PIERRE CURIE*

A. F. Ioffe

Pierre Curie was born almost 100 years ago—in 1859—and lived only 47 years. In 1906, having only just obtained favorable conditions for scientific work, he died, falling under a horse-drawn carriage. Despite the short span of his scientific activity, the heavy teaching load, and the absence of a laboratory suited to scientific research, Curie left a large scientific legacy of enduring significance. His works have been published in the form of a single volume of 600 pages. Yet how many classical results there are here, permanently incorporated into physical science, and not only into physics. If one uses the present-day classification of the sciences, it may be said that Curie’s works enriched crystallography, physics, nuclear physics, medicine, and geology. Curie’s contribution to the development of the methodology of exact knowledge, to the theory and practice of measuring instruments, is also remarkable.

For all the breadth of his interests, Curie was above all a physicist-innovator. It was not by chance, but under the influence of an irresistible urge to seek the new and even the unexpected, that Curie made his discoveries. He continually questioned nature: is the matter really as we are accustomed to think it is; what is actually happening?—and he experienced no disappointment if an experiment merely confirmed what was already known. Pierre Curie strove to understand the general laws governing natural phenomena, not to increase the number of his scientific papers—much, indeed, never saw the light of day. His intuition, anticipating the views of his contemporaries, was astonishing. I shall give three examples:

1. Jacques Curie, Pierre’s brother, evidently with the latter’s closest participation, published in 1888 his doctoral dissertation on the electrical properties of insulating crystals. At that time, and for a long time afterward, it was customary to think that decaying in time

* A report read on April 19, 1956, at a ceremonial meeting of the Academy of Sciences of the USSR, the Soviet Committee for the Defense of Peace, Moscow State University named after M. V. Lomonosov, and the All-Union Society for Cultural Relations with Foreign Countries, devoted to the fiftieth anniversary of the death of Pierre Curie.

currents in these crystals are the result of a slowly established dielectric polarization. Pierre Curie, speaking at a meeting of the Physical Society, gave an entirely different interpretation, according to which the currents were produced by the motion of ions, while the decay of the current was caused by the accumulation of space charges. He correctly assessed the decisive role of impurities and the magnitude of the polarization that arises. Fifteen years later Curie’s views were confirmed by direct experiments carried out by me together with Röntgen.

1. The discovery of radioactivity was regarded by the entire scholarly world as an important step forward both in the knowledge of the world and as a new therapeutic agent. The atomic bomb was still far in the future. And Pierre Curie, in his Nobel lecture of 1903, had already said: “It is not difficult to foresee that in criminal hands radium may become extremely dangerous… but nevertheless new discoveries in the final reckoning bring mankind more benefit than harm.”

2. At the end of the nineteenth century it was considered good form among many scientists to speak of the atomic structure of matter only as a convenient working hypothesis, rejecting it as a reflection of the real world. As is well known, only the discoveries made at the turn of our century and in its first years (Brownian molecular motion, electrons, radioactivity, and the discovery of the diffraction of X-rays) made atoms a reality in the consciousness of physicists. Curie, however, was a convinced supporter of the atomic structure of matter and tried to prove it by an analysis of the symmetry of crystals, which he connected with their atomic structure.

If one attempts to characterize Curie as a physicist, then, alongside his innovativeness, deep analysis of the foundations of science, and powers of observation, one must also note his exceptionally strict exactingness with regard to the conclusions drawn from his experiments and, connected with this, his attention to experimental method.

All this, of course, is only the method of scientific work; its success is due to the enormous talent and intuition that Pierre Curie possessed, as few scientists whose names history has preserved have possessed them.

The best characterization of a scientist is provided by his works. Let us recall, then, what modern physics owes to Pierre Curie. Curie’s first work, that of a twenty-year-old youth, was devoted to infrared rays (or heat waves, as they were then called). Having constructed a diffraction grating from thin wires with a period of \( \frac{1}{4} \) mm, Curie used it to determine wavelengths; the distribution of intensity over the spectrum was given by a rock-salt prism, which made it possible to measure the weakest lines, inaccessible to a diffraction grating.

