, but little headway had been madetoward the general use of these illuminants; the rapid sub-sequent growth of this industry is a matter of generalknowledge.*[135] The employment of storage batteries,which were originally termed secondary batteries or ac-cumulators, began about 1879. Such batteries are nowutilized on a large scale as auxiliaries to the dynamo ma-chine in electric power-houses and substations, in electricautomobiles and in immense numbers in automobile ig-nition and starting systems, also in re alarm telegraphyand other signal systems.*[9]

World's Fair Tesla presentation.

In 1893, the World's Columbian International Expositionwas held in a building which was devoted to electricalexhibits. General Electric Company (backed by Edisonand J.P. Morgan) had proposed to power the electric ex-hibits with direct current at the cost of one million dol-lars. However, Westinghouse proposed to illuminate theColumbian Exposition in Chicago with alternating cur-rent for half that price, and Westinghouse won the bid.It was an historical moment and the beginning of a revo-lution, as George Westinghouse introduced the public toelectrical power by illuminating the Exposition.

4.6 Second Industrial RevolutionMain article: Second Industrial RevolutionBetween 1885 and 1890 Galileo Ferraris in Italy,

Thomas Edison.

18 4 19TH CENTURY

Nikola Tesla in the United States, and Mikhail Dolivo-Dobrovolsky in Germany explored poly-phase currentscombined with electromagnetic induction leading to thedevelopment of practical AC induction motors.*[136]The AC induction motor helped usher in the Second In-dustrial Revolution. The rapid advance of electrical tech-nology in the latter 19th and early 20th centuries led tocommercial rivalries. In the War of Currents in the late1880s, George Westinghouse and Thomas Edison be-came adversaries due to Edison's promotion of direct cur-rent (DC) for electric power distribution over alternatingcurrent (AC) advocated by Westinghouse.Several inventors helped develop commercial systems.Samuel Morse, inventor of a long-range telegraph;Thomas Edison, inventor of the rst commercial electri-cal energy distribution network; George Westinghouse,inventor of the electric locomotive; Alexander GrahamBell, the inventor of the telephone and founder of a suc-cessful telephone business.In 1871 the electric telegraph had grown to large pro-portions and was in use in every civilized country in theworld, its lines forming a network in all directions overthe surface of the land. The system most generally in usewas the electromagnetic telegraph due to S. F. B. Morseof New York, or modications of his system.*[137] Sub-marine cables*[138] connecting the Eastern and West-ern hemispheres were also in successful operation at thattime.*[9]When, however, in 1918 one views the vast applicationsof electricity to electric light, electric railways, electricpower and other purposes (all it may be repeated madepossible and practicable by the perfection of the dynamomachine), it is dicult to believe that no longer ago than1871 the author of a book published in that year, in re-ferring to the state of the art of applied electricity at thattime, could have truthfully written:The most importantand remarkable of the uses which have beenmade of elec-tricity consists in its application to telegraph purposes.*[139] The statement was, however, quite accurate andperhaps the time could have been carried forward to theyear 1876 without material modication of the remarks.In that year the telephone, due to Alexander GrahamBell,was invented, but it was not until several years thereafterthat its commercial employment began in earnest. Sincethat time also the sister branches of electricity just men-tioned have advanced and are advancing with such gigan-tic strides in every direction that it is dicult to place alimit upon their progress. Electrical devices account ofthe use of electricity in the arts and industries.*[9]AC replaced DC for central station power generation andpower distribution, enormously extending the range andimproving the safety and eciency of power distribution.Edison's low-voltage distribution system using DC ulti-mately lost to AC devices proposed by others: Westing-house' AC system, Tesla's AC inventions, and the theo-retical work of Charles Proteus Steinmetz. The success-

Charles Proteus Steinmetz, theoretician of alternating current.

ful Niagara Falls system was a turning point in the ac-ceptance of alternating current. Eventually, the GeneralElectric company (formed by a merger between Edison'scompanies and the AC-based rival Thomson-Houston)began manufacture of AC machines. Centralized powergeneration became possible when it was recognized thatalternating current electric power lines can transport elec-tricity at low costs across great distances by taking advan-tage of the ability to change voltage across the distributionpath using power transformers. The voltage is raised atthe point of generation (a representative number is a gen-erator voltage in the low kilovolt range) to a much highervoltage (tens of thousands to several hundred thousandvolts) for primary transmission, followed to several down-ward transformations, to as low as that used in residentialdomestic use.*[9]The International Electro-Technical Exhibition of 1891featuring the long distance transmission of high-power,three-phase electric current. It was held between 16 Mayand 19 October on the disused site of the three formerWestbahnhfe(Western Railway Stations) in Frankfurtam Main. The exhibition featured the rst long distancetransmission of high-power, three-phase electric current,which was generated 175 km away at Lauen amNeckar.As a result of this successful eld trial, three-phase cur-rent became established for electrical transmission net-works throughout the world.*[9]Much was done in the direction in the improvement ofrailroad terminal facilities, and it is dicult to nd onesteam railroad engineer who would have denied that allthe important steam railroads of this country were not tobe operated electrically. In other directions the progressof events as to the utilization of electric power was ex-pected to be equally rapid. In every part of the worldthe power of falling water, nature's perpetual motion ma-chine, which has been going to waste since the world be-

