PYOTR LEONIDOVICH KAPITSA

(On the Occasion of His Sixtieth Birthday)

E. V. Shpolsky

On July 9 of this year, 60 years had passed since the birth of the outstanding Soviet physicist, Academician Pyotr Leonidovich Kapitsa. P. L. Kapitsa was born in 1894 in Kronstadt, into the family of a military engineer. His father was one of the builders of the Kronstadt fortress, and his mother, Olga Ieronimovna Kapitsa, was a well-known educator and collector of folklore. P. L. received his secondary education at the Kronstadt Realschule, and his higher education in the electromechanical faculty of the Petrograd Polytechnic Institute. Thanks to his outstanding abilities, while still a student he attracted the attention of A. F. Ioffe—at that time a professor at the Polytechnic Institute—who drew him into scientific research work. Thus he belonged to that small group of A. F. Ioffe’s first pupils who formed the nucleus of the Leningrad Physico-Technical Institute, created in 1918 (at that time founded as the physico-technical department of the State Roentgenological Institute).

In 1921 P. L. was sent on a scientific mission abroad to Rutherford’s Cavendish Laboratory. There, while still a very young physicist, he carried out a number of remarkable works. These works, subsequently continued and expanded in the Soviet Union, brought him worldwide fame as one of the most outstanding physicists of our time. It should be noted, moreover, that after working for several years alongside a scientist of such stature as Rutherford, who exerted an enormous influence on the development of science and above all, of course, on the activity of his colleagues and pupils, the young Soviet scientist Pyotr Leonidovich Kapitsa, practically from the very beginning, followed his own path, one entirely original and independent of the ideas and methods of Rutherford’s school.

In this brief article there is no possibility of giving a complete survey of the works of P. L. and of all his many-sided activity as a scientist, engineer, and organizer. We shall therefore confine ourselves only to a brief characterization of this fruitful activity.

Already in P. L.’s first published works the characteristic features of his talent appeared: boldness in setting up an experiment that goes straight to the goal, remarkable acuteness in overcoming the enormous difficulties that usually confront the investigator in such an experimental design, and, along with this, an exact calculation of all the details ensuring the achievement of the set goal. Being, both by education and by inclination, not only a physicist but also an engineer, P. L. is able to give his brilliant physical ideas an equally brilliant technical embodiment. At the same time he always places his work on a large technical scale, far exceeding the laboratory scale customary for physicists.

In what follows, with several examples, we shall try to give concrete illustrations of this general characterization.

Having become interested at the beginning of his scientific activity in the behavior of matter in very strong magnetic fields, P. L. decided first of all to produce experimentally magnetic fields of the order of hundreds of thousands of oersteds. Strange as it may seem at first glance, the obstacle to obtaining such fields is the core of the electromagnet, whose saturation sets a limit on obtaining strong fields, at any rate in a volume sufficient for experimentation. Bearing this in mind, P. L. solves the problem radically: he discards the core—for in principle the possibilities of generating a magnetic field by means of a solenoid without a core are not limited; it is only necessary to pass through the solenoid a current of sufficient strength. However, calculation shows that to obtain fields of the order of 100,000 oersteds, gigantic currents would be required, which no solenoid could withstand. From this difficulty P. L. finds a simple way out, reasoning as follows: for atomic processes, which are precisely what interest him, a time interval of \(10^{-3}\) sec is very large; a magnetic field existing during such an interval of time, from the point of view of “atomic clocks,” may be regarded as constant. P. L. therefore decides to set up the experiment in such a way as to obtain superstrong magnetic fields during very short intervals of time. For this purpose he builds a primitive storage-battery bank with low internal resistance, which discharged completely within a few thousandths of a second, giving a current up to \(10^{4}\) a. In this way P. L. at first obtains fields of about \(10^{5}\) oersteds in a volume of \(2\ \text{cm}^{3}\), which was already an enormous step forward in comparison with what could be achieved with electromagnets. Subsequently he builds

PYOTR LEONIDOVICH KAPITSA

a special generator, operating on short circuit and producing fields of up to 500,000 oersteds.

