Curie, Pierre

Curie, Pierre

(b. Paris France, 15 May 1859; d. Paris 19 April 1906)

physics.

Curie was the son and grandson of physicians. His grandfather came from a protestant family of Alsatian origin and practiced medicine in Mulhouse. His father, Eugèene Curie, had married Sophie-Claire Depoully in 1854; they had two sons, Jacques-Paul, born in 1855, and Pierre. The boys were brought up in a permissive atmosphere. Their father, an enthusiast of science, an idealist, and an ardent republican, opposed any sort of physical or moral servitude. Curie attended neither écolo nor lycée; his pensive spirit was unsuited to academic discipline. He worked at home, first with his mother, then with his father and brother; at fourteen he studied with a mathematics professor.

While a boy Curie observed experiments performed by his father and acquired a taste for experimental research. He received his bachelor of science degree on 9 November 1875, and while still a probationary student in pharmacy he enrolled at the Faculty of Sciences in Paris, receiving the license in physical sciences in November 1877. His brother Jacques was a laboratory assistant to Charles Friedel at the Sorbonne mineralogy laboratory. For the two brothers this marked the beginning of a fruitful collaboration in the physics of crystals. When he was eighteen, Pierre and Jacques discovered the phenomenon of piezoelectricity, which proved to be extremely important in its manifold applications. In 1878 Pierre was appointed laboratory assistant to Paul Desains at the Sorbonne physics laboratory; he remained there until 1882, when he became director of laboratory work at the École Municipale de Physique et Chimie, which had just been founded. In 1883 Jacques Curie was appointed professor of mineralogy at Montpellier; this separation ended the scientific collaboration of the two brothers.

Pierre Curie spent twenty-two years at the Écolede physique et Chimie, in more isolated work dedicated to theoretical and experimental research. He developed new equipment that all physics laboratories were to use; his studies on magnetism culminated in the publication (1895) of his famous dissertation “Propriétés magnétiques des corps à diverses tempéeratures,” which was the subject of his doctoral thesis.

In the spring of 1894 Curie met Maria Sklodowska, who was studying the magnetic properties of certain steels at the École de physique et Chimie. They were married in 1895 and had two daughters: Iréne, born in 1897, who had a brilliant scientific career and married Frédéric Joliot, and Eve, born in 1904. It was Marie Curie who, impressed by Roentgen’s and Becquerel’s discoveries, considered investigating other substances exhibiting the same properties as uranium. For Pierre Curie this was a new period in his scientific career: in close collaboration with his wife, he was to study radioactivity and discover polonium and radium, discoveries that made both of them famous. Until 1905 their work was done in a wretched laboratory at the École de Physique et Chimie. Pierre Curie was appointed professor of physics at the Sorbonne in October 1904 but was unable to realize his dream of working in the new laboratory that he had equipped. He died in his forty seventh year, after being struck by a truck while crossing the rue Dauphine in Paris.

Serious, reserved, and involved in his own thoughts, Curie was unconcerned with life’s material comforts and improvident in his own personal needs; a man of great gentleness and kindly disposition, he had the insatiable need to understand the phenomena of nature and sacrificed all diversions and outside involvements to that end. Aside from his work he liked only outings in the country; he was a superb naturalist and knew where and when various plants, flowers, and insects could be found.

Curie’s concept of scientific research was pure and lofty. He never indulged in the sort of hasty publication that is likely to become dated, and his consistently lucid and cogent papers accurately reflected his precise mind. He showed complete unselfishness, was unconcerned with his career, and steadfastly rejected all compromise and complicated diplomacy; in 1903 he firmly refused to be decorated. Lord Kelvin was perhaps one of the first to recognize Curie’s great ability. Through him Curie’s achievements were acknowledged in British scientific circles more quickly than in France. He maintained a continuing correspondence with Curie from the time Curie discovered piezoelectricity and visited him in his laboratory in October 1893, while Curie was working on magnetism. It was through Kelvin’s intervention that Curie was appointed professor at the École de Physique et Chimie in 1894.