5.1 Lorentz and Poincar 19

gan, is now being converted into electricity and transmit-ted by wire hundreds ofmiles to points where it is usefullyand economically employed.*[9]*[140]The rst windmill for electricity production was built inScotland in July 1887 by the Scottish electrical engineerJames Blyth.*[141] Across the Atlantic, in Cleveland,Ohio a larger and heavily engineered machine wasdesigned and constructed in 188788 by Charles F.Brush,*[142] this was built by his engineering companyat his home and operated from 1886 until 1900.*[143]The Brush wind turbine had a rotor 56 feet (17 m) in di-ameter and was mounted on a 60-foot (18 m) tower. Al-though large by today's standards, the machine was onlyrated at 12 kW; it turned relatively slowly since it had 144blades. The connected dynamo was used either to chargea bank of batteries or to operate up to 100 incandescentlight bulbs, three arc lamps, and various motors in Brush'slaboratory. The machine fell into disuse after 1900 whenelectricity became available from Cleveland's central sta-tions, and was abandoned in 1908.*[144]

5 20th centuryVarious units of electricity and magnetism have beenadopted and named by representatives of the electri-cal engineering institutes of the world, which units andnames have been conrmed and legalized by the govern-ments of the United States and other countries. Thus thevolt, from the Italian Volta, has been adopted as the prac-tical unit of electromotive force, the ohm, from the enun-ciator of Ohm's law, as the practical unit of resistance; theampere, after the eminent French scientist of that name,as the practical unit of current strength, the henry as thepractical unit of inductance, after Joseph Henry and inrecognition of his early and important experimental workin mutual induction.*[145]Dewar and John Ambrose Fleming predicted that atabsolute zero, pure metals would become perfect elec-tromagnetic conductors (though, later, Dewar altered hisopinion on the disappearance of resistance believing thatthere would always be some resistance). Walther Her-mann Nernst developed the third law of thermodynam-ics and stated that absolute zero was unattainable. Carlvon Linde and William Hampson, both commercial re-searchers, nearly at the same time led for patents onthe Joule-Thomson eect. Linde's patent was the cli-max of 20 years of systematic investigation of establishedfacts, using a regenerative counterow method. Hamp-son's design was also of a regenerative method. The com-bined process became known as the Linde-Hampson liq-uefaction process. Heike Kamerlingh Onnes purchaseda Linde machine for his research. Zygmunt FlorentyWroblewski conducted research into electrical proper-ties at low temperatures, though his research ended earlydue to his accidental death. Around 1864, Karol Ol-szewski andWroblewski predicted the electrical phenom-

ena of dropping resistance levels at ultra-cold tempera-tures. Olszewski and Wroblewski documented evidenceof this in the 1880s. A milestone was achieved on 10July 1908 when Onnes at the Leiden University in Leidenproduced, for the rst time, liquied helium and achievedsuperconductivity.In 1900, William Du Bois Duddell develops the SingingArc and produced melodic sounds, from a low to a high-tones, from this arc lamp.

5.1 Lorentz and PoincarMain articles: History of special relativity and Lorentzether theoryBetween 1900 and 1910, many scientists like Wilhelm

Hendrik Lorentz.

Wien, Max Abraham, Hermann Minkowski, or GustavMie believed that all forces of nature are of electromag-netic origin (the so-calledelectromagnetic world view). This was connected with the electron theory developedbetween 1892 and 1904 by Hendrik Lorentz. Lorentzintroduced a strict separation between matter (electrons)and ether, whereby in his model the ether is completelymotionless, and it won't be set in motion in the neighbor-hood of ponderable matter. Contrary to other electronmodels before, the electromagnetic eld of the ether ap-pears as a mediator between the electrons, and changes inthis eld can propagate not faster than the speed of light.In 1896, three years after submitting his thesis on the

20 5 20TH CENTURY

Kerr eect, Pieter Zeeman disobeyed the direct ordersof his supervisor and used laboratory equipment to mea-sure the splitting of spectral lines by a strong magneticeld. Lorentz theoretically explained the Zeeman eecton the basis of his theory, for which both received theNobel Prize in Physics in 1902. A fundamental conceptof Lorentz's theory in 1895 was the theorem of cor-responding statesfor terms of order v/c. This theoremstates that a moving observer (relative to the ether) makesthe same observations as a resting observer. This theo-rem was extended for terms of all orders by Lorentz in1904. Lorentz noticed, that it was necessary to changethe space-time variables when changing frames and in-troduced concepts like physical length contraction (1892)to explain the MichelsonMorley experiment, and themathematical concept of local time (1895) to explain theaberration of light and the Fizeau experiment. That re-sulted in the formulation of the so-called Lorentz trans-formation by Joseph Larmor (1897, 1900) and Lorentz(1899, 1904).*[146]*[147]*[148] As Lorentz later noted(1921, 1928), he considered the time indicated by clocksresting in the aether astruetime, while local time wasseen by him as a heuristic working hypothesis and amath-ematical artice.*[149]*[150] Therefore, Lorentz's theo-rem is seen by modern historians as being a mathematicaltransformation from arealsystem resting in the aetherinto actitioussystem in motion.*[146]*[147]*[148]

Henri Poincar.

