PHYSICS OF ELECTRICAL DISCHARGES IN GASES OVER THIRTY YEARS IN THE USSR

N. A. Kaptsov

I. INTRODUCTION

The study of electrical phenomena in gases at the present time constitutes an extensive field of physics in which, alongside questions directly connected with “electrical discharges,” there are closely intertwined problems of the physics of atomic and electronic phenomena. The study and explanation of the macroscopic phenomena accompanying the passage of electric current through gases is impossible without investigation of the so-called elementary processes that occur in the interaction of molecules, atoms, ions, electrons, and photons both in the volume of the gas and at the boundary between the gas, on the one hand, and metals, semiconductors, and dielectrics, on the other (photoelectric effect, secondary emission, thermoelectronic and autoelectronic emission, etc.). The work done in the USSR in this latter direction has already found its reflection in the article by S. Yu. Lukyanov, “Soviet Electronics over 30 Years”*). This considerably facilitates the task before us. However, in order to assess fully all that has been done over thirty years by Soviet physicists in the field of electrical discharges in gases, it is necessary to take both articles as a whole, since the physics of electrical discharges in gases is inseparable from “electronics.”

Russian physicists concerned themselves with questions of electrical discharges in gases even in prerevolutionary times. Widely known are the observations by the founder of Russian physics, Mikhail Vasilyevich Lomonosov, on thunderstorm discharges. He was the first to establish the possibility of extracting electric charges from the atmosphere in clear weather, in the absence of a thunderstorm. The skillful Russian experimental physicist Vasily Vladimirovich Petrov in 1802 discovered the phenomenon of the electric arc several years before this form of electrical discharge was discovered by foreign scientists. Petrov

) Uspekhi fizicheskikh nauk*, vol. XXXIII, no. 4, p. 549.

not only discovered the electric arc, but also studied the course of a number of physical and chemical phenomena in the region of the arc. In subsequent times Petrov’s arc repeatedly attracted the attention of Russian physicists. Much work on its investigation was done by P. N. Yablochkov, who established the influence of vapors of refractory substances on the electric arc and on its radiation. D. A. Lachinov, together with V. N. Chikolev, studied the dependence of the current strength and the brightness of the radiation of Petrov’s arc on a number of conditions. He also attempted to detect directly in experiment the “electromotive force of polarization,” which both at that time and much later was ascribed to the electric arc.

Finally, V. F. Mitkevich, in the first years of the twentieth century, unequivocally established that the cause sustaining Petrov’s arc is the emission of a large number of electrons from the incandescent cathode, and that this emission of electrons is nothing other than the “Edison effect,” as it was then called, or thermoelectronic emission in our present terminology. After Aleksandr Stepanovich Popov applied Hertz’s spark generator as a transmitter of radio signals, the problem arose of studying the spark that jumps across the discharge gap of the generator, and of the influence of the parameters of this spark on the generation of electromagnetic waves. This problem was taken up by A. S. Popov’s pupil D. A. Rozhanskii. He established that in this case the electric spark is indeed, according to the view expressed by J. J. Thomson, an alternating electric arc. Rozhanskii investigated the variation of the resistance of this arc with time and found this variation to be linear.

Another follower of A. S. Popov, Vladimir Konstantinovich Lebedinskii, comprehensively studied the conditions for the occurrence of the electric spark and established the fact, still not fully explained by contemporary theories, that irradiation of the cathode of the spark gap by ultraviolet radiation does not always lower the sparking voltage (“ignites the spark,” in Lebedinskii’s expression), but under some conditions raises it (“extinguishes the spark,” according to Lebedinskii).

At the present time it has been established that “ignition” occurs at the immediate onset of the spark, whereas “extinction” occurs in the transition into the spark of a corona discharge between a positively charged point and a second electrode in the form of a sphere or disk.

Very interesting are I. I. Borgman’s investigations, dating from 1886–1887, of the silent discharge. I. I. Borgman placed at various points in the space between two electrodes, located in atmospheric air, very small and easily movable magnetic needles. When a high voltage was applied to the electrodes and a silent discharge passed through this gap, the magnetic needles, situated in different places, differently

PHYSICS OF ELECTRIC DISCHARGES IN GASES

deviated from the direction of the magnetic meridian. Thus, Borgman succeeded in showing that, in a silent discharge through air, electric charges move from one electrode to the other along strictly definite paths. In the same work I. I. Borgman pointed out the possibility of using a flame of small dimensions as a reliable probe for determining the electric potential of the air at the point where the tip of the flame is located.

