Famous Physicist

7/29/2019 Famous Physicist

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Georg Simon Ohm

The German physicist Georg Simon Ohm(1789-1854) was the discoverer of thelaw, named for him, which states the

exact relationship of potential andcurrent in electric conduction.

Georg Ohm was born on March 16, 1789, inErlangen, Bavaria, the eldest of seven children.His father, Johann Wolfgang Ohm, was amaster mechanic and an avid reader of bookson philosophy and mathematics. He cultivatedthe obvious mathematical talents of Georg andhis younger brother, Martin, and the two soongained the reputation of being the latter-dayversion of the famed Bernoulli brothers. Due tofinancial difficulties, Georg left the Universityof Erlangen in 1806 after three semesters. Forthe next year and a half he earned his living asprivate tutor in Gottstadt, Switzerland, but by1809 he settled in Neuchtel to continueprivately with his university studies. In 1811 hereturned to Erlangen and obtained hisdoctorate. For the next three semesters Ohmtaught mathematics at the University ofErlangen, but the meagerness of his incomeforced him to take the post of tutor at the

realgymnasium in Bamberg.

Following the publication in 1817 of Ohm's firstbook, a textbook of geometry, he received anappointment as teacher of mathematics andphysics at the Royal Prussian Konsistorium inCologne. The well-equipped laboratory of thelocal Jesuit gymnasium was put at his disposal,and there he began his epoch-makinginvestigations on the characteristics of electriccircuits, a virtually unexplored field at that

time.

In 1825 theJournal fr Chemie undPhysik carried Ohm's first communication onthe laws of the galvanic (electric) circuit,"Preliminary Notice on the Law according toWhich Metals Conduct Contact-electricity."The paper gave an incorrect formula for whatlater became known as Ohm's law, but within ayear Ohm corrected the mistake. The 1826issue of theJournalcarried Ohm's

"Determination of the Law according to WhichMetals Conduct Contact-electricity, Togetherwith the Outlines of a Theory of the VoltaicApparatus and of the Schweigger Multiplicator[Galvanoscope]." In the introductory part of

the paper he noted that the new form of his lawwas not only in perfect agreement with allexperiments but also embodied a unitaryexplanation of a broad range of phenomenaConsequently, he argued, his law or formulahad to be a true law of nature.

These remarks of Ohm are important to note asthey hold the key to some of the subsequentmisunderstanding of his work. Hisexperimental work was unimpeachable. Hisdata fully justified his conclusion that the ratioof V (the change in electromotive force)andX(the electromotive force) wasproportional to the ratio ofh (the change in thelength of the conducting wire) andx(the wire'soriginal length), or V/X = h/ (b+x), where b is aconstant.

Ohm was, however, determined to give the lawa most general if not an a priori justification. In1827 he published his most renownedwork, The Galvanic Circuit MathematicallyTreated. It contains the now familiar formulaI= V/R written in the notationS = A/L, which isfollowed by the historic statement, "Themagnitude of the current in a galvanic circuit isdirectly proportional to the sum of all tensions[potentials] and indirectly to the total reducedlength of the circuit." By "reduced" he meantthe appropriate resistances of all parts of thecircuit.

Ohm's Galvanic Circuitwas greeted with some

appreciation but largely with indifference andwith some hostility. He withdrew from theacademic world for 6 years. In 1833 he becameprofessor of physics at the Polytechnic Schoolin Nuremberg. But the real turning point in hislife came when the Royal Society of Londonawarded him the Copley Medal in 1841. Ohmdedicated to the Royal Society the first volumeof hisContribution to Molecular Physics, awork in which he planned to elucidate theinternal constitution of matter with the same


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success Isaac Newton had achieved in celestialdynamics.

Apart from the gigantic demands of the plan,Ohm's teaching duties stood in the way of its

execution. In 1835 he assumed, in addition tohis duties in Nuremberg, the chair of highermathematics at the University of Erlangen.Shortly afterward, he became inspector ofscientific education in the state of Bavaria. Heachieved his lifelong dream, a position with amajor university, in 1849 as professor at theUniversity of Munich. He was working on themanuscript of his textbook on optics when hedied on July 6, 1854.


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Alessandro Volta

The Italian physicist Alessandro Volta(1745-1827) invented the electric battery,or "voltaic pile," thus providing for the

first time a sustained source of currentelectricity.

Alessandro Volta was born on Feb. 18, 1745, inComo. He resisted pressure from his family toenter the priesthood and developed instead anintense curiosity about natural phenomena, inparticular, electricity. In 1769 he published hisfirst paper on electricity. It contained no newdiscoveries but is of some interest as the mostspeculative of all Volta's papers, his subsequentones being devoted almost exclusively to thepresentation of specific experimentaldiscoveries.

Early Investigations and Inventions

In 1774 Volta was appointed professor ofphysics at the gymnasium in Como, and thatsame year he made his first importantcontribution to the science of electricity, theinvention of the electrophorus, a device whichprovided a source of electric potential utilizingthe principle of electrostatic induction. Unlikeearlier source of electric potential, such as theLeyden jar, the electrophorus provided asustained, easily replenishable source of staticelectricity. In 1782 Volta announced theapplication of the electrophorus to thedetection of minute electrical charges. Hisinvention of the so-called condensingelectroscope culminated his efforts to improvethe sensitivity of earlier electrometers.

During these same years Volta also conductedresearches of a purely chemical nature. He hadfor some time been experimenting withexploding various gases, such as hydrogen, inclosed containers and had observed that whenhydrogen and air were exploded there was adiminution in volume greater than the volumeof hydrogen burned. In order to measure suchchanges in volume, he developed a graduatedglass container, now known as a eudiometer, in

which to explode the gases. Utilizing thiseudiometer he studied marsh gas, or methaneand distinguished it from hydrogen by itsdifferent-colored flame, its slower rate ofcombustion, and the greater volume of air and

larger electric spark required for detonation.In 1779 Volta was appointed to the newlycreated chair of physics at the University ofPavia. In 1782 he became a correspondingmember of the French Academy of Sciences. In1791 he was elected a fellow of the RoyalSociety of London, and in 1794, in recognitionof his contributions to electricity andchemistry, he was awarded the society'scoveted Copley Medal. However, his mostsignificant researchesthose which were tolead to the discovery of current electricitywere yet to be undertaken.

Discovery of Current Electricity

Until the last decade of the 18th centuryelectrical researchers had been primarilyconcerned with static electricity, with theelectrification produced by friction. Then, in1786, Luigi Galvani discovered that the musclesin a frog's amputated leg would contractwhenever an electrical machine was dischargednear the leg. As a result of his initialobservations, Galvani undertook a long seriesof experiments in an effort to more thoroughlyexamine this startling phenomenon. In thecourse of these investigations he discoveredthat a frog's prepared leg could be made tocontract if he merely attached a copper hook tothe nerve ending and then pressed the hookagainst an iron plate on which the leg wasresting so as to complete an electrical circuit

even though no electrical machines wereoperating in the vicinity. Galvani concluded thecontraction was produced in the organism itselfand referred to this new type of electricity as"animal electricity."

Galvani's experiments and interpretation weresummarized in a paper published in 1791, acopy of which he sent to Volta. Although, likemost others, initially convinced by Galvani'sarguments, Volta gradually came to the


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conclusion that the two metals were not merelyconductors but actually generated theelectricity themselves. He began by repeatingand verifying Galvani's experiments but quicklymoved beyond these to experiments of his own,

concentrating on the results of bringing intocontact two dissimilar metals. By 1794 he hadconvinced himself that the metals, in his ownwords, "are in a real sense the exciters ofelectricity, while the nerves themselves arepassive," and he henceforth referred to thisnew type of electricity as "metallic" or "contact"electricity.

The announcement of Volta's experiments andinterpretation touched off one of the greatcontroversies in the history of science.Although other factors were important as well,the physiologists and anatomists tended tosupport Galvani's view that the electricity wasproduced by the animal tissue itself whereasthe physicists and chemists, like Volta, tendedto see it as produced by the external bimetalliccontacts. The resulting rivalry not only took oninternational dimensions but died out onlygradually after more than a decade. AlthoughGalvani withdrew from the arena, allowingothers to carry his standard, Volta took an

active role in the controversy and vigorouslypursued his research.

Volta discovered that not only would twodissimilar metals in contact produce a smallelectrical effect, but metals in contact withcertain types of fluids would also produce sucheffects. In fact, the best results were obtainedwhen two dissimilar metals were held incontact and joined by a moist third body which,in modern terminology, completed the circuit

between them. Such observations led directlyto the construction in 1800 of the electricbattery, or "pile" as Volta called it, the firstsource of a significant electric current.

Volta announced his discovery in a letter to SirJoseph Banks, then president of the RoyalSociety of London. The letter, dated March 20,1800, created an instant sensation. Here forthe first time was an instrument capable ofproducing a steady, continuous flow of

electricity. All previous electrical machinesincluding Volta's electrophorus, had producedonly short bursts of static electricity. The abilityto create at will a sustained electrical currentopened vast new fields for investigation, and

the significance of Volta's discovery wasimmediately recognized.