In the course of these works P. L., with astonishing brilliance and acumen, overcame enormous difficulties—from the calculation and construction of a powerful generator, in which those properties had to be hypertrophied that are in every way avoided in the ordinary design of electric machines, to the realization of a solenoid whose turns had to withstand tensile mechanical stresses reaching 1 t per 1 cm². In addition, he had to develop an entirely new method for studying the processes that interested him in strong magnetic fields. The conditions that this method had to satisfy were extraordinarily unusual. Since the fields were established for a thousandth of a second, all the results of the action of these fields on matter had to be recorded automatically. This recording was carried out by low-inertia, very delicate instruments, which, however, had to give correct readings under conditions in which a powerful elastic wave, propagating through the ground at the moment the current was switched on in the solenoid, shook the entire institute building despite the careful damping of the machine.

No less vivid examples of P. L.’s talent as an experimental physicist and engineer are furnished by his work in the field of low temperatures. In connection with his studies of the properties of matter in strong magnetic fields, P. L. first became interested in obtaining extremely low temperatures—hydrogen and helium temperatures. Here he immediately discovered that the considerable technical difficulties involved were being overcome by no means in the most perfect way in the installations then in existence. Thus, for example, it turned out that, in order to obtain liquid hydrogen, use was being made of the method employed by Dewar as early as 1898. A fundamental difficulty of this method is the necessity of using the purest hydrogen. Even the best hydrogen technically obtainable—99.5% pure—is unsuitable for liquefaction in the machines in use: impurities (for example, air) at the temperature of liquid hydrogen solidify and block the narrow tubes of the regenerator.

P. L. overcame this difficulty with astonishing ease. In his hydrogen liquefier there are two circuits: in one, hydrogen is liquefied in the usual manner, using “chemically pure” hydrogen. But this circuit is once and for all completely closed, and the hydrogen does not escape from it anywhere. In the other circuit ordinary technical hydrogen circulates; this hydrogen is precooled by liquid nitrogen and enters an “exchanger,” where it is further cooled to the point of liquefaction by the hydrogen of the first circuit. Since the solidifying impurities are heavier than liquid hydrogen, they sink to the bottom of the exchanger and do not interfere with the circulation of hydrogen in the second circuit.

Still more boldly and ingeniously were the difficulties overcome in the helium liquefier built by P. L. following the hydrogen one. To appreciate the idea of this liquefier, one should recall that in principle there exist two methods of cooling and liquefying gases. In the first, the gas is cooled by doing external work during adiabatic expansion. In the second, the cooling is due to the performance of internal work, i.e., to the Joule–Thomson effect. In the Leiden cryogenic laboratory of Kamerlingh Onnes, where helium was first brought into the liquid state and which, until the work of P. L., practically monopolized the field of low temperatures, the liquefaction of helium was achieved by the second method. The efficiency of this method, however, is negligibly small: calculation shows that the yield it gives is approximately 1% of the yield that should be given by the method of adiabatic expansion. If, nevertheless, the latter method was not used for the continuous liquefaction of helium before the work of P. L., the reason for this was connected with a serious technical difficulty standing in the way of its use. This difficulty consists in designing an expansion machine (i.e., that part of the apparatus where the gas is cooled by doing external work) which would operate at extreme degrees of cold. In this machine there is a cylinder with a piston, and the serious difficulty lies in finding a suitable lubricant which, on the one hand, would allow the piston to fit tightly to the walls of the cylinder and thereby prevent gas leakage, and, on the other hand, would retain its lubricating properties at very low temperatures. The French engineer G. Claude, who built an apparatus based on the principle of adiabatic expansion for obtaining liquid air, bypassed this difficulty by using liquid air itself for lubrication. This method, however, is unsuitable for a helium machine, since at the temperature of liquid helium any substance will be in the solid state. An attempt to use liquid helium itself for lubrication (by analogy with Claude’s method) is doomed in advance to failure, since, owing to the extremely small value of its surface tension, liquid helium in general has no lubricating properties. P. L. overcame this difficulty just as boldly and radically as the difficulties connected with the properties of the core in obtaining strong magnetic fields: if it is impossible to find a lubricant suitable for operation at extremely low temperatures, then one must simply eliminate the need for lubrication. This can be achieved by leaving a clearance between the piston and the cylinder, so that the piston moves freely, without touching the cylinder walls. But in that case the gas must escape from under the piston! Yet P. L. overcomes the resulting difficulty by making the gas expand very rapidly. Calculation shows in this case that if the stroke of the piston