Questions of spectroscopy evidently interested the young Curie little. His thoughts were occupied with the problem of symmetry in the structure of matter, on the one hand, and with the symmetry properties of physical phenomena, on the other.

In that same year, 1880, together with his brother—the crystallographer—he reported their discovery of piezoelectricity, which he consid-

is revealed as a generalization and further development of the already known pyroelectric properties of tourmaline. Both phenomena are caused by a change in the dimensions of the crystal. Whether tourmaline is compressed under the influence of external pressure or cooling, charges of the same sign appear on its faces; heating and stretching produce opposite charges. Two weeks later the Curie brothers report to the Academy their investigations of the piezoelectric properties of a large series of crystals, among which, along with tourmaline, we find quartz and Rochelle salt. Another six months later the quantitative laws of piezoelectricity are formulated and the conclusions following from the symmetry properties of piezoelectric crystals for the question of an atomic or continuum theory of matter are discussed.

Pierre Curie returns more than once to this question: in the symmetry properties of the crystalline medium he sees a manifestation of its molecular structure and tries to give this connection the character of convincing proof. While he has not succeeded in doing so, he examines every phenomenon from the standpoint both of an atomic and of a continuous filling of space, and objectively sets against one another the conclusions from both conceptions.

Another year passes, and the Curie brothers report the discovery by them of the inverse of piezoelectricity—the phenomenon of electrical deformation of piezoelectric crystals. The necessity of such a phenomenon had been foreseen by Lippmann. As had been supposed, the deformation obeys Lenz’s rule: the secondary effect of electrization caused by it is opposite in sign to the cause that produced it. During the following year the quantitative laws of the new phenomenon are established and their connection with the quantitative characteristics of the piezoelectric properties of the same crystals is determined; the piezoelectric constants of a number of crystals are measured.

Later, in 1889, the Curie brothers publish the data they had obtained as early as 1881 and a detailed description of the experiments. Here a whole series of new methods is set forth, with the aid of which Pierre Curie succeeded in measuring with great accuracy the exceedingly small deformations of quartz. Instruments based on piezoelectricity are described. Especially interesting is the electric-charge meter: two glued quartz plates with opposite directions of their electrical axes. Their bending during electrization deflects a pointer connected with them, provided with a micrometric scale.

In connection with the study of electrization, Curie also develops and describes an improved quadrant electrometer, an absolute condenser, and a number of other instruments; he did not even consider it necessary to publish their description—it was found after his death among his papers.

One cannot fail to mention here the theory of the oscillation of balances created by Pierre Curie, and the new type of aperiodic precision balance developed by him.

Pierre Curie’s instrument for measuring the quantity of electricity by means of piezoelectric quartz retained great importance for many years after his death. It is a thin plate cut perpendicular to the electric axis and suspended along the principal axis of the crystal. After the piezoelectric constant of quartz had been measured with great accuracy, it became possible, with the same accuracy, to measure the charge appearing on the surfaces of the quartz by the magnitude of the weight placed on the pan suspended from the quartz plate.

This instrument was still being used in Madame Curie’s laboratory in the 1930s, when I was there. And, it must be said, for measuring the quantity of electricity it is a splendid instrument—simple and accurate.

With Curie’s piezoquartz I associate the memory of my first scientific work. I was asked to determine what is the cause of elastic aftereffect—deformation or the stress causing the deformation. It seemed possible to separate them with the aid of piezoquartz. The stress, determined by the suspended load, remains unchanged; if the deformation continues to increase, it will create an additional charge on the electrodes. When the experiment was carried out, systematic errors were found to be present, distorting the result; it was necessary to turn to observing elastic aftereffect in the bending of Curie’s plate. But here too the aftereffect proved to be the result of the appearance of piezoelectric charges, which in this case filled the entire volume of the crystal. In order to eliminate the possibility of charges appearing during bending, the plate had to be cut in another crystallographic direction.

I wrote about this to Pierre Curie and asked him, if he considered my considerations correct, to transmit to the workshop an order for such a plate.