Acclaim and Retirement

Volta was summoned to Paris by Napoleon andin 1801 gave a series of lectures on hisdiscoveries before the National Institute ofFrance, as the Academy of Sciences was thencalled. A special gold medal was struck tohonor the occasion, and the following yearVolta was distinguished by election as one ofthe eight foreign associates of the institute.

Although only in his mid-50s when heannounced the discovery of the "pile," Voltatook no part in applying his discovery to any ofthe immense new fields it opened up. Duringthe last 25 years of his life he demonstratednone of the intense creativity that hadcharacterized his earlier researches, and hepublished nothing of scientific significanceduring these later years. He continued, at theurging of Napoleon, to teach at the Universityof Pavia and eventually became director of thephilosophy faculty there. In 1819 he retired tohis family home near Como. He died there onMarch 5, 1827, little realizing that currentelectricity would eventually transform a way oflife.


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Andr-Marie Ampre

Andr-Marie Ampre (1775-1836), was aFrench physicist, natural philosopher, andmathematician who is best known for his

important contributions to the study ofelectrodynamics. He invented the astaticneedle, a critical component of the modernastatis galvanometer, and was the first todemonstrate that a magnetic field is generatedwhen two parallel wires are charged withelectricity. He is generally credited as one ofthe first to discover electromagnetism. BornJanuary 20, 1775, Ampre was the son of asuccessful businessman and local governmentofficial in Polemieux-auMont-d'Or, a smallcommunity near Lyon, France. As a childAmpere spent a great deal of time reading inthe library of his family home, and hevoraciously consumed books of history,geography, literature, philosophy and thenatural sciences. His father taught him Latinand encouraged Ampre to pursue his passionfor mathematics. Some historians write thatthe young Ampre was a math prodigy at a veryearly age and that he used to work out longmathematical formulas, just for his ownpersonal entertainment, using small pebbles or

breadcrumbs to represent groups of numbers.

Even without any formal education Amprebegan a career as a science teacher. Afterteaching for a while in Lyon he acceptedpositions at institutions of higher learningincluding the College of France and thePolytechnic School at Paris, where he was aprofessor of mathematics. It was there that hefirst conducted important research andexperiments into the nature of electrical and

magnetic forces. In the early 1820s, afterlearning about the electromagnetismexperiments of Hans Christian Oersted,Ampre began to formulate a combined theoryof electricity and magnetism, doing severaldemonstrations involving magnetic andelectrical forces. His work confirmed andvalidated the discoveries of Oersted while alsoexpanding upon them, helping to acceleratework in the field of electromagnetism aroundthe world.

Ampre's most significant scholarly paper onthe subject of electricity and magnetism, titledMemoir on the Mathematical Theory ofElectrodynamic Phenomena, was published in1826. The theoretical foundation presented in

this publication served as the basis for otherideas of the 19th century regarding electricityand magnetism. It helped to inspire researchand discoveries by scientists including FaradayWeber, Thomson, and Maxwell.

Ampre was elected to the prestigious NationalInstitute of Sciences in 1814, and was awardeda chair at the University of France in 1826.There he taught electrodynamics and remaineda member of the faculty until his death. He wasalso a member of the Fellows of the RoyalSociety of London

Despite his celebrated accomplishmentsAmpre led a rather tragic life. When Lyonswas taken over by rebels during the FrenchRevolution, his beloved father was a districtjudge. Because of his political affiliationsAmpres father was taken as a politicaprisoner and then publicly executed byguillotine, an event that severely scarred theyoung Ampre and led to a period of

psychological depression. Later in lifeAmpres first wife met with an early deathafter a prolonged illness, and although heremarried, his second marriage was unhappyand unsuccessful.

Ampre died June 10, 1836 in MarseillesFrance, and was buried in the MontmartreCemetery in Paris. When Gustave Eiffel builthis famous Eiffel Tower in Pairs in 1889, heincluded the names of 72 prominent French

scientists on plaques around the first section atthe base of the structure. The name of Andr-Marie Ampre is included in that distinguishedmemorial.

The ampere the unit for measuring electriccurrent was named in honor of Ampre. Inthe past, an ampere was understood as theforce generated between parallel electricallycharged wires, but as scientific knowledgeevolves over time, the definition of ampere


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sometimes changes slightly also. The currentmodern definition of ampere describes theability of a specified current to deposit aprecise amount of a substance on an electrodeduring electrolysis.


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Henry Cavendish

The English physicist and chemist HenryCavendish (1731-1810) determined the

value of the universal constant of

gravitation, made noteworthy electricalstudies, and is credited with thediscovery of hydrogen and thecomposition of water.

Henry Cavendish was born on Oct. 10, 1731, theelder son of Lord Charles Cavendish and LadyAnne Grey. He entered Peterhouse, Cambridge,in 1749 and left after 2 years without taking adegree. He never married and was so reservedthat there is little record of his having anysocial life except occasional meetings withscientific friends. His death (Feb. 24, 1810) hefaced with the same equanimity with which hefaced the unavoidable breaking of apparatus inthe course of increasing knowledge. He wasburied in All Saints Church, Derby.

Cavendish's work and reputation have to beconsidered in two parts: the one relating to hispublished work, the other to the large amounthe did not publish. During his lifetime he madenotable discoveries in chemistry mainlybetween 1766 and 1788 and in electricitybetween 1771 and 1788. In 1798 he published asingle notable paper on the density of the earth,but interest in this subject was evidently of longstanding.

Contributions to Chemistry

At the time Cavendish began his chemicalwork, chemists were just beginning torecognize that the "airs" which were evolved in

many chemical reactions were distinct entitiesand not just modifications of ordinary air.Cavendish reported his own work in ThreePapers Containing Experiments on FactitiousAir in 1766. These papers added greatly toknowledge of the formation of "inflammableair" (hydrogen) by the action of dilute acids onmetals. Cavendish also distinguished theformation of oxides of nitrogen from nitricacid. Their true chemical character was not yet

known, but Cavendish's description of hisobservations had almost the same logicalpattern as if he were thinking in modern terms,the principal difference being that he used theterminology of the phlogiston theory (that is, a

burning substance liberates into itssurroundings a principle of inflammability).

Cavendish's other great merit is hisexperimental care and precision. He measuredthe density of hydrogen, and although hisfigure is half what it should be, it is astonishingthat he even found the right order ofmagnitude, considering how difficult it was tomanage so intractable a substance. Not that hisapparatus was crude; where the techniques ofhis day allowed, his apparatus (like thesplendid balance surviving at the RoyalInstitution) was capable of refined results.

Cavendish investigated the products ofermentation, showing that the gas from thefermentation of sugar is indistinguishable fromthe "fixed air" characterized as a constituent ofchalk and magnesia by Black (both are, inmodern language, carbon dioxide).

Another example of Cavendish's technicalexpertise wasExperiments on Rathbone-PlaceWater (1767), in which he set the highestpossible standard of thoroughness andaccuracy. It is a classic of analytical chemistryIn it Cavendish also examined thephenomenon of the retention of "calcareousearth" (chalk, calcium carbonate) in solutionand in doing so he discovered the reversiblereaction between calcium carbonate andcarbon dioxide to form calcium bicarbonatethe cause of temporary hardness of water. He

also found out how to soften such water byadding lime (calcium hydroxide).

In his study of the methods of gas analysisCavendish made one remarkable observationHe was sparking air with excess oxygen (toform oxides of nitrogen) over alkali until nomore absorption took place and noted that atiny amount of gas could not be furtherreduced, "so that if there is any part of thephlogisticated air of our atmosphere which


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differs from the rest, and cannot be reduced tonitrous acid, we may safely conclude, that it isnot more than 1/120 part of the whole." As isnow known, he had observed the noble gases ofthe atmosphere.

One of Cavendish's researches on the currentlyengrossing problem of combustion made anoutstanding contribution to fundamentaltheory. Without seeking particularly to do so,in 1784 Cavendish determined the compositionof water, showing that it was a compound ofoxygen and hydrogen ("dephlogisticated air"and "inflammable air"). Joseph Priestley hadreported an experiment of Warltire in whichthe explosion of the two gases had left a dew onthe sides of a previously dry vessel. Cavendishstudied this, prepared water in measurablequantity, and got an approximately correctfigure for its volume composition.

Electrical Researches

Cavendish published only a fraction of theexperimental evidence he had available tosupport his theories, but his contemporarieswere convinced of the correctness of hisconclusions. He was not the first to profoundan inverse-square law of electrostaticattraction, but Cavendish's exposition, based inpart on mathematical reasoning, was the mosteffective. He founded the study of theproperties of dielectrics and also distinguishedclearly between quantity of electricity and whatis now called potential.