Continuing the work of Lorentz, Henri Poincar be-tween 1895 and 1905 formulated on many occasions thePrinciple of Relativity and tried to harmonize it withelectrodynamics. He declared simultaneity only a con-

venient convention which depends on the speed of light,whereby the constancy of the speed of light would be auseful postulate for making the laws of nature as sim-ple as possible. In 1900 he interpreted Lorentz's localtime as the result of clock synchronization by light sig-nals, and introduced the electromagnetic momentum bycomparing electromagnetic energy to what he called actitious uidof mass m = E/c2 . And nally inJune and July 1905 he declared the relativity principlea general law of nature, including gravitation. He cor-rected some mistakes of Lorentz and proved the Lorentzcovariance of the electromagnetic equations. Poincaralso suggested that there exist non-electrical forces to sta-bilize the electron conguration and asserted that grav-itation is a non-electrical force as well, contrary to theelectromagnetic world view. However, historians pointedout that he still used the notion of an ether and distin-guished betweenapparentandrealtime and thereforedidn't invent special relativity in its modern understand-ing.*[148]*[151]*[152]*[153]*[154]*[155]

5.2 Einstein's Annus MirabilisMain article: Annus Mirabilis PapersIn 1905, while he was working in the patent oce,

Albert Einstein, 1905.

Albert Einstein had four papers published in the Annalender Physik, the leading German physics journal. Theseare the papers that history has come to call the AnnusMirabilis Papers:

His paper on the particulate nature of light put for-

5.3 Latter half of the 20th Century 21

ward the idea that certain experimental results, no-tably the photoelectric eect, could be simply un-derstood from the postulate that light interacts withmatter as discrete packets(quanta) of energy,an idea that had been introduced by Max Planck in1900 as a purely mathematical manipulation, andwhich seemed to contradict contemporary wave the-ories of light (Einstein 1905a). This was the onlywork of Einstein's that he himself called revolu-tionary.

His paper on Brownian motion explained the ran-dom movement of very small objects as direct ev-idence of molecular action, thus supporting theatomic theory. (Einstein 1905b)

His paper on the electrodynamics of moving bod-ies introduced the radical theory of special relativ-ity, which showed that the observed independenceof the speed of light on the observer's state ofmotionrequired fundamental changes to the notion of si-multaneity. Consequences of this include the time-space frame of a moving body slowing down andcontracting (in the direction of motion) relative tothe frame of the observer. This paper also arguedthat the idea of a luminiferous aetherone of theleading theoretical entities in physics at the timewas superuous. (Einstein 1905c)

In his paper on massenergy equivalence (previ-ously considered to be distinct concepts), Einsteindeduced from his equations of special relativity whatlater became the well-known expression: E = mc2, suggesting that tiny amounts of mass could beconverted into huge amounts of energy. (Einstein1905d)

All four papers are today recognized as tremendousachievementsand hence 1905 is known as Einstein's"Wonderful Year". At the time, however, they were notnoticed by most physicists as being important, and manyof those who did notice them rejected them outright.Some of this worksuch as the theory of light quantaremained controversial for years.*[156]*[157]

5.3 Latter half of the 20th Century

The rst formulation of a quantum theory describingradiation and matter interaction is due to Paul AdrienMaurice Dirac, who, during 1920, was rst able tocompute the coecient of spontaneous emission of anatom.*[158] Paul Dirac described the quantization of theelectromagnetic eld as an ensemble of harmonic oscil-lators with the introduction of the concept of creationand annihilation operators of particles. In the followingyears, with contributions from Wolfgang Pauli, EugeneWigner, Pascual Jordan, Werner Heisenberg and an el-egant formulation of quantum electrodynamics due to

Paul Adrien Maurice Dirac.

Enrico Fermi,*[159] physicists came to believe that, inprinciple, it would be possible to perform any compu-tation for any physical process involving photons andcharged particles. However, further studies by FelixBloch with Arnold Nordsieck,*[160] and Victor Weis-skopf,*[161] in 1937 and 1939, revealed that such com-putations were reliable only at a rst order of perturbationtheory, a problem already pointed out by Robert Op-penheimer.*[162] At higher orders in the series inni-ties emerged, making such computationsmeaningless andcasting serious doubts on the internal consistency of thetheory itself. With no solution for this problem known atthe time, it appeared that a fundamental incompatibilityexisted between special relativity and quantum mechan-ics.In December 1938, the German chemists OttoHahn and Fritz Strassmann sent a manuscript toNaturwissenschaften reporting they had detectedthe element barium after bombarding uranium withneutrons;*[163] simultaneously, they communicatedthese results to Lise Meitner. Meitner, and her nephewOtto Robert Frisch, correctly interpreted these resultsas being nuclear ssion.*[164] Frisch conrmed thisexperimentally on 13 January 1939.*[165] In 1944,Hahn received the Nobel Prize for Chemistry for thediscovery of nuclear ssion. Some h