Curie’s work in radioactivity assured his fame. The Académic de Science awarded him the La Caze prize in Physics in 1901 and elected him a member on 3 July 1905; and he was elected a corresponding member of the Academy of Sciences and Letters of Cracow on 15 February 1903. The Royal Society of London awarded the Curies the Davy medal on 5 November 1903, and on 12 December the Nobel Prize in physics was divided among them and Henri Becquerel. Their honors also included honorary membership in the Royal Institution of Great Britain and in the department of physical sciences in August 1904; and corresponding membership in the Batavian Society of Experimental Philosophy of Rotterdam.

Curie’s scientific work and his private life were closely linked. His entire life, basically, was spent in his laboratory; it comprised three periods. The first, begun with his brother Jacques, concerned the physics of crystals, particularly piezoelectricity; then came the years of more isolated but nonetheless fruitful work that mark the complete physicist: he built new apparatus and worked on experimental and theoretical problems of crystallography and magnetism: the third period largely involved the study of radioactivity with his wife.

Piezoelectricity. Jacques Curie, under the direction of Charles Friedel, undertook research on pyroelectricity, a phenomenon that had been known for quite some time and consisted of the appearance of electrical charges in certain crystals when they were heated. This research, conducted in various laboratories, led to experimental observations and interpretations that were often contradictory. Revealing a rare sense of geometry, Pierre and Jacques Curie, by means of simple considerations of symmetry in crystals, discovered the novel phenomenon of piezoelectricity, a property of nonconducting crystals that have no center of symmetry.

These crystals, including zinc sulfide, sodium chlorate, boracite, tourmaline, quartz, calamine, topaz, sugar, and Rochelle salt, were cited in their first publication (1880). These so-called hemihedral crystals may possess axes of symmetry which are polar; in quartz, which they studied extensively, the polar axes are the three binary axes perpendicular to the ternary axis; and in tourmaline it is the ternary axis. By compressing a thin plate cut perpenticular to a binary axis in quartz (still called the electric axis) or perpendicular to the ternary axis in tourmaline, the two faces on which two tin sheets are fastened become charged with equal amounts of electricity of opposite signs, these amounts being proportional to the pressure exerted. For a decrease in pressure of the same value the two faces are charged with the same amounts of electricity but with opposite signs. The amounts of electricity are proportional to the surface of the plates. These measurements were accurate because of the use of Kelvin’s electrometer.

As soon as this research was published. Lippmann observed that thermodynamics demanded the existence of the inverse phenomenon, i.e., the strain of the piezoelectric crystals under the action of an electric field.

In 1881 the two brothers proved, with quartz and tourmaline, that the piezoelectric plates of these two substances underwent either contraction or expansion, depending on the direction of the electrical field applied. They showed this extremely slight strain, indirectly at first, by using it to compress another quartz, which exhibited the direct piezoelectric effect, and then directly, with a microscope, amplifying the strain by using a lever. Having established the experimental laws of piezoelectricity, the Curie brothers built a remarkable piece of equipment, the piezoelectric quartz balance, which supplied amounts of electricity proportional to the weights suspended from it. This device, which was immediately used by laboratories engaged in electrical research, was later used by Pierre and Marie Curie in their work on radioactivity.

At first the discovery of piezoelectricity was of only speculative interest. It permitted removal of the contraditions found in pyroelectasric observations; thus, in 1882 Friedel and Jacques Curie showed that the electri charges observed in a heated quartz plate originated in internal tensions due to heterogeneous heating. If the temperature remained homogeneous, no electric charge developed; therefore quartz was piezoelectric and not pyroelectric. The industrial uses of piezoelectricity were to come much later. During World War I. Constatin Chilovsky and Paul Langevin, a student of Pierre Curie’s, had the idea of placing piezoelectric quartz in an alternating electric field; under the effect of inverse piezoelectricity, predicted by Lippmann and verified by the Curies, the crystal expands and contracts, vibration is especially intense when the frequency of the field is the same as that of one of the natural vibration modes of the quartz, i.e. when there is resonance. This is a convenient method of producing high-frequency sound waves, first used to lacate enemy submarines and later for underwater soundings. The applications of piezoelectric crystals are innumerable; one of the most important is their use in frequency stabilization of oscillating electromagnetic cirasciots for radio broadcasting stations. They are used in most piezometers for measuring with great precision either very strong pressure variations, such as those of a cannon at the moment of firing, or very weak ones, such as artery pulsations. These applications have led to the creation of a new industry, the manufacture of large “mono” such as quartz obtained hydrothermally around 500°C. under high water pressures, or crystals such as Rochelle salt, obtained from aqueous solutions. These two substances were mentioned in the Curie brothers’ report announcing the discovery of piezoelectricity.