Soon I received a reply confirming my assumptions, and thereafter the plate as well, which indeed showed the absence of true elastic aftereffect in quartz.

Another recollection connected with Pierre Curie belongs to the very beginning of my work with Röntgen. This was in March 1903. Röntgen called me in and said that a paper by Curie had appeared in which he reported that he had discovered large quantities of heat continuously emitted by radium. “I would not have believed it,” said Röntgen, “for there are so many sensational reports that are not confirmed; but this is Pierre Curie, one of the best experimenters of our time and a cautious man. The result is too important and unusual—it must be checked. Here is an ampoule containing 63 milligrams of radium chloride—see how much heat it gives off.”

A short time later I was asked to present at the seminar Rutherford’s paper, in which, proceeding from the heat emission discovered by Curie, he put forward the idea of the disintegration of atoms and their transformations—and thus the idea of atomic energy was born.

The impression made by Curie’s note was exceptionally great. In the obituary printed in Nature after Curie’s death, there is recalled the prediction that future humanity would take as the beginning of its era Pierre Curie’s discovery, in March 1903, of the heat emitted by radium.

However, let us return to Curie’s scientific life in the 1880s—the years of the discovery of piezoelectricity. He had not yet had time to publish his investigations in full when Voigt’s generalizing work devoted to piezoelectricity appeared, and Curie’s monograph was never written. Later, in his lecture course, Curie wished to fill this gap and began to set forth the question of piezoelectricity, but death did not allow him to complete this work, which was close to him.

For Pierre Curie, the laws of piezoelectricity were closely intertwined with the problem of symmetry, which was fundamental for him. He generalized and supplemented the doctrine of the elements of symmetry and formulated the general proposition that both the presence of symmetry and its absence in any phenomenon can be caused only by the same properties of symmetry in the causes that produce this phenomenon. He devoted a profound analysis to the question of the interrelation between the elements of symmetry of the medium in which a phenomenon occurs and the symmetry properties of the phenomenon itself. In particular, Curie examined in detail the question of the symmetry of electric and magnetic fields. In crystals he saw not only their faceting, but above all an anisotropic medium, owing to the corresponding arrangement of the molecules composing it.

Curie also devoted much time and labor to the problem of the external form of the crystal. He carefully studied the growth and dissolution of individual faces and discovered a law connecting the rate of growth of a face with the magnitude of its surface energy, with the density of the arrangement of molecules in it.

Becoming acquainted with this cycle of Curie’s works, one sees in him a crystallographer who possessed the full wealth of accumulated experience and was creatively developing it. Curie’s crystallographic laws appear to be as fundamental as his laws in the field of piezoelectricity.

But still more striking is his work devoted to magnetism. Each subsequent step of Curie’s was larger than the preceding one and raised him as a scientist to a great height. So it was further, too, when he took part in investigations of radioactivity.

Curie’s work on magnetism became his doctoral dissertation; it was printed in 1895. Having investigated 20 different materials—diamagnetic, paramagnetic, and ferromagnetic—over a wide temperature range up to 1400° C, Curie gave an exhaustive description of their properties. He established Curie’s law for paramagnetic substances, according to which paramagnetic susceptibility is inversely proportional to the absolute temperature. He showed the independence, with respect to temperature and to the intensity of the magnetic field, of diamagnetic

susceptibility. Later his student Langevin created a theory explaining Curie’s results. The study of ferromagnetic materials led Pierre Curie to establish the Curie temperature, at which ferromagnetic properties pass into paramagnetic ones. Since then the Curie point has become a concept extending far beyond the limits of magnetism.

Curie correctly concluded from his experiments that the physical nature of diamagnetism and paramagnetism is entirely different, whereas the nature of paramagnetism is close to ferromagnetism. The reason for this was clarified later by Langevin.

It is difficult in a short article to convey even the most important results of Curie’s magnetic investigations, the distinctive character of each of the materials he studied, and the influence on them of pressure, temperature, and changes in state of aggregation. A simple enumeration of the substances for each of which a careful study was made of the influence of temperature, field, pressure, state of aggregation, and treatment shows the scale and thoroughness of the work carried out by Curie.