...is completed in a small fraction of a second, the gas leakage is so small that it has no noticeable effect on the output of the machine. A machine in which lubrication is not required should, obviously, operate successfully at any temperature. It goes without saying that, on the way to the practical realization of such a machine, many particular difficulties arose; these, however, P. L. successfully overcame, and the helium liquefier he built is a record-holder in simplicity and output. It does not require preliminary cooling with liquid hydrogen (liquid nitrogen alone is sufficient) and is started up within one hour, whereas in the Leiden laboratory the preparation for starting a helium liquefier required several days. The influence of this work on the development of techniques for obtaining low temperatures was enormous. It is enough to recall that in the early thirties, when P. L. began work in this field, there were only two places on the entire globe where liquid helium was obtained: the famous Leiden laboratory of Kamerlingh Onnes and MacLennan’s laboratory in Toronto (Canada). But already in 1938, in Moscow, liquid helium was being used in a student laboratory course.

The successful development of methods for obtaining extremely low temperatures, which are of enormous importance for purely physical investigations, prompted P. L. to turn to the improvement of industrial methods for using low temperatures and, in particular, to the development of effective methods for obtaining large quantities of liquid oxygen, which at present has such important applications in metallurgy and in other fields of technology. The most important result of this work was the creation of a highly efficient turbo-expander.

Already in his work on the helium liquefier (1934), P. L. noted that the most attractive method of adiabatic extraction of heat from a compressed gas is the use of a turbine to perform external work. However, a detailed discussion of the question and the corresponding calculations led him to the conclusion that the turbine method in the case of helium was “impractical, unless there were a need to obtain liquid helium on a large scale.” But precisely this latter condition holds in the case of liquid air. Therefore, subsequently setting himself the task of developing highly efficient methods for obtaining liquid air, chiefly for the purpose of its subsequent separation into oxygen and nitrogen, P. L. decided to use the turbine method for cooling. Although the possibility of using a turbine for this purpose had already been indicated in 1898 by Rayleigh, nevertheless this idea had not found practical application before P. L.’s work. The reason was that the turbines used for cooling had an extremely low coefficient of efficiency. In detail...

Having analyzed the operating conditions of a turbine driven by compressed air at a temperature of about \(100^\circ\) K, P. L. showed that its efficiency could be significantly increased if centrifugal forces were used. Having built a turbine in which, along the rotor, the air moves in the radial direction from the periphery toward the center, P. L. achieved a record efficiency of \(83\%\). This experimental turbine was, in certain respects, a marvel of engineering. It had a diameter of 8 cm, weighed only 250 g, and operated at 40,000 revolutions per minute. For comparison, P. L. notes in his work that the compressor supplying air to this turbine (from 500 to 1000 \(m^3\) per hour) weighs 3 tons and consumes 50–80 kW. It goes without saying that achieving stable rotation of the turbine at the indicated ultra-high speeds was a serious problem, which was solved both theoretically and experimentally in a specially undertaken investigation. On the basis of this turbine P. L. developed and built an experimental installation for obtaining liquid air. In his machine the air is first compressed only up to 4–5 atmospheres, whereas in the well-known Linde installations, where cooling is achieved by means of the Joule–Thomson effect, the initial compression is 200 atmospheres. Thus a new principle was introduced into low-temperature technology—the principle of low-pressure machines. The efficiency of these machines, confirmed by the practice of their industrial application, is extremely high. At present, throughout the world, the technology for obtaining large quantities of oxygen has proceeded precisely along the path of using low-pressure machines indicated by P. L. Kapitsa.

While devoting such serious attention to work in the field of technical physics, P. L. always remained a physicist. This means that the powerful experimental methods he developed were, for him, mainly either a means of studying physical phenomena or—as in the case of the development of methods for obtaining oxygen—his contribution, as a physicist, to the great cause of the industrial development of our country. In any case, the development of new experimental methods in P. L.’s work was always immediately accompanied by the application of these methods to the solution of physical problems and always brought important results. Thus, having implemented a method for obtaining superstrong magnetic fields, he carried out a number of important studies on the electrical properties of metals placed in such fields, and in doing so discovered a linear law of dependence of resistance on field strength instead of the previously established quadratic law for ordinary fields; he studied the Zeeman effect in superstrong magnetic fields and carried out a number of other works. But perhaps of the broadest interest are his works devoted to the study of the properties of liquid helium. As is known, at temperatures below \(2.19^\circ\) K helium passes