Isolated work. The study of piezoelectric and pyroelectric phenomena led Pierce Curie to research on crystal symmetry after his brother Jacques left for Montpellier in 1883. As early as 1884 he published several purely geometric reports on symmetry; and in 1885 he published a brief note on the morphology of crystals, in which he used the capillary constants of the different faces of a developing crystal to derive the ideal shape. This theory of Curie’s on the growth of Crystals is often cited. His ideas on symmetry, which have been fundamental to crystallographers, always gave direction to his experimental research. In 1894 he published an important work on symmetry in physical phaenomoena, in which he set forth what are today called Curie’s laws of symmetry. After discovering radium he returned to this area of study, the results of which he set forth in his course at the Sorbonne.

Curie’s laws of symmetry express, in a new and fruitful manner, the principle of causality; When certain causes produce certain effects, the symmetry of the causes reappears, in its entirety, in the effects; if an effects includes an asymmetry, this asymmetry appears, of necessity, in the effective cause.

The important factor in the production of a phenomenon or the realization of a physical state is not the presence, but rather the absence, of certain elements of symmetry. As Curie said, “It is asymmerty that creates the phenomenon,” A given phenomenon possesses characteristic symmetry that is the maximum symmetry compatible with the existence of that phenomenon; and this phenomenon cannot appear in an environment that contains an element of symmetry (center, axis, or plane) not included in its characteristic symmetry. The affirmation of this impossibility made possible a considerable economy of research. For example, it establishes that the characteristic symmetry of an electrical field is that of the frustum of a cone of revolution that has neither a plane nor an axis of symmetry perpendicular to the axis of revolution. Which crystals could be pyroelectric and the direction of the electrical field could therefore be predicted. Thus tourmaline is pyroelectric, since its symmetry group (generated by a ternary axis and three planes of reflection passing through this axis) forms a subgroup of the symmetry group of an electrical field; on the other hand, quartz, which has a ternary axis and three perpendicular binary axes, is not. If a crystal is compressed, this compression introduces an asymmetry that homogeneous heating does not introduce; quartz, like all crystals not having a center of symmetry, is piezoelectric. It follows that all pyroelectric crystals are also piezoelectric.

For the magnetic field, the characteristic symmetry is that of a cylinder of revolution turning on its axis, which explains the phenomenon of rotatory magnetic polarization.

Between 1890 and 1895 Curie devoted a great deal of effort to studying the magnetic properties of substances at various temperatures, at that time one of the most obscure areas of physics. The results, presented in a doctoral thesis that he defended on 6 March 1895, form the basis of all modern theories of magnetism. If the work of Curie’s first period, relating to crystallography and the principle of symmetry, is essentially of theoretical importance, his studies on magnetism exhibit his experimental ability. Skill and patience were required to measure the force to which a sample was subjected in a nonuniform magnetic field of which the variation in relation to space must be known in its absolute value. The force was measured with a torsion balance and was proportional to the mass of the substance under study, to its magnetic susceptibility, and to the derivative of the square of the field in the direction of the displacement. Heating presented serious difficulties and the influence of convection currents had to be eliminated, lest they interfere with the measurement of extremely small forces; Curie used an electric furnace that allowed him to operate up to 1370°C.

In terms of their magnetic properties the substances investigated by Curie may be divided into three distinct groups: (1) ferromagnetic substances, such as iron, that always magnetize to a very high degree; (2) low magnetic (paramagnetic) substances, such as oxygen, palladium, platinum, manganese, and manganese, iron, nickel, and cobalt salts, which magnetize in the same direction as iron but much more weakly: and (3) diamagnetic substances, which include the largest number of elements and compounds, whose very low magnetization is in the inverse direction of that of iron in the same magnetic field. He had to take great precautions to purify his diamagnetic substances because the very weak forces could be either greatly changed or completely hidden by the presence of traces of a ferromagnetic substance such as iron.