Cavendish had the ability to make anapparently limited study yield far-reachingresults. An example is his study of the origin of

the ability of some fish to give an electric shock.He made up imitation fish of leather and wood,soaked in salt water, with pewter attachmentsrepresenting the organs of the fish whichproduced the effect. By using Leyden jars tocharge the imitation organs, he was able toshow that the results were entirely consistentwith the fish's being able to produce electricity.This investigation was among the earliest inwhich the conductivity of aqueous solutionswas studied.

Cavendish began to study heat with his father,then returned to the subject in 1773-1776 with astudy of the Royal Society's meteorologicainstruments, in the course of which he workedout the most important corrections to be

employed in accurate thermometry. In 1783 hepublished a study of the means of determiningthe freezing point of mercury. In it he added agood deal to the general theory of fusion andfreezing and the latent heat changesaccompanying them.

Cavendish's most elaborate (and celebrated)investigation was that on the density of theearth. He took part in a program to measurethe length of a seconds pendulum in thevicinity of a large mountain (Schiehallion)Variations from the period on the plain wouldshow the attraction exerted by the mountainfrom which the density of its substance couldbe calculated. Cavendish also approached thesubject in a more fundamental way bydetermining the force of attraction of a verylarge, heavy lead ball for a very small, light ball.The ratio between this force and the weight ofthe light ball would furnish the mass of theearth. His results were unquestioned andunsurpassed for nearly a century.

Unpublished Works

Had Cavendish published all his work, his greatinfluence would undoubtedly have beengreater, but in fact he left in manuscript form avast amount which often anticipated that of hissuccessors. It came to light only bit by bit untilthe thorough study undertaken by Maxwell(published in 1878) and by Thorpe (publishedin 1921). In these notes is to be found such

material as the detail of his experiments toexamine the law of electrostatic force, theconductivity of metals, and many chemicalquestions such as a theory of chemicalequivalents. He had a theory of partialpressures before Dalton.

However, the history of science is full ofinstances of unpublished works which mighthave influenced others but in fact did notWhatever he did not reveal, Cavendish gave his


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colleagues enough to help them on the road tomodern conceptions. Nothing he did has beenrejected, and for this reason he is still, in aunique way, part of modern life.


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Coulomb, Charles

FRENCH ENGINEER AND PHYSICIST17361806

Charles-Augustin de Coulomb was born toaffluent parents in Angoulme, France. Hisfather's family was prominent in the legalprofession and involved in the administrationof the Languedoc region of France. Hismother's family was quite wealthy. After beingraised in Angoulme, Charles moved with hisfamily to Paris, where he entered the CollgeMazarin and pursued a classical education.

After a brief stay in Montpellier, Coulombreturned to Paris to study at the cole du Gnieat Mzires. This was one of the first schools ofengineering; it specifically focused on militaryengineering. Coulomb graduated in 1761 with adegree in engineering and the rank oflieutenant in the Corps du Gnie. Over the nexttwenty years, he was posted to a variety oflocations where he became involved in thestructural design of forts and fortifications, andsoil mechanics.

In 1777 his work on torsion balances (amongother subjects) won Coulomb a share of theGrand Prix of the Acadmie des Sciences.Historically, all measurements of weight hadbeen obtained by using a two-pan balance,which is simply a bar centered on a fulcrum .Coulomb's torsion balance replaced thefulcrum with a fine silk thread or hair, andrather than the up-and-down motion of the panbalance, he used a twist or torsion around thisthread. He was able to show that the amount oftorsion is proportional to the amount of force;

thus he devised a method for measuring verysmall interactions.

With his very fine torsion balance, Coulombwas able to demonstrate that the repulsiveforce between two small spheres electrifiedwith the same type of electricity is inverselyproportional to the square of the distancebetween the centers of the two spheres. At thetime, the electron had not yet been discovered,

so the underlying reason for this remained amystery but Coulomb was able to demonstratethat both repulsion and attraction followedthis principle. He was not able to make thequantitative step to show that the force was

also directly proportional to the product of thecharges, but he did complete some experimentsexploring this relationship. As a consequencethe law governing one of the four fundamentalforces of nature is named Coulomb's law:

F= kq1q2/r2

For his work in setting physics on a course ofdiscovery, the fundamental unit of charge wasnamed the "coulomb" in his honor.
http://www.encyclopedia.com/topic/Paris.aspxhttp://www.encyclopedia.com/topic/Paris.aspx


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Antoine Henri Becquerel

The French physicist Antoine Henri

Becquerel (1852-1908) was thediscoverer of natural radioactivity.

Antoine Henri Becquerel was born in Paris onDec. 15, 1852. Both his father, AlexandreEdmond Becquerel, and his grandfather,Antoine Csar Becquerel, were scientists.Following his graduation from the colePolytechnique in 1874, Antoine Henri workedas a civil engineer, but he also retained a stronginterest in scientific problems. In 1878 hesucceeded in the chair of his father who wasprofessor of applied physics at theConservatoire des Arts et Mtiers. Ten yearslater Becquerel earned his doctor's degree witha dissertation on the absorption of light incrystals. He then became professor of appliedphysics at the Museum of Natural History inParis in 1892 and professor of physics at thePolytechnique in 1895.

Prior to 1895 Becquerel did research onphosphorescence. He had inherited from hisfather a supply of uranium salts, which wereknown to be phosphorescent when exposed tolight. Upon learning in January 1896 about W.C. Roentgen's discovery of x-rays, Becquerel'sinterest immediately turned to the question ofwhether all phosphorescent materials acted assources of similar rays.

The results did not justify his hopes, butBecquerel stumbled on an unexpectedphenomenon. After placing sheets of sulfate of

uranium on photographic plates wrapped inblack paper, he exposed the package to light forseveral hours. On developing the plates heobtained distinct pictures of the uraniumsheets. Later he obtained pictures of medalswhich had been placed between the uraniumand the plates. The uneven thickness of themedals blocked in varying degrees theeffectiveness of the radiation from uranium. Healso discovered that part of the radiation could

be deflected by a magnetic field and that theradiations had an ionizing effect on thesurrounding air.

For the discovery of natural radioactivity

which for a number of years was calledBecquerel rays, he won the Nobel Prize inphysics in 1903. In his Nobel lecture Becquerelnoted that the new radiation indicated thepossible modification of atoms which "themethods at our disposal are unable to bringabout (but which) could certainly releaseenergy in sufficiently large quantities toproduce the observed effects, without thechanges in matter being large enough to bedetectable by our methods of investigation." Asa cause of that modification, he held out thepossible existence of "an external radiation"hitherto undetected but which, when absorbedby radioactive materials, would be transformedinto radioactivity without bringing about thetransformation of the atoms themselves.

Becquerel's election as perpetual secretary ofthe Academy of Sciences in 1908 was one of thenumerous honors bestowed on him. His deathon Aug. 25, 1908, at Le Croisic did not signalthe end of the lineage of scientists in the

Becquerel family. From Becquerel's marriage toLucie Zo Marie Jamin a son, Jean, had beenborn; he became the fourth Becquerel tooccupy the chair of physics at the Museum ofNatural History and was also an ableinvestigator of radioactivity.


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Michael Faraday

The English physicist and chemistMichael Faraday (1791-1867) discovered

benzene and the principles of currentinduction.

One of a blacksmith's 10 children, MichaelFaraday was born on Sept. 22, 1791, inNewington, Surrey. The family soon moved toLondon, where young Michael picked up therudiments of reading, writing, and arithmetic.At the age of 14 he was apprenticed to abookbinder and bookseller. He read ravenouslyand attended public lectures, including someby Sir Humphry Davy.

Faraday's career began when Davy, temporarilyblinded in a laboratory accident, appointedFaraday as his assistant at the RoyalInstitution. With Davy as a teacher in analyticalchemistry, Faraday advanced in his scientificapprenticeship and began independentchemical studies. By 1825 he discoveredbenzene and had become the first to describecompounds of chlorine and carbon. He adoptedthe atomic theory to explain that chemicalqualities were the result of attraction andrepulsion between united atoms. This provedto be the theoretical foundation for much of hisfuture work.

Faraday had already done some work inmagnetism and electricity, and it was in thisfield that he made his most outstanding

contributions. His first triumph came when hefound a solution to the problem of producingcontinuous rotation by use of electric current,thus making electric motors possible. HansOersted had discovered the magnetic effect of acurrent, but Faraday grasped the fact that aconductor at rest and a steady magnetic fielddo not interact and that to get an inducedcurrent either the conductor or the field has to

move. On Aug. 29, 1831, he discoveredelectromagnetic induction.

During the next 10 years Faraday explored andexpanded the field of electricity. In 1834 he

announced his famous two laws of electrolysisBriefly, they state that for any given amount ofelectrical force in an electrochemical cellchemical substances are released at theelectrodes in the ratio of their chemicalequivalents. He also invented the voltameter, adevice for measuring electrical charges, whichwas the first step toward the laterstandardization of electrical quantities.