Among diamagnetic materials there were studied: water, a number of salts, quartz, sulfur, selenium and tellurium, bromine, iodine, mercury, phosphorus, antimony, and bismuth. Curie established the difference of solid bismuth from all other bodies with respect to the influence of temperature. Among paramagnetic substances Curie studied oxygen, air, magnetic salts and their solutions, palladium, glass and phosphorus, and ferromagnetics above the Curie point. Ferromagnetics were studied comprehensively over a broad range of magnetic fields and temperatures, and the quantitative regularities taking place here were clarified. The decrease of the magnetic induction of a ferromagnetic as it approaches the Curie point he regarded as a prelude to the decrease of paramagnetism above that point. The transition from the paramagnetic state to the ferromagnetic one reminded him of the process of condensation. It was not for nothing that he simultaneously analyzed the Van der Waals equation and the law of corresponding states, in which he found many features in common with the regularities he had discovered in magnetic phenomena. Curie established a similarity between the curves of changes in vapor density and magnetic induction with temperature. In 100 pages of his work on magnetism Curie managed to set forth such a wealth of facts and conclusions from them that another investigator would have required a thick volume.

Of course, here too it was not possible to dispense with the development of new methods and new instruments, ensuring the great accuracy, which Curie himself estimated at 1–2%, which, given the extreme smallness of the effects being measured, was an outstanding success.

Curie extended the ideas of symmetry also to the phenomena of magnetism. Not confining himself to his observations, he asked himself whether the existence of free magnetism was possible and what its symmetry properties would be. To this question he had to give a negative answer.

Guided, apparently, by similar considerations of symmetry, Curie persistently sought an analogue of ferromagnetism among diamagnetics; he did not find it. But did not superconductivity, discovered five years after Curie’s death, lead to superdiamagnetism, of which he dreamed, to the complete expulsion of the magnetic field? One may regret that at that time the phenomenon of antiferromagnetism was unknown; it would have completed the picture of magnetic properties that Curie had outlined. Antiferromagnetism is, perhaps, the only substantially new fact introduced by science into the study of magnetism in the 60 years following the publication of Curie’s work.

The last period of Pierre Curie’s activity after 1895, when he married Maria Skłodowska and, together with her, began the investigation of radioactivity, is better known. In their collaboration it is difficult to single out the share of each separately, just as in the earlier work in collaboration with his brother Jacques. There is no doubt that to the union of the talents of Pierre and Marie Curie humanity owes the discovery of radium and polonium, the analysis of radioactive radiations—$\alpha$-, $\beta$-, and $\gamma$-rays, which they always called Becquerel rays. Pierre Curie established the deflection in a magnetic field and the transport of charge by $\beta$-rays, discovered and studied with exceptional thoroughness the phenomenon of induced activity, and the emission of emanation. To him belong the principal results of the investigation of secondary rays produced by X-rays. Curie discovered the influence of radioactive rays on the electrical conductivity of liquids. He studied the chemical and physiological actions of radium. The independence of the rate of decay of radioactive substances from any external conditions suggests to him the idea of using this phenomenon to define a unit of time.

We have already mentioned the discovery of the heat emitted by radium. In the very first brief note Curie correctly estimated the amount of heat emitted by one gram-atom of radium and the conclusions following from this fact. However, alongside the hypothesis he expressed in that very first note, and even a year earlier, of the liberation of intra-atomic energy during the transformation of elements, he considered it necessary first to make sure whether radium does not absorb energy of some as yet unknown kind from the surrounding space—for example, radiation imperceptible to us. As we know, the energy liberated by radium is the result of its decay into emanation and a helium nucleus—an $\alpha$-particle; but the then unknown source of energy—cosmic rays, as we now know, does nevertheless exist and likewise causes intranuclear transformations, although of a different order of magnitude in comparison with the spontaneous decay of radium.