Among pre-revolutionary works in Russia on gas discharge, special note must be made of the investigations by the Moscow University professor Aleksandr Grigor’evich Stoletov on the “actino-electric effect” (1888–1890). This effect was a combination of two phenomena: the photo-effect at the cathode and a non-self-sustained discharge in a gas. Stoletov investigated the actino-electric effect in air not only at atmospheric pressure, but also at reduced pressure, down to 0.002 mm of mercury. He established that, other conditions being equal, the strength of the discharge current, as the pressure is lowered, first increases, passes through a maximum, and then decreases again. Varying not only the gas pressure but also the voltage between the cathode and the anode, as well as the distance between them, Stoletov found that the maximum value of the current always corresponded to one and the same constant ratio of the electric-field strength in the discharge gap \(E\) to the gas pressure \(p\). The English physicist Townsend, who around 1900 constructed the first quantitative theory of the non-self-sustained discharge, used the results of Stoletov’s investigations as experimental material. Townsend called the effect of pressure on the current strength the “Stoletov effect,” and the value of the ratio

\[ \frac{E}{p}, \]

corresponding to the maximum value of the current, the “Stoletov constant.”

Having measured the coefficient of volume ionization of air \(\alpha\) at various pressures and various field strengths, Townsend calculated, on the basis of his theory, the “Stoletov constant.” The value he obtained for this constant for air proved to be in complete agreement with the value found experimentally by Stoletov. The phenomena of electric discharges in gases remained at the center of A. G. Stoletov’s attention to the end of his days. One day before his death, at his last meeting with him, P. N. Lebedev, while A. G. Stoletov was gravely ill, “started a conversation on his favorite subject, gas discharges.” Taking leave, he added, barely audibly: “I advise you to take up these questions—they are very interesting and very important”*). Soviet physicists have now fulfilled this behest of A. G. Stoletov.

) P. N. Lebedev, Collected Works*, pp. 285–6, Moscow, 1913.

II. STUDY OF PROCESSES IN THE PLASMA OF A GAS DISCHARGE

The first decade after the October Revolution coincided with a period of rapid development of the physics of electrical phenomena in gases, which had received a powerful impetus from the classical experiments of 1913 in the field of ionization and excitation of atoms in inelastic collisions of electrons, as well as from the successes of Bohr’s theory of the atom. An important stage in this development was the method of probe characteristics proposed by Langmuir for studying the discharge and the notion, introduced by him, of the plasma of a gas discharge.

At the same time, a new branch of industry that had begun to develop in the USSR, aimed at the manufacture of electric-vacuum devices, confronted Soviet physicists with a number of questions closely connected with electrical phenomena in gases. Both in Leningrad and in Moscow, groups of physicists were formed who worked in this field. In Leningrad they maintained close contact with the “Svetlana” plant, which at that time manufactured mainly vacuum devices. The attention of the Leningrad physicists was directed toward phenomena of all kinds of electron emission, which were very important for vacuum devices. The corresponding works and achievements of the Leningrad physicists are set forth in the above-mentioned article by S. Yu. Lukyanov. Questions of gas discharge and, in particular, questions concerning the study of plasma were taken up by D. A. Rozhansky with his students L. A. Sena and V. F. Kovalenko. In contrast to foreign investigators, who merely mechanically continued Langmuir’s work, blindly following the method indicated by the latter, D. A. Rozhansky, V. F. Kovalenko, and L. A. Sena subjected Langmuir’s probe method to criticism on one of its fundamental points—namely, on the question of methods for extrapolating the ion current. This extrapolation is necessary for determining the electron current to the probe at a small negative potential of the latter relative to the surrounding gas, since the strength of the electron current is obtained from the results of measuring the total current to the probe, which is the difference between the electron and ion currents.