At the beginning of his research Curie stated the problem thus:

At first glance, these three groups are completely separate, but will this separation bear a closer examination? Do transitions between these groups exist? Is this a question of entirely different phenomena, or are we dealing only with a single phenomenon modified in various degrees? These questions were of great concern to Faraday, who often referred to them in his memoirs. He performed one important experiment in this field: it was believed for a long time that iron loses its magnetic properties when it becomes red-hot; Faraday showed that iron remains weakly magnetic when subjected to high temperatures [“Propriétés magnétiques des corps a diverses températures,” in Annales de chimie et de physique,, 7th ser., 5 (1895), 289].

In order to resolve these questions Curie studied, at various temperatures, the diamagnetic substances water, rock salt, potassium chloride, potassium sulfate, potassium nitrate, quartz, sulfur, selenium, tellurium, iodine, phosphorus, antimony, and bismuth; the paramagnetic substances oxygen, palladium, and iron sulfate; and the ferromagnetic substances iron, nickel, magnetite, and cast iron. The large number of measurements taken allowed him to confirm that no parallel can be drawn between the properties of diamagnetic substances and those of paramagnetic substances. The negative susceptibility of diamagnetic substances remains invariable when the temperature varies within wide ranges. This property does not depend on the physical state of the material, since neither fusion (in the case of potassium nitrate) nor allotropic modification (in the case of sulfur) affects the diamagnetic properties of the respective substances. Diamagnetism must therefore be a specific property of atoms. It must result from the action of the magnetic field on the movement of the particles inside the atom, which explains the extreme weakness of the phenomenon and its independence of thermal disturbances or changes of phase, such as fusion and polymorphic transformations. Diamagnetism is thus a property of all matter; it exists also in ferromagnetic or paramagnetic substances but is little apparent there because of its weakness.

Ferromagnetism and paramagnetism, on the other hand, are properties of aggregates of atoms and are closely related. The ferromagnetism of a given substance decreases when the temperature rises and gives way to a weak paramagnetism at a temperature characteristic of the substance and known as its “Curie point.” Paramagnetism is inversely proportional to the absolute temperature. This is Curie’s law. A little litter Paul Langevin, who had been Curie’s student at the Ecole de Physique et Chimie, proposed a theory that satisfied these facts by postulating a thermal excitation of the atoms in the phenomena of magnetization. Curie’s experimental laws and a quantum mechanical version of Langevin’s theory still constitute the basis of modern theories of magnetism.

Curie devoted much of his energy in the laboratory to inventing and perfecting measuring devices, in the design of which his abilities both as a theorist and as a skilled experimentalist are apparent. Among the most notable was the employment of piezoelectric quartz in an instrument for making absolute measurements of very small quantities of electricity. He built a quadrant electrometer that improved upon the one devised by Kelvin by adding an ingenious magnetic damper. Also worthy of note was a standard condenser built of two perfectly parallel glass planes coated with silver and ingeniously mounted so as to eliminate any insulation difficulty. These devices brought Curie, young though he was, to the attention of Lord Kelvin and became the subject of correspondence between the two scientists. In addition he constructed the very sensitive and precise aperiodic balance, with direct reading, which was the first modern balance. All of these devices played an important role in the work on radioactivity that followed.

Radioactivity. It was the research in radioactivity that excited the greatest interest and immortalized the names of Pierre and Marie Curie. The success of their collaboration was assured by the complementary nature of their talents. Pierre Curie appeared to be the complete physicist—theoretician, philosopher, experimenter, and builder of equipment—who was interested in the entire world of nature and had countless ideas in his head; Marie Curie, on the other hand, was trained mainly as a chemist and had a persevering firmness that would not allow for a single moment of discouragement once she attacked a problem. It was Marie who, after having completed a study of the magnetic properties of tempered steel (1898), selected the radiation of uranium for her doctoral thesis.