Faraday continued to work in his laboratorybut his health began to deteriorate and he hadto stop work entirely in 1841. Almostmiraculously, however, his health improvedand he resumed work in 1844. He began asearch for an interaction between magnetismand light and in 1845 turned his attention fromelectrostatics to electromagnetism. Hediscovered that an intense magnetic field canrotate the plane of polarized light, aphenomenon known today as the Faradayeffect. In conjunction with these experimentshe showed that the magnetic line of force is

conducted by all matter. Those which weregood conductors he called paramagneticswhile those which conducted the force poorlyhe named diamagnetics. Thus, the energy of amagnet is in the space around it, not in themagnet itself. This is the fundamental idea ofthe field theory.

Faraday was a brilliant lecturer, and throughhis public lectures he did a great deal topopularize science. Shortly after he became

head of the Royal Institution in 1825, heinaugurated the custom of giving a series oflectures for young people during the Christmasseason. This tradition has been maintainedand over the years the series have frequentlybeen the basis for fascinating, simply writtenand informative books.

On Aug. 25, 1867, Faraday died in London.


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The admiration of physicists for Faraday hasbeen demonstrated by naming the unit ofcapacitance the farad and a unit of charge, thefaraday. No other man has been doublyhonored in this way. His name also appears

frequently in connection with effects, laws, andapparatus. These honors are proper tribute tothe man who was possibly the greatestexperimentalist who ever lived.


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Ernest Rutherford

The British physicist Ernest Rutherford,1st Baron Rutherford of Nelson (1871-1937), discovered transmutation of theelements, the nuclear atom, and a hostof other phenomena to become the mostprominent experimental physicist of histime.

In searching for an experimental physicist tocompare with Lord Rutherford, it is natural tothink of Michael Faraday. Like Faraday,Rutherford instinctively knew whatexperiments would yield the most profoundinsights into the operations of nature; unlikeFaraday, however, Rutherford established aschool of followers by training a large numberof research physicists. One of his colleaguesobserved that Rutherford always appeared tobe on the "crest of the wave." Rutherford, withno sense of false modesty, replied, "Well! Imade the wave, didn't I?" Then, after amoment's reflection, he added, "At least tosome extent." Most physicists would agree thatit was to a very large extent.

Ernest Rutherford was born on Aug. 30, 1871,in Spring Grove (Brightwater), near Nelson,New Zealand. His father, a Scot, was awheelwright, farmer, timberman, and large-scale flax producer. Rutherford attendedNelson College, a secondary school (1886-

1889), and then studied at Canterbury Collegein Christchurch, receiving his bachelor's degreein 1892. The following year he took his master'sdegree with honors in mathematics andphysics.

First Research

Rutherford's interest in original researchinduced him to remain at Canterbury for an

additional year. Using the rather primitiveresearch facilities available to him, he provedthat iron can be magnetized by the rapidlyoscillating (and damped) electric fieldproduced during the discharge of a Tesla coil.

This indicated that electromagnetic(Maxwellian or Hertzian) waves might bedetectable if they were allowed to demagnetizea magnetized wire, and by the end of 1894 hewas sending and receiving these "wireless"signals in the laboratory.

In 1895 Rutherford arrived in Cambridgewhere he became the first research student towork under J. J. Thomson at the CavendishLaboratory. He improved his earlierinstrumentation and was soon transmittingand receiving electromagnetic signals up to 2miles' distance, a great achievement in thosedays. Thomson asked Rutherford to assist himin his own researches on the x-rayinducedconduction of electricity through gases. Withina year these studies led Thomson to hisdiscovery of the electron.

Rutherford then explored still another recentfind, A. H. Becquerel's 1896 discovery oradioactivity. Rutherford soon determined that

the uranium rays were capable of ionizinggases. He also discovered something newnamely, that uranium emits two different typesof radiation, a highly ionizing radiation of lowpenetrating power, which he termed alpharadiation, and a much lower ionizing radiationof high penetrating power, which he termedbeta radiation.

Rutherford remained with Thomson at theCavendish Laboratory until 1898; he was

therefore extremely fortunate in being atprecisely the right place at precisely the righttime. His scientific horizons broadenedenormously during these years; and hisconfidence increased greatly owing toThomson's open recognition of his exceptionalability.


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Radioactive Transformations

Rutherford's first professorship was theMacdonald professorship of physics at McGillUniversity in Montreal. In 1900 he married

Mary Newton; the following year their onlychild, Eileen, was born.

Concerning research, Rutherford knewprecisely the area he wished to study:radioactivity. On his suggestion, R. B. Owens, ayoung colleague in electrical engineering, hadprepared a sample of thorium oxide to studythe ionizing power of thorium's radiations.Owens found, oddly enough, that the ionizationthey produced apparently depended upon thepresence or absence of air currents passingover the thorium oxide. Nothing similar hadever been observed with uranium. It was thismystery that Owens, going on vacation, left forRutherford to solve.

Rutherford designed a series of masterfulexperiments from which he concluded thatthorium somehow produces a gas, which hecalled "thorium emanation." It was this gasthat Owens's air currents had transported,thereby influencing the recorded ionization.Rutherford also found that any thoriumemanation produced soon disappeared beforehis very eyes! By passing some thoriumemanation through a long tube at a constantrate, Rutherford discovered that half theamount present at any given time disappeared("decayed") roughly every minuteits "half-life." He also found that, if thorium emanationcame into contact with a metal plate, the platewould acquire an "active deposit" which alsodecayed but which had a half-life of roughly 11

hours. Further studies revealed that pressureor other external conditions did not influencethese half-lives. In addition, the "activities" ofthe substances as a function of time decayedexponentially, which Rutherford realized waspossible only if the activity was directlyproportional to the number of "ions" (atoms)present at any given time. In this wayRutherford discovered the first knownradioactive gas, thorium emanation, andexplored its behavior.

In 1900 Rutherford was joined by FrederickSoddy, a member of McGill's chemistrydepartment. Together they resolved to isolatethe sources of thorium's radioactivity bychemical separation techniques. By the end of

1901 their most important conclusions werefirst, that thorium emanation is an inert gaslike argon and, second, that thoriumemanation is produced, not by thoriumdirectly, but by some unknown, and apparentlychemically different, element which theytermed "thorium X." This was a key insight intothe understanding of radioactivity, for itsuggested that one element, thorium, can decayinto a second element, thorium X, which inturn can decay into a third element, thoriumemanation.

Item after item now fell into place. Soddyturning from thorium to uranium, found that itdecayed into a new radioactive element"uranium X." Next, Rutherford came tounderstand the crucial fact that eachradioactive transformation is accompanied bythe instantaneous emission of a single alpha orbeta particle. Rutherford also proved by asimple calculation that in radioactivetransformations enormous quantities of energy

are released, which, he argued could be derivedonly from an internal atomic source.

Although some links were still missingRutherford's revolutionary theory ofradioactive transformations was essentiallycomplete by early 1904. He summarized theresults of all of his own researches, as well asthose of the Curies and other physicists, in hisBakerian lecture, "The Succession of Changesin Radioactive Bodies," of May 19, 1904, which

he delivered before the Royal Society ofLondon. In this lecture, one of the classics inthe literature of physics, he presented thecomplete mathematical formulation of histheory, identified the four radioactive seriesuranium, thorium, actinium, and radium(neptunium)and established the principlealbeit tacitly, that any radioactive element canbe uniquely identified by its half-life.


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Rutherford also delivered a lecture at the RoyalInstitution in which he dwelled at some lengthon an important consequence of his theoryitsimplications for the age of the earth. Herealized that lead, a stable element, is the end

product of each radioactive series. This meantthat, by determining the relative amounts of,say, uranium and lead in a sample of rock, itsage can be calculatedwhich is the basis of theradioactive dating method.

Rutherford's researches attracted a number ofscientists to McGill. His activities thereteaching, experimenting, writing his famousbookRadioactivitywere prodigious.Recognition came to Rutherford early: he waselected a Fellow of the Royal Society in 1902,was awarded the society's Rumford Medal in1905, and delivered the Yale UniversitySilliman Lectures and received his firsthonorary degree in 1906. In 1908 he receivedthe Nobel Prizein chemistry! Rutherfordlater remarked that he had in his day observedmany transformations of varying periods oftime, but the fastest he had ever observed washis own from physicist to chemist. He refusedto disappoint the Nobel Committee, however,and titled his Nobel lecture "The Chemical

Nature of the Alpha-Particles from RadioactiveSubstances."

Nuclear Atom and ArtificialTransmutations

In 1907 Rutherford arrived at the University ofManchester to succeed Sir Arthur Schuster asLangworthy professor of physics. Rutherfordseems to have enjoyed teaching at Manchestermore than at McGill. As he later wrote to his

friend B.B. Boltwood of Yale University: "I findthe students here regard a full professor aslittle short of Lord God Almighty. It is quiterefreshing after the critical attitude ofCanadian students."