Instead of the linear extrapolation introduced by Langmuir, which leads to absurd values of the ion current when the probe potential is equal to the potential of the surrounding gas, D. A. Rozhansky, V. F. Kovalenko, and L. A. Sena proposed extrapolation by a parabola, free from the shortcoming indicated above. The sudden death of D. A. Rozhansky interrupted the intensive work on the plasma of a gas discharge being carried out in the laboratory he had created. These works were continued in the following years by L. A. Sena. In his works Sena continued the critical study of Langmuir’s probe method and established a number of factors that often lead to great inaccuracy in the quantitative data obtained with the aid of the probe method, especially

in determining such an essential internal parameter of the discharge as the concentration of free electrons in the plasma. In recent years Saha introduced into the theory of discharge plasma an essentially new element. Langmuir initially assumed that the thermal motion of positive ions corresponds to the Maxwellian distribution of velocities and can be characterized by specifying the “temperature” of this distribution in the same way as the temperature of the electron gas is specified. However, all attempts to determine, by means of the probe method, the temperature of positive ions led to data not only mutually contradictory, but also incompatible with ideas about the character of the exchange of energy between ions and neutral gas particles. Thus, the question of the specific nature of the motion of positive ions in plasma remained open.

L. A. Sena investigated the question of the effective cross section of gas particles in plasma with respect to various kinds of interactions of these particles with positive ions, and came to the conclusion that the most frequent type of interaction between positive ions and neutral gas particles is by no means an elastic collision leading to enhanced exchange of energies, but a charge-transfer process. As a result of this process, the positive ion turns into a neutral particle, rapidly losing its excess energy in elastic collisions, while the particle that has captured the ion’s charge (having given the latter a valence electron) turns into a positive ion. The new positive ion is accelerated under the action of the potential gradient in the discharge and moves with high velocity in a straight line in the direction of the field until the next act of charge transfer. Only a very weak chaotic motion in various directions is superposed on this specific motion of the ions. Therefore comparison of the mean energy of positive ions with some ion temperature loses its meaning. This picture of ion motion and theoretical calculations of their mean velocity have recently been confirmed in the experiments of S. E. Frisch and Yu. M. Kagan, who developed an optical method for investigating the motion of positive ions. The question of the velocity distribution among electrons in a gas-discharge plasma was developed theoretically by B. I. Davydov. B. I. Davydov and A. I. Zmanovskaya gave a theory of the electron current to a probe with allowance for the phenomena of electron diffusion in the region surrounding the probe.

In Moscow, B. N. Klyarfeld (All-Union Electrotechnical Institute) worked on questions of gas-discharge plasma and the theory of probes. In his doctoral dissertation B. N. Klyarfeld clarified in detail the limits of applicability of the probe method. Together with V. L. Granovskii and V. A. Fabrikant he gave, in a special article, a critique of a number of works in which the probe theory had been applied incorrectly. At the same time B. N. Klyarfeld, taking into account all the precautions necessary in applying the probe method, himself carried out

a large number of measurements of various plasma parameters as a function of different conditions. He succeeded in establishing the influence of the Ramsauer effect on the potential gradient in the positive column. He determined the conditions under which processes of stepwise ionization begin to play a substantial role in the plasma, and developed an experimental method making it possible to judge unambiguously whether these processes are present.

In their time Langmuir and Tonks did not give a complete solution of the problem of plasma, but only sketched the general outlines of the theory, solving incidentally a number of particular questions. B. N. Klarfeld verified some conclusions of Langmuir’s theory (in potassium vapor and in mercury vapor) and indicated the limits of applicability of these conclusions. Then B. N. Klarfeld, applying a number of ingenious and successful simplifications, solved the plasma problem completely for the case of low gas pressures. The quantitative comparison of theory with experiment carried out by him confirms the correctness of the theoretical relations obtained. On the question of the energy balance of the plasma, the discussion of which Langmuir had at one time declined as still premature, B. N. Klarfeld generalized and processed the extensive material available in the literature and substantially supplemented it with his own measurements. On this basis he constructed a successful scheme of the power balance in the positive column at various pressures, both for the case of small (milliamperes) and for the case of large (several amperes) current strength.

According to this scheme, the power expended per one centimeter of length of the discharge tube is composed of the power dissipated on the walls of the tube, the power dissipated in the volume of the gas, the power of radiation of the resonant lines of the spectrum, and the power of radiation of the nonresonant lines. The scheme makes it possible to judge the relative magnitudes of all these power components over a wide range of pressures, from extremely low to very high.