In 1896 Henri Becquerel had discovered that uranium compounds constantly emitted radiation that was capable of exposing a photographic plate and making air conduct electricity; this radiation had not yet attracted the attention of the scientific world when Marie Curie began her research. On 12 April 1898 she presented, on her own, to the Academie des Sciences a preliminary note that marked the first step in the discovery of radium and opened to mankind the immense world of nuclear physics. Impressed with the importance of this subject, Pierre dropped experiments he had in hand on the growth of crystals (temporarily, he thought) to join in his wife’s research. On 18 July 1898 they published their first joint report, “Sur une substance nouvelle radioactive contenue dans la pechblende,” in which they announced the discovery of polonium.

The reason for their success may be found in the new method of chemical analysis based on the precise measurement of radiation emitted, a method still in use. It shows the trademark of Pierre Curie. Each product was placed on one of the plates of a condenser, and the, conductibility acquired by the air was measured with the aid of the electrometer and piezoelectric quartz he had built. This value is proportional to the quantity of active substance, such as uranium or thorium, present in the product. They examined a great number of compounds of almost all known elements and found that only uranium and thorium were definitely radioactive. Nevertheless, they decided to measure the radiation emitted by the ores from which uranium and thorium were extracted, such as pitchblende, chalcolite (torbernite), and uraninite.

In her first paper Marie Curie had pointed out that emission of radiation seemed to be an atomic property persisting in all physical and chemical states of matter and that in pitchblende it could be even greater than that of known chemical elements, including uranium and thorium. She suggested that a small amount of a very active substance might be found in the ores examined. Pierre and Marie Curie thought that by using the classical methods of analysis and chemical separations, they might be able to isolate new elements that were more active than uranium and thorium.

In that wretched shed at the École Municipale de Physique et Chemie Industrielles they set to work processing huge quantities of pitchblende ore, manhandling the vats and carboys of reagents, carrying out hundreds of fractionations by recrystallization. After each operation they followed up the trace of active fractions remaining in the pitchblende by accurate and rapid measurements of the radioactivity. The importance of this “new method of chemical research,” as Pierre Curie called it, is well-known today, with the appearance of radiation detectors such as the Geiger-Muller and scintillation counters.

The pitchblende used recorded, on their plate apparatus, an activity around two and a half times greater than that of uranium. After treating the residue in classical fashion with acid, and the resulting solution with hydrogen sulfide, they discovered that an active substance was present with bismuth in all reactions carried out in solution. Still, they did achieve partial separation by observing that bismuth sulfide was less volatile than the sulfide of the new element, which they called polonium, in honor of Marie Curie’s native land.

On 26 December 1898 the Curies and G. Bémont, head of studies at the École Municipale de Physique et Chimie Industrielles, announced in a report to the Académie des Sciences the discovery of a new element, which they called radium, which is present with barium in all chemical separations carried out in solution.

Since radium chloride is less soluble than barium chloride, the Curies were able, by means of fractional crystallizations, to isolate the new element in small quantities; in order to obtain just a few centigrams of pure radium chloride, they had to process two tons of the residue of uranium ore. Almost immediately (October 1899) André Debierne, a student of Pierre Curie’s, announced the discovery of a third new radioactive element, actinium. Polonium, and even more so radium, showed enormous radioactivity compared with that of uranium and thorium; for example, the Curies proved that the same effect was produced on a photographic plate by a thirty-second exposure to radium and polonium as could be obtained only after several hours of exposure to uranium and thorium. Further discoveries followed like so many coups de theatre.

First came the announcement in 1899 by Marie Curie of induced radioactivity, brought about by the action of polonium or radium on inactive substances. The induced radioactivity persisted over a considerable period of time, a phenomenon of great concern to Pierre Curie. He took up the question with Debierne, with whom he published two papers in 1901; their experiments could be explained by Rutherford’s theory of emanation (radon), a radioactive gas emitted by radium. With J. Danne, Curie measured the diffusion coefficient of radium emanation in the air and proved, as Rutherford had done, that it liquefies at - 150°C. In order to clarify the nature of the emanation he studied the law of diminution of the activity of a solid after having removed it from a chamber in which a radium salt was present. In two notes presented to the Academy on 17 November 1902 and 26 January 1903, Curie showed that this activity diminishes according to an exponential law characterized by a time constant that, for the emanation, is equal to 5,752 days, regardless of the conditions of the experiment. The importance of this discovery, which marks the point of departure for all modern measurements of archaeological and geological dating, did not escape his, for at a meeting of the Société Française de Physidque in 1902 he defined a standard for the absolute measurement of time on the basis of radioactivity. Almost immediately Rutherford and Soddy showed that the exponential diminution was caused by the transmutation of radioactive elements.