By early 1908 Rutherford was ready to testsome new ideas. One of the first questions hewanted to settle was the nature of alphaparticles. He devised a very simple scheme forcapturing alpha particles, from purified radium

emanation, in a glass enclosure. There thealpha particles acquired free electrons andformed a gas which spectroscopic analysisproved to be helium. This work took on muchbroader significance as a result of another

observation, namely, that alpha particles canbe scattered by various substances. Hiscoworkers, H. Geiger and E. Marsden, allowedalpha particles to strike various metal foils (forexample, gold and platinum) and counted thatbetween 3 and 67 alpha particles per minuteor about 1/8000 of those present in theincident beamwere scattered backward, thatis, through more than a right angle.

Two years elapsed before Rutherford achievedthe insights necessary for a satisfactoryexplanation of Geiger and Marsden'sexperiments. He had to realize that the alphaparticle is not of atomic dimensions but that itcan be considered to be a point charge inscattering theoretical calculations and that thenumber of electrons per atom is relativelysmallon the same order of magnitudenumerically speaking, as the atom's atomicweight. He also had to realize the extremeimprobability of obtaining Geiger andMarsden's results if the alpha particle was

multiply scattered by presumably widelyseparated electrons in the atom, as a 1904atomic model, as well as a 1910 scatteringtheory, of Thomson's suggested. In early 1911Rutherford became convinced, through ratherextensive calculations, that Geiger andMarsden's alpha particles were being scatteredin hyperbolic orbits by the intense electric fieldsurrounding a dense concentration of electriccharge in the center of the atomthe nucleusThe nuclear atom had been born.

No one, however, noticed the new arrival. Itwas apparently not even mentioned, forexample, at the famous 1911 Solvay Conferencein Brussels, which Rutherford, Albert EinsteinMax Planck, and many other prominentphysicists attended. Whatever noveltycontemporary physicists attached toRutherford's paper seems to have been to hisscattering theory rather than to his model ofthe atom which was only one of many models


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present in the literature. Only after Niels Bohrexploited the nucleus in developing his famous1913 quantum theory of the hydrogen atom,and only after H.G.J. Moseley attached to thenucleus a unique atomic number through his

well-known 1913-1914 x-ray experiments, wasthe full significance of Rutherford's nuclearmodel generally appreciated. Only then, forexample, did the concept of isotopes becomegenerally and clearly recognized.

The researches that Rutherford fostered atManchesterpartly for which he was knightedin 1914were not confined to alpha scatteringand atomic structure. For example, he and hiscoworkers studied the chemistry and modes ofdecay of the radioactive elements; thescattering, the wavelengths, and the spectra ofgamma rays; and the relationship between therange of alpha particles and the lifetime of theelements from which they are emitted.

Most of this immense activity was brought to ahalt at the outbreak of World War I. Rutherfordbecame associated with the Admiralty Board ofInvention and Research early in the war, andhe carried out experiments relating to thedetection of submarines, devising a variety of

microphones, diaphragms, and underwatersenders and receivers to study underwatersound propagation. He supplied Americanscientists with a vast amount of informationwhen the United States entered the war in 1917.

In 1919 Rutherford and William Kay found, asthe culmination of a long series of investigations, that when alpha particles strikehydrogenor, in a more famous experiment,nitrogenrecoil "protons" (Rutherford's term)

are produced. Rutherford realized at once thathe had achieved the first artificial nucleartransmutation (alpha particle + nitrogen toproton + oxygen) known to man. He gave a fullaccount of his and Kay's work in 1920 in hissecond Bakerian lecture, "Nuclear Constitutionof Atoms." One surprising prediction he madein this lecture was that of a "kind of neutraldoublet," perhaps a faint premonition of theneutron. Rutherford's discovery of artificial

transmutation was, in general, a fittingcapstone to his brilliant career at Manchester.

Cambridge and Honors

In 1919 Rutherford became CavendishProfessor of Physics and Director of thelaboratory and, a bit later, Fellow of TrinityCollege, Cambridge. As the occupant of themost prestigious chair of physics in Englandand, concurrently, as the holder of aProfessorship of Natural Philosophy at theRoyal Institution (1921), Rutherford was moreand more called upon to deliver public lecturesand serve in various professional offices. In1923 he was elected President of the BritishAssociation for the Advancement of Science; in1925, the same year in which he gainedadmittance into the coveted Order of Merit, hebecame President of the Royal Society for thecustomary 5-year term. In 1933 he accepted thepresidency of the Academic Assistance Councilformed to aid Nazi-persecuted Jewish scholarsHe died on Oct. 19, 1937, in Cambridge.

Portrait of the Man

C. P. Snow has provided the following portraitof Rutherford in mature life: "He was a big,rather clumsy man, with a substantial baywindow that started in the middle of the chestI should guess that he was less muscular thanat first sight he looked. He had large staringblue eyes and a damp and pendulous lower lipHe didn't look in the least like an intellectualCreative people of his abundant kind never doof course, but all the talk of Rutherford lookinglike a farmer was unperceptive nonsense. Hiswas really the kind of face and physique that

often goes with great weight of character andgifts. It could easily have been the soma of agreat writer. As he talked to his companions inthe streets, his voice was three times as loud asany of theirs, and his accent was bizarre. Itwas part of his nature that, stupendous as hiswork was, he should consider it 10 per centmore so. It was also part of his nature thatquite without acting, he should behaveconstantly as though he were 10 per cent largerthan life. Worldly success? He loved every


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minute of it: flattery, titles, the company of thehigh official world."

Gustav Robert Kirchhoff

The German physicist Gustav RobertKirchhoff (1824-1887) is bestremembered for his pioneeringwork in

spectroscopy that permittedinvestigation of the chemicalcomposition of stars.

Gustav Kirchhoff was born on March 12, 1824,in Knigsberg, East Prussia, the son of alawyer. He attended the local gymnasium andentered the University of Knigsberg at the ageof 18. Among his teachers were FranzNeumann, the noted theoretical physicist, andFriedrich Richelot, the mathematician. Shortlyafter he received his doctorate in 1847, hemarried Richelot's daughter, Clara; they hadtwo sons and two daughters. Also in 1847, hereceived a rarely awarded travel grant from theuniversity for a study trip to Paris, but thepolitical situation forced him to cancel theplans. In 1848 Kirchhoff became privatdozentin Berlin, and 2 years later he obtained the postof extraordinary (associate) professor atBreslau. It was there that he first met RobertBunsen. By 1854 both Kirchhoff and Bunsenwere working together in Heidelberg.

The investigation of spectra with prisms hadbeen going on for decades. There had also beenseveral guesses made as to the identity betweensome lines in the solar spectrum and in spectraproduced in laboratories. But it was Kirchhoffwho, one afternoon in the summer of 1859,looked at the interaction of sunlight and thelight of table salt burning in the flame of theBunsen burner and said, "There must be a

fundamental story here." When he returned tothe laboratory the next day, he had the solutionto his observation. It is known as Kirchhoff'slaw of radiation: the relation between thepowers of emission and the powers of

absorption for rays of the same wavelength isconstant for all bodies at the sametemperature. This law also implies that thebodies absorb more readily the radiation ofsuch wavelengths as they tend to emitFurthermore, the law implies that the greaterthe opacity of a body, the more complete itsspectrum, and that the true emission spectrumof a substance is obtained in its gaseous stateKirchhoff's now famous paper, written withBunsen and published in 1859, also stated that"the dark lines [Fraunhofer lines] of the solarspectrum which are not caused by theterrestrial atmosphere, arise from the presencein the glowing solar atmosphere of thosesubstances which in a flame produce brightlines in the same position."

Kirchhoff and Bunsen became celebritiesovernight. Subsequent scientific developmentsdid full justice to the elation of the moment.Spectroscopy turned out to be the magic key toa great number of practical discoveries, and

half a century later it ushered in the era ofmodern atomic physics. In a sense, Kirchhoff'sgreat success in spectroscopy drew attentionaway from his varied contributions to everybranch of physics. He occupied the chair oftheoretical physics at the University of Berlinfrom 1875 until his death on Oct. 17, 1887.


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Joseph Henry

Joseph Henry (1797-1878), Americanphysicist and electrical experimenter,

was primarily important for his role inthe institutional development of sciencein America.

Joseph Henry was born Dec. 17, 1797, inAlbany, N. Y. He attended the common schooluntil the age of 14, when he was apprenticed toa jeweler. He later studied at the AlbanyAcademy and in 1826 became professor ofmathematics there. He immediately beganresearching a comparatively new fieldtherelation of electric currents to magnetism. Theimportant result of this work was Henry'sdiscovery of induced currents. In 1832 he wasappointed professor of natural philosophy(chemistry and physics) in the College of NewJersey at Princeton.

In 1846 Henry became the first secretary anddirector of the Smithsonian Institution inWashington, D.C., a position he held for therest of his life. Under his direction theinstitution encouraged and supported original

research. Although a large portion of theincome settled on the institution by Congresswas for the support of the museum, art gallery,laboratory, and library, Henry took everyopportunity to divest the institution of suchburdens.