These works of B. N. Klarfeld made it possible to approach quantitatively the question of the causes of the greater or lesser luminous efficiency of a gas discharge, and are of substantial importance for the development of light sources of the greatest possible economy.

In the experimental works first of D. V. Zernov, and then of Klarfeld and Taraskov, an extensive investigation was carried out of the dependence of the luminous efficiency of a discharge in sodium vapor on the discharge parameters, on the pressure of sodium vapor, and on the presence in the discharge tube of one or another inert gas. These works established the quantitative dependence of the luminous efficiency of a sodium lamp on the diameter of the tube, on the current density and on the pressure of sodium vapor, and consequently also on the temperature of the walls of the discharge tube. Since the optimum current strength does not, under the usual conditions of burning of the discharge in a glass tube, lead to the optimum wall temperature, it follows from Klarfeld’s experiments that, in order to obtain the highest possible luminous efficiency, special conditions of heat exchange must be created,

PHYSICS OF ELECTRICAL DISCHARGES IN GASES

by arranging suitable thermal protection for the tube. Having developed the design of sodium lamps in this direction, Klarfeld achieved luminous efficacy in experimental specimens of up to 120 lumens per watt, which is seven times greater than the luminous efficacy of powerful incandescent lamps and ten times that of low-power ones.

In high-pressure mercury lamps, the dependence of luminous efficacy on various conditions was studied by Klarfeld and Plokhotskii.

G. V. Spivak and E. M. Reikhrudel approached the question of the further development of the probe method and of plasma research from a somewhat different angle. Experimenting with probes around which a magnetic field was artificially created, they came to the conclusion that the presence of a magnetic field substantially changes the conditions under which gas-discharge processes proceed. Therefore, for example, it is impossible to study the behavior of positive ions in a plasma by deflecting, with the aid of a magnetic field, the electrons striking the probe. Relying on the laws of electron motion in electric and magnetic fields, Spivak and Reikhrudel constructed a generalized theory of probes. The theory developed by Langmuir for the case in which magnetic fields are absent is a special case of this generalized theory. Having clarified the influence of a magnetic field on the course of probe characteristics, Spivak and Reikhrudel were thereby able to proceed, with the aid of the generalized probe method, to the study of the behavior of a gas-discharge plasma in a magnetic field. In particular, they studied the question of the contraction (pinching) of the positive column in a uniform and nonuniform magnetic field and the accompanying change in the laws of distribution of the concentration of free electrons along the radius of the tube—the work of G. V. Spivak, E. M. Reikhrudel, Kh. N. Fatalieva, and O. N. Repkova. Another question that was resolved in the work of Spivak and Reikhrudel, with the participation of A. A. Zaitsev, was the question of the influence of metastable atoms on the electron current to the probe. Such an influence must take place owing to the emission of electrons from the surface of the probe at the expense of the energy of metastable atoms striking the probe. The results of earlier investigations did not make it possible to judge unambiguously either the magnitude of this effect or the distortions introduced. The experiments of Spivak and Reikhrudel showed that this effect has an appreciable magnitude and cannot be neglected.

The method applied by them to solve the problem (the destruction of metastable neon atoms by absorption of nonresonant radiation from a powerful neon lamp) enabled the same authors to establish the role of metastable atoms in another case as well. They, and also A. A. Zaitsev, clearly established the influence of metastable atoms on the magnitude of the cathode fall of potential in a glow discharge in neon. A. A. Zaitsev investigated the influence of foreign impurities on the longitudinal potential gradient in the positive column of a discharge in mercury vapor and in noble gases both in

in pure gases, and also in the presence of various kinds of impurities. M. I. Rodin, studying the current to several probes placed along a long discharge tube at different distances from a heated oxide cathode, established that near the cathode the Maxwellian velocity distribution in the plasma is distorted by the presence of fast electrons accelerated in the cathode layer. The farther the probe was from the cathode, the smaller was the distortion of the rectilinear semilogarithmic characteristic of the electron current to the probe. At a certain distance from the cathode the distortion disappeared completely.