PETER LEONIDOVICH KAPITSA

into a state called helium II and characterized by many remarkable properties. One of the most striking properties of helium II is its unusually high thermal conductivity, discovered by Keesom. From the measurements of Keesom and his daughter it followed that the thermal conductivity of helium II is roughly a million times greater than the thermal conductivity of copper. By analogy with superconductivity, Keesom therefore attributed to helium II a property that he called superthermal conductivity. But the presence of superthermal conductivity would entail the necessity of high viscosity. Meanwhile experiments carried out in Canada showed that, quite the contrary, liquid helium II has a very low viscosity. P. L. resolved this contradiction in a very radical way, asserting that the thermal conductivity observed by Keesom was apparent, and that the rapid transfer of heat in this case is due not to thermal conductivity but to convection, so that helium II is a liquid that is not superthermally conducting but possesses negligible viscosity. This assertion was then justified by a long series of excellent experiments, remarkable for their clarity and beauty. Thus an entirely new phenomenon was discovered, which was called superfluidity. This phenomenon was subsequently subjected to detailed experimental investigation by P. L. himself and by a number of his collaborators at the Institute of Physical Problems of the Academy of Sciences of the USSR, while L. D. Landau, at the same institute, constructed its complete theory. In this connection, as a consequence of the theory, the existence of yet another new phenomenon, called second sound, was predicted, and the experiments of V. P. Peshkov confirmed the reality of this phenomenon. As a result, it must be noted that the work of P. L. and his collaborators in the field of low temperatures constitutes one of the most brilliant pages in the history of Soviet physics.

A characteristic feature of P. L.’s talent as a physicist is his remarkable versatility. He is a brilliant experimenter, and at the same time he carries out his most difficult experiments himself, with his own hands. However, P. L. is not only an experimenter but also a theoretician. Moreover, the matter is not limited to the fact that he does not need the help of a mathematician for carrying out the necessary calculations preceding the setting up of an experiment; he also carries out investigations that must be assigned to the field of theoretical physics. In this respect, a series of his recent works in a field unexpected for the circle of P. L.’s previous interests is characteristic—namely, in the field of hydrodynamics. We have in mind investigations devoted to the wave flow of thin layers of a viscous liquid. In these works the problem of the stable regime of flow of a thin layer of liquid was first solved theoretically, and then the wave flow was subjected to detailed quantitative experimental study.

Among P. L.’s “unexpected” works is a purely mathematical communication in which expressions, previously unnoticed by anyone, were derived for the sums of powers of the roots of Bessel functions. To the same category belongs a work, carried out quite in passing, on the rocking of a pendulum on a vibrating suspension. Mathematically this problem had been solved by an unusually cumbersome method with the aid of Mathieu’s equation, solved by infinite determinants, while experimental work was not generally known. Having become interested in this problem, P. L. began by reformulating it mathematically; in doing so he arrived at an equation more general than Mathieu’s equation and gave an elementarily simple solution of the resulting equation—though an approximate one, yet yielding results with an accuracy quite sufficient for practical purposes. Subsequently a series of simple experiments was carried out, vividly demonstrating the paradoxical properties of the dynamical stability of this system—properties, as P. L. rightly observed, no less interesting than the usually demonstrated properties of the dynamical stability of gyroscopes.

Devoting his energies and attention chiefly to research work in the laboratory and at the institute, P. L., however, by no means belongs to the number of so-called “armchair scientists.” He responds keenly to practical questions, to the needs of socialist construction. The best proof of this is his work in the field of low temperatures. Here his activity was far from being limited to the development of highly productive methods for obtaining liquid air and, subsequently, oxygen. Having created economically advantageous methods for producing oxygen, he expended much effort on introducing the results obtained into industry, i.e., on finding varied applications of oxygen in industry.

P. L.’s services to Soviet science and technology have been highly appreciated by the government. He has been awarded three Orders of Lenin and the Order of the Red Banner, and since 1945 he has been a Hero of Socialist Labor. The Stalin Prize has been awarded to him twice. Since 1939 P. L. has been a full member of the Academy of Sciences of the USSR. About 20 academies and learned societies of the world, including the Royal Society of Great Britain, the Academy of Sciences of Denmark, the National Academy of the USA, the Indian Academy, and a number of others, have elected him their full or honorary member. In 1942 he was awarded the Faraday Medal, and in 1945—the Franklin Medal.

Having reached the age of 60, Pyotr Leonidovich is in the prime of his powers. He continues his scientific work with enthusiasm and is full of creative plans. It remains for us to wish him vigor and health for many more years, so that he may continue his many-sided activity, directed above all to the benefit of our Motherland.