As the Smithsonian's director, Henry acted asone of the major coordinators of governmentscience. Among the projects he originated was

the system of receiving simultaneous weatherreports by telegraph and basing weatherpredictions on them. From these beginningscame the U.S. Weather Bureau. During theCivil War he served on the Navy's permanent

commission to evaluate inventions and on theLighthouse Board.

Henry was elected to the AmericanPhilosophical Society in 1835. He helpedorganize the American Association for theAdvancement of Science in 1847 and was anoriginal member of the National Academy ofSciences, chartered by Congress in 1863. Hebecame vice president of the National Academyin 1866 and was president from 1868 until hisdeath. He was responsible for reorganizing theacademy and transforming it from a societythat emphasized governmental service to anhonorary organization which recognized"original research."

Henry died on May 13, 1878. By concurrentresolution a memorial service was held in hishonor on the evening of Jan. 16, 1879, in thehall of the House of Representatives, and by actof Congress a bronze statue was erected atWashington in his memory.


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Jean Bernard Lon Foucault

The French physicist Jean Bernard LonFoucault (1819-1868) is remembered forthe Foucault pendulum, by which hedemonstrated the diurnal rotation of theearth, and for the first accuratedetermination of the velocity of light.

Lon Foucault, son of a Paris bookseller, wasborn on Sept. 18, 1819. He began to studymedicine but turned to physics, probably as aresult of becoming assistant to Alfred Donn,who was developing a photoengraving processby etching daguerreotypes in connection withhis anatomy lectures. This brought Foucaultcontact with the physicist Hippolyte Fizeau,who was at that time attempting to improve thedaguerreotype process, and they collaboratedfor several years on optical topics. From 1845Foucault was editor of the scientific section oftheJournal de dbats. In 1855 he wasappointed physicist at the Paris Observatory; in1864 he was elected a foreign member of theRoyal Society of London; and in 1865 he

became a member of the Acadmie desSciences.

Rotating Frames of Reference

Foucault's first important experimentaldemonstration was of the earth's rotation, forwhich he used a pendulum. The plane ofmotion of a freely suspended simple pendulumappears to rotate; in fact, it is spatially fixed

while the earth rotates. Foucault published hisaccount of this in 1851, together with anequation connecting the apparent angularrotation of the pendulum's plane with theangular velocity of the earth and the latitude of

the place of the experiment. It created greatinterest, and the experiment, readily repeatablewith simple apparatus, was, and still isfrequently performed in public. In 1852Foucault gave a further demonstration of theearth's rotation with a freely mountedgyroscope and derived some laws describing itsbehavior. These experiments, in combinationwith earlier theoretical work by GustaveCoriolis, led to a clearer understanding ofrotating frames of reference. For his workFoucault was awarded the Copley Medal of theRoyal Society in 1855.

Determining the Velocity of Light

In 1850 Foucault joined the debate over thethen-competing particle and wave theories oflight. D. F. J. Arago had demonstrated in 1838that a crucial test could be made by comparingthe velocities of light in air and in a densemedium, and he was experimenting todetermine the velocity of light with a rotating-

mirror method devised by Charles Wheatstonein 1834. Lack of success and ill health led Aragoto pass the task on to Foucault in 1850. Successcame in the same year, when Foucaultobserved a retardation of the velocity of light inwater, giving support to the wave theory. Hethen saw how the rotating-mirror methodcould be refined to measure the absolutevelocity of light in a restricted space. Foucaultovercame the technical problems and in 1862obtained a value of 2.98 x 1010 centimeters per

second, the first accurate measure of thisfundamental physical constant.

From 1855, as physicist at the ImperiaObservatory, Foucault worked to improve thedesign of telescopes. As a member of theBureau of Longitudes from 1862 he improvedcertain surveying instruments, particularly thecentrifugal governor, which aided timekeepingin the use of field-transit instruments. The1860s saw Foucault turning toward precision


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engineering and electricity, but he wasincapacitated by a stroke in July 1867 and diedin Paris on Feb. 11, 1868.

Foucault's ability to recognize fruitful lines of

research, so sadly lacking in many of hiscontemporary countrymen, was combined withan experimental ability of the first order. Hisearly death was a great loss to French science.

Enrico Fermi

The Italian-American physicist EnricoFermi (1901-1954) discovered "Fermistatistics," described beta decay,established the properties of slowneutrons, and constructed the firstatomic pile.

In Enrico Fermi, the theorist andexperimentalist were combined in a supremelyintimate, complementary, and creative way. Hepossessed an almost uncanny physical intuitionwhich, together with his personal simplicity,made him universally admired and respected.

Fermi was born on Sept. 29, 1901, in Rome, thethird child of an official in the Ministry ofRailroads. At about the age of 10 his interest inmathematics and physics awakened. Aperceptive colleague of his father's, theengineer A. Amidei, recognized Fermi's trulyexceptional intellectual qualities and guided hismathematical and physical studies betweenages 13 and 17.

By the time Fermi received his doctorate fromthe University of Pisa in 1922, he had writtenseveral papers on relativistic electrodynamics,using the methods of Albert Einstein's generaltheory. Fermi received a fellowship to study atthe University of Gttingen. In spite of the factthat he attacked problems of interest to theGttingen physicists, his 8 months there werenot very satisfactory. In 1924, on George E.

Uhlenbeck's urging, Fermi went to study at theUniversity of Leiden with Uhlenbeck's teacherPaul Ehrenfest. Several years later, whenUhlenbeck was at the University of Michiganhe arranged for Fermi to spend the summers of

1930, 1933, and 1935 at Michigan's SummerSchool for Theoretical Physics.

Fermi Statistics

Late in 1924, after leaving Leiden, Fermi wentto the University of Florence, where he taughtmathematical physics and theoreticalmechanics. In 1926 he published his first majordiscovery, namely, the quantum statistics nowuniversally known as Fermi-Dirac statisticsThe particles obeying these statistics are nowknown as fermions.

Fermi's discovery did not stem basically fromthe concurrently emerging quantum theory, asmight be expected, but rather from his ownstudies in statistical mechanics. These studiesbegan as early as 1923 but were frustratedbecause a key concept, Wolfgang Pauli'sexclusion principle, was still missing. Fermsaw immediately that all particles (fermions)obeying Pauli's exclusion principle wouldbehave in a definite way, quantum-mechanically and statistically speaking. Fermi'sdiscovery led to an understanding of certainimportant features of gas theory, of howelectrons in metals conduct electricity, of whyelectrons do not contribute to the specific heatsof substances, and of many other phenomenaIt also undergirded Fermi's widely used 1927statistical model of the atom, an approximatemodel in which the atom is envisioned as astatistical assemblage of electrons.

Theory of Beta Decay

The years between 1926 and 1938 constitutedFermi's "golden age." He accepted the chair oftheoretical physics at the University of Rome in1926 and only 3 years later became one of thefirst 30 members (and sole physicist) to beelected to the Royal Academy of Italy. In 1928


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he married Laura Capon; they had a son and adaughter.

Fermi made significant contributions to a widevariety of problems in atomic, molecular, and

nuclear spectroscopy; in particle scatteringtheory; in atomic and nuclear structure; and inquantum electrodynamics. His most celebratedtheoretical work of this period was his 1933theory of nuclear beta decay, a theory thatnicely supplemented the theory of nuclearalpha decay of George Gamow, R. W. Gurney,and Edward U. Condon.

In beta decay a negatively charged particle(beta particle), known to be identical to anelectron, is emitted from the nucleus of anatom, thereby increasing the atomic number ofthe nucleus by one unit. Fermi worked out in ashort time an elegant theory of beta decaybased on the idea that a neutron in the nucleusis transformed (decays) into three particles: aproton, an electron (beta particle), and aneutrino. Actually, the neutrinoan elusive,massless, chargeless particlewas not detectedexperimentally until the 1950s.

Slow Neutrons

In the late 1920s Fermi decided to attackexperimental problems in nuclear physicsrather than continue his ongoing spectroscopicresearches. By mixing beryllium powder withsome radon gas, he had a source of neutronswith which to experiment and determinewhether neutrons could induce radioactivity.He constructed a crude Geiger-counterdetector and, methodically, he startedbombarding hydrogen, then went on to

elements of higher atomic number. All resultswere negative until he bombarded fluoriumand detected a weak radioactivity. This keydate in neutron physics was March 21, 1934.

With high excitement Fermi and his coworkerscontinued. By summer 1934 they hadbombarded many substances, discovering, forexample, that neutrons can liberate protons aswell as alpha particles. In addition, they had

detected a slight radioactivity whenbombarding uranium, and they attemptedwithout success, to understand why aluminum,when bombarded with neutrons, could notdecide, in effect, which of two different nuclear

reactions to undergo.Their next discovery was a milestone. Theyfound that the level of radioactivity induced ina substance was increased if a paraffin filterwas placed in the beam of neutrons irradiatingthe substance. Fermi's hypothesis for thismiracle, which he immediately confirmed, wasthat in passing through the paraffin, acompound containing a large amount ohydrogen, the neutrons had their velocity muchreduced by collisions with the hydrogen nuclei;and these very slow neutronscontrary to allexpectationsinduced a much higherradioactivity in substances than did fastneutrons. Furthermore, the old aluminummystery had been solved: slow neutronsproduce one kind of reaction, fast neutronsanother. The discovery of the remarkableproperties of slow neutrons was the keydiscovery in neutron physics.