Of essential importance in the operation of electrovacuum devices in which an electric discharge in gases is used (mercury rectifiers, gasotrons, thyratrons, ignitrons) is the deionization time, i.e., the time during which the plasma practically disappears. Questions of deionization were studied in detail by V. L. Granovskii at the All-Union Electrotechnical Institute. Making certain assumptions about the mechanism of the deionization process and taking into account the laws of ambipolar diffusion of ions and electrons to the walls of the discharge tube, V. L. Granovskii gave a quantitative theory of deionization and calculated the course of deionization for a number of special cases. At low pressure the recombination of ions and electrons occurs exclusively at the walls; its course, according to Granovskii’s theory, depends on the distribution of ion velocities. At medium pressure the motion of charged particles toward the walls already has the character of diffusion, the rate of which slows with time. A substantial role in this case is played by the decrease in the electron temperature. Definite quantitative conclusions are drawn about this decrease. On the basis of all this, V. L. Granovskii indicated the law according to which deionization of the gas proceeds with time. At low pressures this law is an exponential dependence. At higher pressures there are deviations in the direction of a slowing of the deionization process. V. L. Granovskii also gave a calculation of the course of deionization at high pressures, when the principal factor is the recombination of ions and electrons in the volume of the gas. Further, he considered the influence of an electric field on deionization. A logarithmic dependence of the deionization time on the applied voltage was found. In the experimental part of V. L. Granovskii’s work, good agreement of theory with experiment was obtained for the case of mercury vapor, and a number of practical conclusions essential for the theory of the mercury rectifier and the thyratron were made. He calculated in detail (jointly with L. Reverdatto) the deionizing action of grids in mercury rectifiers; reverse currents in multiphase mercury rectifiers were investigated, and certain specific phenomena occurring at individual grid apertures were also discovered. These phenomena led to the conception of gas rarefaction and the accompanying complete ionization in the places where the discharge gap narrows. They were subjected to special investigation by V. N. Klyar-

... by Klyarfeld in connection with the question of the causes of the “arc break” observed in high-power rectifiers, and also by V. L. Granovskii. Making use of this phenomenon, the latter constructed special generators of electrical oscillations of the relaxation type.

The determination of the discharge parameters in mercury vapor as applied to glass (low-power) mercury rectifiers was studied in the laboratory of the Research Institute of Physics of Moscow State University, under the direction of S. D. Gvozdovers, by D. R. Konaskov, who in particular investigated the diffusion of electrons in mercury vapor.

Closely connected with the question of the deionization time of the gas-discharge gap is the question of the inertia of the discharge glow. This question is of practical interest in connection with attempts that were being made to realize an intense, sufficiently sharply modulated light source for use at the receiving station in television transmission. In this direction several works were carried out at the All-Union Electrotechnical Institute by V. I. Romanov and B. N. Klyarfeld, who obtained sharp modulation in the frequency range from 50 to 14,000 hertz.

III. AVALANCHE DISCHARGES.

IGNITION OF A “SELF-SUSTAINING DISCHARGE”

Besides the plasma, an essential role in gas discharges is played by regions in which beams of fast electrons, emitted by the cathode and accelerated by the field of the discharge, produce the enhanced ionization of gas particles necessary for the formation of the plasma. With an incandescent cathode, thermionic emission is of principal importance. The region lying between the cathode and the plasma is thereby contracted into a very thin “Langmuir” layer. In the case of a cold cathode, in the so-called cathode dark space, whose length is the greater the lower the gas pressure, electron avalanches develop. A small number of electrons leaving the cathode in this case rapidly increases in the direction toward that region of the discharge gap where there is a plasma or directly the anode. Along the path of motion of the avalanche, not only are new free electrons and positive ions formed, but excitation of gas particles also occurs, leading to the emission of photons. The incidence of positive ions and photons (as well as excited atoms) on the cathode causes the emission from the cathode of electrons that sustain the discharge. This emission of electrons from the cathode is characterized by the coefficient of surface ionization \(\gamma\) (\(\gamma\)-processes).