In order properly to assess Pierre Curie as one of the most outstanding experimental physicists, it is necessary to take into account not only the remarkable results in every field of knowledge to which he devoted his attention, but also the conditions under which his scientific activity proceeded. Without a specially equipped laboratory,

in a corner under the stairs, among the student practical laboratory, in a shed, with the help of homemade instruments Curie made one discovery after another, carried out the most delicate precision investigations, and created new instruments.

Even the Nobel Prize, which he received together with his wife Marie Curie and Becquerel, did not bring a decisive change in his working conditions. After overcoming numerous obstacles, Curie obtained a small laboratory and the necessary funds for equipping it. His absurd death cut short his activity at the period of its greatest flourishing.

Pierre Curie’s work was continued by his friends and pupils. Under Marie Curie’s direction there grew up and gained world renown the Radium Institute, where more than 500 investigations on radioactivity were carried out. His pupil Paul Langevin created, on the basis of results obtained by Pierre Curie and in full agreement with them, a theory of magnetism; and the discovery of piezoelectricity was used for producing and receiving ultrasound in the sea. Curie’s crystallographic ideas also found continuators, among them the Russian scholar Yuri Viktorovich Wulff.

Among the biographical facts about Pierre Curie it should be pointed out that he received his education at home and never attended school. His father—a physician with an inclination toward scientific activity—evidently early understood the distinctive character of his son’s scientific gifts and chose for him a teacher whom Curie the scientist more than once recalled with gratitude.

Immediately after passing the examinations for secondary school, Pierre Curie became an assistant in Desains’s laboratory, where he also carried out his first scientific work. Even before that, for several years he had helped his elder brother Jacques in laboratory work; he was bound to him by a touching friendship.

Together with the mineralogist Jacques, Pierre conducted his investigations in the field of crystallography, piezoelectricity, and the electrical properties of insulating crystals. In long conversations with Jacques, Pierre’s scientific outlook took shape, in which symmetry occupied so large a place; here his interest in crystals grew, and a deep knowledge of their properties was accumulated.

The first half of Pierre Curie’s scientific activity, brief in time but rich in content, was closely connected with his brother and friend Jacques Curie. The second half, beginning in 1895, proceeded in collaboration with his wife and friend Marie Curie. To the latter belongs the best biography of Pierre Curie, published twice in the USSR. She also wrote the preface to the collected works of Pierre Curie, published in 1908 by the French Physical Society. Almost all of Pierre Curie’s investigations were published jointly with one or another of his pupils or friends.

Recalling Pierre Curie’s brilliant scientific activity, one cannot forget his personality, his modesty, his high principledness

and of progressive convictions. He resolutely rejected all forms of awards and honors that were offered to him. He spoke with revulsion of the custom of obtaining scientific posts by candidates’ visits to influential persons. He and his wife spent the Nobel Prize on organizing treatment with radium. Curie knew no compromises; he was truthful to the end not only in science, but also in life.

In political questions, and in his attitude toward religion, Pierre Curie was one of the most progressive people of his time. His democratic convictions, his complete absence of national chauvinism, his sympathy for oppressed peoples and hatred of oppressors, like his scientific ideas, found further development in the person of his wife—one of the best friends of the Soviet Union—and of his pupil Langevin, who joined the Communist Party.

His daughter Irène, who recently died at her post of selfless service to science, was a major scientist and a prominent public figure in France, an active fighter for peace among peoples; and Irène’s husband, Frédéric Joliot-Curie, who together with her laid the broad foundations for research in the field of artificial radioactivity, is widely known as one of the leaders of the progressive movement of advanced humanity for peace.

Fifty years have passed since the death of Pierre Curie. Centuries more will pass, and the memory of Curie the scientist, of Curie the teacher, will not be erased from the history of science. The founder of the doctrine of magnetism, of radioactivity, of nuclear energy; the author of leading ideas on symmetry in nature; the discoverer of the interrelation between deformation and the electrization of crystals—Curie will remain a vivid figure in the physics of the late nineteenth and early twentieth centuries, even if the prediction is not fulfilled that humanity will declare Curie’s work of 1903 the beginning of a new era. In honor of the remarkable scientists—the Curies—one of the recently discovered elements of the periodic system has lately been given the name curium.