By 1937 Fermi's wife and their children became

directly affected by the racial laws in FascistItaly. In December 1938 the Fermi family wentto Stockholm for the presentation of the NobelPrize in physics to Fermi. He and his familythen left for the United States, arriving in NewYork on Jan. 2, 1939, where Fermi accepted aposition at Columbia University.

Atomic Age

With the assistance of Herbert L. Anderson

Fermi produced a beam of neutrons with theColumbia cyclotron, thus verifying the fissionof uranium. Then he quantitatively exploredthe conditions governing its production. Heand his coworkers also proved, using a minutesample, that the fissionable isotope of uraniumis U 235. By mid-1939 there was clear evidencethat a self-sustaining chain reaction might berealizable. Furthermore, the stupendousmilitary importance of nuclear fission hadbecome clear. By July 1941 Arthur H. Compton


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chairman of a special committee of theNational Academy of Sciences, could report thepossibility not only of a uranium bomb but alsoof a plutonium bomb.

Fermi was asked to assume the hugeresponsibility of directing the construction ofthe first atomic pile. He, and other keyphysicists, moved to the University of Chicagoin the spring of 1942; by early October theirresearches had progressed to the point whereFermi was confident he knew how to constructthe pile, and the project (the "ManhattanProject") was under way. Construction of thepile began in mid-November 1942, and onDecember 2 Fermi directed the operation of thefirst self-sustaining chain reaction created byman. The actual length of time it was operatedon that historic day was 40 minutes; itsmaximum power was 1/2 watt, enough toactivate a penlight. It was the opening of a newage, the Atomic Age.

Fermi's experiment was far more than anexperiment in pure research. Huge nationallaboratories were constructed, one of which,Los Alamos, had immediate responsibility forthe construction of the nuclear bomb. Its

director was J. Robert Oppenheimer. InSeptember 1944 he brought Fermi fromChicago primarily to have him on hand duringthe last, critical stages in the construction ofthe bomb. By early 1945 the project hadproceeded to the point where the greatestamount of new information could be obtainedonly by actually exploding the fearsomeweapon. The test, which bore the code name"Project Trinity," was successfully carried outon July 16, 1945, in the desert near

Alamogordo in southern New Mexico.

Last Years

On Dec. 31, 1945, Fermi became Charles H.Swift distinguished service professor of physicsand a member of the newly establishedInstitute (now the Enrico Fermi Institute) forNuclear Studies at the University of Chicago.This was the beginning of a period duringwhich his reading and range of interests

always confined largely to physics contractedconsiderably. For a few years he continuedworking in the fields of nuclear and neutronphysics. In 1949 he demonstrated theoreticallythat the extremely high cosmic-ray energies

can be accounted for by the accelerationsimparted to them by vast interstellar magneticfields. At about the same time his interestshifted away from nuclear physics to high-energy (particle) physics. In a number of hisresearches he used the Chicagosynchrocyclotron to explore pi-mesoninteractions in an effort to discover the meansby which the nucleus is held together in astable configuration.

Fermi died in Chicago on Nov. 29, 1954.


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Albert Einstein

The German-born American physicistAlbert Einstein (1879-1955)revolutionized the science of physics. Heis best known for his theory of relativity.

In the history of the exact sciences, only ahandful of menmen like Nicolaus Copernicusand Isaac Newtonshare the honor that wasAlbert Einstein's: the initiation of a revolutionin scientific thought. His insights into thenature of the physical world made it impossiblefor physicists and philosophers to view thatworld as they had before. When describing theachievements of other physicists, the tendencyis to enumerate their major discoveries; whendescribing the achievements of Einstein, it ispossible to say, simply, that he revolutionizedphysics.

Albert Einstein was born on March 14, 1879, inUlm, but he grew up and obtained his earlyeducation in Munich. He was not a childprodigy; in fact, he was unable to speak fluentlyat age 9. Finding profound joy, liberation, andsecurity in contemplating the laws of nature,already at age 5 he had experienced a deepfeeling of wonder when puzzling over theinvisible, yet definite, force directing the needleof a compass. Seven years later he experienced

a different kind of wonder: the deep emotionalstirring that accompanied his discovery oEuclidean geometry, with its lucid and certainproofs. Einstein mastered differential andintegral calculus by age 16.

Education in Zurich

Einstein's formal secondary education wasabruptly terminated at 16. He found life inschool intolerable, and just as he was schemingto find a way to leave without impairing hischances for entering the university, his teacherexpelled him for the negative effects hisrebellious attitude was having on the morale ofhis classmates. Einstein tried to enter theFederal Institute of Technology (FIT) in ZurichSwitzerland, but his knowledge ofnonmathematical disciplines was not equal tothat of mathematics and he failed the entranceexamination. On the advice of the principal, hethereupon first obtained his diploma at theCantonal School in Aarau, and in 1896 he wasautomatically admitted into the FIT. There hecame to realize that his deepest interest andfacility lay in physics, both experimental andtheoretical, rather than in mathematics.

Einstein passed his diploma examination at theFIT in 1900, but due to the opposition of one ofhis professors he was unable to subsequentlyobtain the usual university assistantship. In1902 he was engaged as a technical expert,third-class, in the patent office in BernSwitzerland. Six months later he marriedMileva Maric, a former classmate in ZurichThey had two sons. It was in Bern, too, thatEinstein, at 26, completed the requirements forhis doctoral degree and wrote the first of his

revolutionary scientific papers.

Academic Career

These papers made Einstein famous, anduniversities soon began competing for hisservices. In 1909, after serving as a lecturer atthe University of Bern, Einstein was called asan associate professor to the University ofZurich. Two years later he was appointed a full


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professor at the German University in Prague.Within another year and a half Einsteinbecame a full professor at the FIT. Finally, in1913 the well-known scientists Max Planck andWalter Nernst traveled to Zurich to persuade

Einstein to accept a lucrative researchprofessorship at the University of Berlin, aswell as full membership in the PrussianAcademy of Science. He accepted their offer in1914, quipping: "The Germans are gambling onme as they would on a prize hen. I do not reallyknow myself whether I shall ever really layanother egg." When he went to Berlin, his wiferemained behind in Zurich with their two sons;after their divorce he married his cousin Elsa in1917.

In 1920 Einstein was appointed to a lifelonghonorary visiting professorship at theUniversity of Leiden. During 1921-1922Einstein, accompanied by Chaim Weizmann,the future president of the state of Israel,undertook extensive worldwide travels in thecause of Zionism. In Germany the attacks onEinstein began. Philipp Lenard and JohannesStark, both Nobel Prize-winning physicists,began characterizing Einstein's theory ofrelativity as "Jewish physics." This callousness

and brutality increased until Einstein resignedfrom the Prussian Academy of Science in 1933.(He was, however, expelled from the BavarianAcademy of Science.)

Career in America

On several occasions Einstein had visited theCalifornia Institute of Technology, and on hislast trip to the United States Abraham Flexneroffered Einsteinon Einstein's termsa

position in the newly conceived and fundedInstitute for Advanced Studies in Princeton. Hewent there in 1933.

Einstein played a key role (1939) in mobilizingthe resources necessary to construct the atomicbomb by signing a famous letter to PresidentFranklin D. Roosevelt which had been draftedby Leo Szilard and E.P. Wigner. WhenEinstein's famous equation E mc2 was finallydemonstrated in the most awesome and

terrifying way by using the bomb to destroyHiroshima in 1945, Einstein, the pacifist andhumanitarian, was deeply shocked anddistressed; for a long time he could only utter"Horrible, horrible." On April 18, 1955

Einstein died in Princeton.Theory of Brownian Motion

From numerous references in Einstein'swritings it is evident that, of all areas inphysics, thermodynamics made the deepestimpression on him. During 1902-1904 Einsteinreworked the foundations of thermodynamicsand statistical mechanics; this work formed theimmediate background to his revolutionarypapers of 1905, one of which was on Brownianmotion.

In Brownian motion (first observed in 1827 bythe Scottish botanist Robert Brown), smallparticles suspended in a viscous liquid such aswater undergo a rapid, irregular motionEinstein, unaware of Brown's earlierobservations, concluded from his theoreticastudies that such a motion must exist. Guidedby the thought that if the liquid in which theparticles are suspended consists of atoms ormolecules they should collide with the particlesand set them into motion, he found that whilethe particle's motion is irregular, fluctuatingback and forth, it will in time neverthelessexperience a net forward displacementEinstein proved that this net forwarddisplacement of the suspended particles isdirectly related to the number of molecules pergram atomic weight. This point created a gooddeal of skepticism toward Einstein's theory atthe time he developed it (1905-1906), but when

it was fully confirmed many of the skepticswere converted. Brownian motion is to this dayregarded as one of the most direct proofs of theexistence of atoms.