The growth of electron avalanches is determined by the coefficient of volume ionization \(\alpha\) (the number of ionizations produced in a given gas by one electron in moving one centimeter from the cathode to the anode). The positive space charge, which accumulates along the path of the avalanche because the velocity of motion of the electrons is much greater than the velocity of motion of the posi-

of positive ions in an electric field of the same intensity, constitutes, according to Poisson’s law, a distortion of the electrostatic field between the electrodes, caused by the gas discharge. For the course of all phenomena in the avalanche regions of a discharge, and also for the processes of “ignition” of an independent discharge, the dependence of the coefficient \(\alpha\) on various conditions is of essential importance. This question, as well as the closely related question of the ignition voltage of a discharge, was considerably advanced by the work of a group of physicists at Moscow University. S. K. Moralev developed a theory of the coefficient of volume ionization \(\alpha\). Moralev’s theory makes it possible to calculate the coefficient \(\alpha\) on the basis of quantitative data on various kinds of interactions between electrons and gas particles, and explains the influence of various factors on the coefficient \(\alpha\) and on the ignition voltage of a discharge. Moralev’s theory brought great clarity to the understanding of experimental results and serves as a reliable basis for practical action on the ignition voltage. The experimental works of I. I. Glotov and I. I. Balog confirmed Moralev’s conclusions and provided much practically important data on the influence of small foreign impurities on the ignition voltage of a discharge in noble gases. The works of B. N. Klyarfeld on the same subject should also be included here. At the same time N. A. Kaptsov gave a theoretical calculation of the growth of space charges and of the distortion of the field during the passage of a series of successive avalanches in the period of ignition of a discharge. This calculation provided a quantitative substantiation for the explanation, given shortly before by foreign physicists, of the short duration of the time of formation of a discharge at atmospheric pressure. In this cycle of works one should also note the work of I. I. Glotov on the quantitative influence of collisions of the second kind in neon on the current strength of a non-self-sustained discharge. The avalanche-discharge field includes the works carried out in the same laboratory at Moscow State University by P. V. Timofeev, I. F. Kvarts-khava, and N. Penin on the study of the causes of the inertia of discharges in gas-filled photocells. In these works a correct estimate was given of that part of the inertia which is caused by the time of flight of ions from anode to cathode. It was shown experimentally that this cause of inertia occurs to a greater or lesser degree in all cases and is supplemented, and sometimes even overlapped, by the action of metastable atoms with a long lifetime.

When the diameter of the discharge tube is small, the proximity of the walls to the path of the avalanches may affect the process of discharge ignition. Such conditions of a “hindered discharge” occur in a large number of discharge devices. The ignition of a discharge under these conditions was investigated by Katysheva. One of the causes making ignition of the discharge difficult in these cases is the formation of negative surface charges on the walls of the tube. The influence of external…

electric and magnetic fields on the ignition of a discharge was investigated by E. L. Stolyarova and G. V. Spivak. The influence of a magnetic field on the course of the initial stages of a discharge should differ greatly from the influence of a magnetic field on the behavior of the discharge plasma, owing to the fact that in the first case the directed motion of electrons predominates, while in the second their random thermal motion predominates. The investigation undertaken by G. V. Spivak and E. L. Stolyarova of the influence of magnetic fields of various configurations on the development of a discharge in long tubes confirmed the correctness of this point of view. In the course of the investigation, a periodic variation of the ignition voltage in the presence of a magnetic field was established when the distance between the electrodes was changed. This phenomenon was interpreted by Spivak and Stolyarova as periodic focusing of electrons; it was also shown that, in the presence of a magnetic field, there exists a definite correspondence between the laws governing the development of a gas discharge and the formulae of electron optics.

The experimental works mentioned here on ignition voltage concern gases at low pressures. The ignition voltage of various gases at high pressures was studied at the P. N. Lebedev Physical Institute of the Academy of Sciences of the USSR by B. M. Vul, partly together with I. Goldman. These investigations established that, in the case of a nonuniform field, all other conditions being equal, the ignition voltage passes through a maximum as the pressure is increased. Vul and Goldman also found that the breakdown voltage increases with increasing temperature. This occurs, as they point out, because of the influence of diffusion on the density of the positive space charge arising during breakdown. In the same institute, Kovalenko investigated the influence of various polyatomic gases on the ignition voltage in air. A persistent effect of increasing the breakdown voltage is produced by an admixture of carbon tetrachloride.

The works of B. M. Gokhberg, E. Ya. Zondberg, Ya. M. Oksman, and the work of A. G. Ryankin also belong to this same field of investigation of the breakdown of compressed gases.