Curie published two papers on the physiological action of radium rays. In the first, written with Henri Becquerel, the two scientists described burns they had sustained from radioactive barium chloride; in the second, written with the physiologists Charles Bouchard and Victor Balthazard, he participated in experiments on the toxic effect of radium emanation on mice and guinea pigs. On 16 March 1903, in a short paper to the Academy, Curie and his student A. Laborde announced a basic discovery on the heat released spontaneously by radium salts. He showed that a gram atom of radium releases 22,500 calories per hour, a quantity of heat comparable with that of the combustion of a gram atom of hydrogen in oxygen. He concluded from this that the energy involved in the radioactive transformation of atoms is extraordinarily large. This was the first appearance, in human affairs, of atomic energy in the familiar form of heat.

Curie did not fail to reflect upon the misuse that mankind might make of his discoveries. In Stockholm, when he received the Nobel Prize, he said:

We might still consider that in criminal hands radium might become very dangerous; and here we must ask ourselves if mankind can benefit by knowing the secrets of nature, if man is mature enough to take advantage of them, or if this knowledge will not be harmful to the world.

Nevertheless, he concluded in optimistic fashion: “I am among those who believe that humanity will derive more good than evil from new discoveries.”

BIBLIOGRAPHY

I Original Works. Many of Curie’s writings were brought together in Oeuvres de Pierre Curie (Paris, 1908). His papers include “Développement, par pression, de l’électricité polaire dans les cristaux hémièdres à faces inclinées,” in Comptes rendus hebdomadaires des séances de l’Académie des sciences, 91 (1880), 294, written with Jacques Curie; “Sur I’électricité polaire dans les cristaux hémièdres à faces inclinées,” ibid., 383, written with Jacques Curie; “Lois du dégagement de l’électricité par pression dans la tourmaline,” ibid., 92 (1881), 186, written with Jacques Curie; “Sur les phénomènes électriques de la tourmaline et des cristaux hémièdres à faces inclinées,” ibid., 350, written with Jacques Curie: “Les cristaux héemièdres à faces inclinées, comme sources constantes d’électricitè,” ibid., 93 (1881), 204, written with Jacques Curie; “Contractions et dilatations produites par des tensions électriques dan les cristaux hémièdres à faces inclinées,” ibid., 1137, written with Jacques Curie: “Déformations électriques du quartz,” ibid., 95 (1882), 914, written with Jacques Curie; “Symétrie dans les phénomènes physiques, symétrie d’un champ électrique et d’un champ magnétique,” in Journal de physique, 3 (1894); “Propriétés magnétiques des corps à diverses températures,” ibid., 4 (1895); “Nouvelle substance fortement radioactive, conteny dans la pechblende,” in Comptes rendus hebdomadaires des séances de l’Académie des sciences, 127 (1898), written with Mme. Curie and G. Bémont; “Sur une substance nouvelle radioactive dans la pechblende,” ibid., written with Mme. Curie; “Action physiologique des rayons du radium,” ibid., 132 (1901), 1289, written with Henri Becquerel; “Conductibilité des diélectriques liquides sous l’influence des rayons du radium et des rayons de Röntgen,” ibid., 134 (1902); “Chaleur dégagée spontanément par les sels de radium,” ibid., 136 (1903), written with A. Laborde; and “Action physiologique de l’émanation du radium,” ibid., 138 (1904), 1384, written with Charles Bouchard and Victor Balthazard.

II. Secondary Literature. On Curie or his work see P. Langevin, “Notice sur les travaux de Monsieur P. Curie,” in Revue du mois (10 July 1906); and Pierre et Marie Curie (Paris, 1967), a booklet for visitors to an exhibit held at the Bibliothèque Nationale.

Jean Wyart