Light Quanta and Wave-ParticleDuality

The most common misconceptions concerningEinstein's introduction of his revolutionary


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light quantum (light particle) hypothesis in1905 are that he simply applied Planck'squantum hypothesis of 1900 to radiation andthat he introduced light quanta to "explain" thephotoelectric effect discovered in 1887 by

Heinrich Hertz and thoroughly investigated in1902 by Philipp Lenard. Neither of theseassertions is accurate. Einstein's arguments forhis light quantum hypothesisthat undercertain circumstances radiant energy (light)behaves as if it consists not of waves but ofparticles of energy proportional to theirfrequencies were absolutely fundamentaland, as in the case of his theory of Brownianmotion, based on his own insights into thefoundations of thermodynamics and statisticalmechanics. Furthermore, it was only afterpresenting strong arguments for the necessityof his light quantum hypothesis that Einsteinpursued its experimental consequences. One ofseveral such consequences was thephotoelectric effect, the experiment in whichhigh-frequency ultraviolet light is used to ejectelectrons from thin metal plates. In particular,Einstein assumed that a single quantum oflight transfers its entire energy to a singleelectron in the metal plate. The famousequation he derived was fully consistent with

Lenard's observation that the energy of theejected electrons depends only on thefrequency of the ultraviolet light and not on itsintensity. Einstein was not disturbed by the factthat this apparently contradicts James ClerkMaxwell's classic electromagnetic wave theoryof light, because he realized that there weregood reasons to doubt the universal validity ofMaxwell's theory.

Although Einstein's famous equation for the

photoelectric effectfor which he won theNobel Prize of 1921 appears so natural today,it was an extremely bold prediction in 1905.Not until a decade later did R.A. Millikanfinally succeed in experimentally verifying it toeveryone's satisfaction. But while Einstein'sequation was bold, his light quantumhypothesis was revolutionary: it amounted toreviving Newton's centuries-old idea that lightconsists of particles.

No one tried harder than Einstein to overcomeopposition to this hypothesis. Thus, in 1907 heproved the fruitfulness of the entire quantumhypothesis by showing it could at leastqualitatively account for the low-temperature

behavior of the specific heats of solids. Twoyears later he proved that Planck's radiationlaw of 1900 demands the coexistence oparticles and waves in blackbody radiation, aproof that represents the birth of the wave-particle duality. In 1917 Einstein presented avery simple and very important derivation ofPlanck's radiation law (the modern laser, forexample, is based on the concepts Einsteinintroduced here), and he also proved that lightquanta must carry momentum as well asenergy.

Meanwhile, Einstein had become involved inanother series of researches having a directbearing on the wave-particle duality. In mid-1924 S.N. Bose produced a very insightfuderivation of Planck's radiation lawthe originof Bose-Einstein statisticswhich Einsteinsoon developed into his famous quantumtheory of an ideal gas. Shortly thereafter, hebecame acquainted with Louis de Broglie'srevolutionary new idea that ordinary material

particles, such as electrons and gas moleculesshould under certain circumstances exhibitwave behavior. Einstein saw immediately thatDe Broglie's idea was intimately related to theBose-Einstein statistics: both indicate thatmaterial particles can at times behave likewaves. Einstein told Erwin Schrdinger of DeBroglie's work, and in 1926 Schrdinger madethe extraordinarily important discovery of wavemechanics. Schrdinger's (as well as C. Eckart)then proved that Schrdinger's wave

mechanics and Werner Heisenberg's matrixmechanics are mathematically equivalent: theyare now collectively known as quantummechanics, one of the two most fruitfulphysical theories of the 20th century. SinceEinstein's insights formed much of thebackground to both Schrdinger's andHeisenberg's discoveries, the debt quantumphysicists owe to Einstein can hardly beexaggerated.


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Theory of Relativity

The second of the two most fruitful physicaltheories of the 20th century is the theory ofrelativity, which to scientists and laymen alike

is synonymous with the name of Einstein. Onceagain, there is a common misconceptionconcerning the origin of this theory, namely,that Einstein advanced it in 1905 to "explain"the famous Michelson-Morley experiment(1887), which failed to detect a relative motionof the earth with respect to the ether, themedium through which light was assumed topropagate. In fact, it is not even certain thatEinstein was aware of this experiment in 1905;nor was he familiar with H.A. Lorentz's elegant1904 paper in which Lorentz applied thetransformation equations which bear his nameto electrodynamic phenomena. Rather,Einstein consciously searched for a generalprinciple of nature that would hold the key tothe explanation of a paradox that had occurredto him when he was 16: if, on the one hand, oneruns at, say, 4 miles per hour alongside a trainmoving at 4 miles per hour, the train appearsto be at rest; if, on the other hand, it werepossible to run alongside a ray of light, neitherexperiment nor theory suggests that the ray of

lightan oscillating electromagnetic wavewould appear to be at rest. Einstein eventuallysaw that he could postulate that no matter whatthe velocity of the observer, he must alwaysobserve the same velocity c for the velocity oflight: roughly 186,000 miles per second. Healso saw that this postulate was consistent witha second postulate: if an observer at rest and anobserver moving at constant velocity carry outthe same kind of experiment, they must get thesame result. These are Einstein's two

postulates of his special theory of relativity.Also in 1905 Einstein proved that his theorypredicted that energy E and mass mare entirelyinterconvertible according to his famousequation, E=mc2.

For observational confirmation of his generaltheory of relativity, Einstein boldly predictedthe gravitational red shift and the deflection ofstarlight (an amended value), as well as thequantitative explanation of U. J. J. Leverrier's

long-unexplained observation that theperihelion of the planet Mercury precessesabout the sun at the rate of 43 seconds of arcper century. In addition, Einstein in 1916predicted the existence of gravitational waves

which have only recently been detectedTurning to cosmological problems thefollowing year, Einstein found a solution to hisfield equations consistent with the picture (theEinstein universe) that the universe is static,approximately uniformly filled with a finiteamount of matter, and finite but unbounded(in the same sense that the surface area of asmooth globe is finite but has no beginning orend).

The Man and His Philosophy

Fellow physicists were always struck withEinstein's uncanny ability to penetrate to theheart of a complex problem, to instantly see thephysical significance of a complexmathematical result. Both in his scientific andin his personal life, he was utterly independenta trait that manifested itself in his approach toscientific problems, in his unconventionaldress, in his relationships with family andfriends, and in his aloofness from university

and governmental politics (in spite of hisintense social consciousness). Einstein loved todiscuss scientific problems with friends, but hewas, fundamentally a "horse for singleharness."

Einstein's belief in strict causality was closelyrelated to his profound belief in the harmony ofnature. That nature can be understoodrationally, in mathematical terms, never ceasedto evoke a deepone might say, religious

feeling of admiration in him. "The mostincomprehensible thing about the world," heonce wrote, "is that it is comprehensible." Howdo we discover the basic laws and concepts ofnature? Einstein argued that while we learncertain features of the world from experiencethe free inventive capacity of the human mindis required to formulate physical theoriesThere is no logical link between the world ofexperience and the world of theory. Once atheory has been formulated, however, it must


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be "simple" (or, perhaps, "estheticallypleasing") and agree with experiment. Onesuch esthetically pleasing and fully confirmedtheory is the special theory of relativity. WhenEinstein was informed of D.C. Miller's

experiments, which seemed to contradict thespecial theory by demanding the reinstatementof the ether, he expressed his belief in thespuriousness of Miller's resultsand thereforein the harmoniousness of naturewith anotherof his famous aphorisms, "God is subtle, but heis not malicious."

This frequent use of God's name in Einstein'sspeeches and writings provides us with afeeling for his religious convictions. He oncestated explicitly, "I believe in Spinoza's Godwho reveals himself in the harmony of allbeing, not in a God who concerns himself withthe fate and actions of men." It is not difficultto see that this credo is consistent with hisstatement that the "less knowledge a scholarpossesses, the farther he feels from God. Butthe greater his knowledge, the nearer is hisapproach to God." Since Einstein's Godmanifested Himself in the harmony of theuniverse, there could be no conflict betweenreligion and science for Einstein.

To enumerate at this point the many honorsthat were bestowed upon Einstein during hislifetime would be to devote space to the kind ofpublic acclamation that mattered so little toEinstein himself. How, indeed, can otherhuman beings sufficiently honor one of theirnumber who revolutionized their conception ofthe physical world, and who lived his life in theconviction that "the only life worth living is alife spent in the service of others"? When

Einstein lay dying he could truly utter, as hedid, "Here on earth I have done my job." Itwould be difficult to find a more suitableepitaph than the words Einstein himself usedin characterizing his life: "God is inexorable inthe way He has allotted His gifts. He gave methe stubbornness of a mule and nothing else;really, He also gave me a keen scent."

Famous Physicist

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