The positive ions formed in the course of the development of an electron avalanche are accelerated by the electric field of the discharge in the direction toward the cathode. One of the consequences of bombardment of the surface of a cold cathode by positive ions is cathode sputtering of the cathode material.

In devices using a glow avalanche discharge, cathode sputtering is a harmful factor leading to increased surface conductivity of the glass walls of the device. Cathode sputtering has an especially harmful effect on the useful service life of gas-discharge light sources, since it leads to a reduction in the emitted luminous flux. A useful application of cathode sputtering is the production of thin metallic films. The investigation of the laws and conditions of cathode sputter-

repeatedly been carried out at the Institute of Physics of the AS UkrSSR by N. D. Morgulis and his collaborators (Bernadiner, Patiokha), both along the line of refining the theory of this phenomenon and in establishing a number of experimental data of practical importance. Another series of works on the study of cathode sputtering belongs to Yu. P. Maslakovets.

IV. CORONA DISCHARGE. HIGH-FREQUENCY DISCHARGES

A special variety of avalanche discharges is the corona discharge. In this case the region of growth of electron avalanches is concentrated in the part of the large field strength in the so-called coronating layer enveloping an electrode with a small radius of curvature (a point, a thin wire, a sharp edge). In the remaining “external” region of the discharge gap there are ions of only one sign, the same as the sign of the coronating electrode. The corona discharge leads to energy losses if it arises on high-voltage power-transmission lines. The same form of discharge finds useful application in the technique of depositing foreign particles suspended in a gas in electrostatic precipitators (purification of flue gases, recovery of valuable chemical products “flying out into the stack”) and in the technique of electroseparation of finely ground ores and other materials. The old theory of the corona discharge, on which the ideas about the corona discharge of practical engineers were based and which regarded the coronating layer as a simple continuation of the metallic conductor in the absence of an electric field in this region, was refuted by N. A. Kaptsov, who constructed a theory of the coronating layer on the basis of modern discharge theories and showed that in the coronating layer there exists a high potential gradient.

In the work of N. A. Kaptsov and S. K. Moralev, the transition of a corona discharge into a spark was investigated, and methods were indicated for calculating the “spark-over voltage of the corona.” In subsequent works by Kaptsov, a method was given for calculating the thickness of the coronating layer, and it was established that the thickness of the coronating layer at the positive electrode, other conditions being equal, is always greater than the thickness of the coronating layer at the negative electrode. On the practical question of the limiting charge and the rate of charging of foreign particles suspended in the external region of the corona discharge, N. A. Kaptsov gave a synthesis of the theory of charging of these particles as a result of the motion of ions under the action of the electric field and the theory of the same charging under the action of the random thermal motion of the ions. At the NIIF of Moscow State University, I. I. Glotov, S. K. Moralev, and A. A. Vlasova experimentally investigated the influence on the corona discharge of a layer, applied to the surface of the non-coronating electrode, of a poorly electrically conducting substance, as well as investigated the potential,

arising at the boundary of this layer and the gas. In the dissertation of E. M. Balabanova a theory was developed for the deposition of particles charged in a corona discharge on a moving collecting electrode—a theory that makes it possible deliberately to develop methods of electroseparation. In the works of Meyer, Morokhovskii and Kaptsov, and subsequently of M. Ya. Vasil’eva, it was established that, contrary to the opinion previously widespread, the initial field strength of a corona discharge depends, other conditions being equal, on the distance between the electrodes both in the wire–plane and point–plane cases, and in the case of two coaxial cylinders.

A number of studies on the corona discharge were carried out by S. P. Zhebrevskii and V. I. Popkov in the laboratory of NIOGAZ of the Gazoochistka trust. These include a series of research works connected with the processes taking place in electrofilters, the study of the dependence of the volt-ampere characteristic of a corona discharge on the humidity of the air, and so on. These works yielded many valuable data for the practice of designing and operating electrofilters. Especially noteworthy are works on determining the distribution of the field in the outer region of the corona discharge in particular cases, for example with a dust-covered collecting electrode, carried out by V. I. Popkov by Sato’s probe method.

A special case of the corona discharge occurs when both electrodes of the discharge gap are coronating (for example, two cylindrical wires of the same diameter stretched parallel to one another). In this case, in the outer region of the corona discharge ions of both signs move toward one another. This creates specific conditions leading to the fact that the corona discharge arises at a lower field strength at each of the wires than in the case of a unipolar corona, and the discharge-current strength is greater than the sum of the currents from two wires coronating independently of one another. Experimental investigations of this case, which is of substantial importance in transmitting electrical energy by high-voltage direct-current lines, were undertaken at the NIIF of Moscow State University—M. Ya. Vasil’eva’s dissertation—and, on a broader scale, by V. I. Popkov at the Power Engineering Institute of the Academy of Sciences of the USSR.

The initial outline of the theory of the corona between two wires was sketched by V. A. Kaptsov. V. I. Popkov studied in detail the course of the volt-ampere characteristic of the corona discharge in the case of two coronating wires, and also, by means of Sato’s probe method, investigated the features of the field distribution in the discharge gap. This enabled him to draw conclusions about the distribution of space negative and positive charges in the gap between the wires, and also, on this basis, to construct a detailed approximate theory of the bipolar corona, making it possible to judge with confidence the magnitude of losses to corona under practical—

under conditions of direct-current electric transmission. The calculation data obtained by V. I. Popkov agree not only with the observed course of the volt-ampere characteristic, but also with data drawn from the literature, obtained on experimental lines at very high voltages.

The methods of electrostatic separation of materials by means of charging finely divided particles*) passing through the outer region of a unipolar corona discharge were developed by E. M. Balabanov, P. M. Ryvkin, and N. F. Olofinsky, with the participation of S. P. Zhebrovsky, and were brought by them to the point of practical implementation.

A special type of corona discharge is the impulse corona, which arises in practice under overvoltages caused on transmission lines by lightning strokes. The study of the impulse corona is of great importance because the conditions of spark flashover of such a corona differ greatly from those of an ordinary spark breakdown between a line conductor and the so-called “protective cable.” At the same time, the processes of streamer formation, which occur in part also in other cases of corona discharge in atmospheric air and which do not fit into the theory of corona discharge as a phenomenon continuous in space and time, are especially sharply expressed in the impulse corona. Works by I. K. Fedchenko, as well as by Borisoglebsky and others, are devoted to the impulse corona. V. V. Gei and S. L. Zaitsev carried out studies of the impulse corona with the aid of a cathode oscillograph and investigated the delay of ignition of the discharge in this case. The corona discharge and its spark flashover acquire a distinctive character at high frequencies. In the frequency region corresponding to the range of short and ultrashort waves, the high-frequency corona passes over into the still very little-studied torch discharge. The conditions for the occurrence of this type of discharge and its course were established in general outline in the works of S. I. Zilitinkevich. The influence of an external electric field on “torch efflux” was investigated by A. M. Prokofiev, who also proposed a torch ion counter. The development of the electric torch with change in pressure was investigated by Ya. Matveev.

V. SPARK DISCHARGE AND LIGHTNING

Systematic investigations of questions connected with lightning were begun by I. S. Stekolnikov at the VEI and were then continued by him with great success at the Power Engineering Institute of the Academy of Sciences of the USSR. One of the main tasks posed in these investigations was the question of the causes and conditions of the selective susceptibility of various objects to lightning strikes. This question was solved by I. S. Stekolnikov by

*) Author’s certificate of N. F. Olofinsky, P. M. Ryvkin, and M. V. Bochevsky No. 189553, 1936.

PHYSICS OF ELECTRIC DISCHARGES IN GASES

of studying the location of objects most frequently struck by lightning in different areas of the earth’s surface in relation to the electrical conductivity of the soil, the proximity of watercourses and bodies of water, the relief of the terrain, etc., and also by modeling lightning strokes in laboratory experiments with spark discharge. The result of the investigation was the conclusion that the greater or lesser probability of a given object being struck by lightning depends mainly on the electrical conductivity of the soil. Thus, if in a narrow valley between two hills with steep slopes and with dry sandy, poorly conducting soil there flows a stream, then the probability of a pole situated at some height on the slope of either of these hills being struck by lightning is less than the probability of a pole situated on the lower part of the slope, near the stream, being struck. This conclusion is of substantial importance for choosing the route of electric-power transmission lines and was repeatedly verified during a number of expeditions to various regions of the USSR, including mountainous regions. During these expeditions, organized under the general direction of I. S. Stekolnikov, there were carried out, on the one hand, a detailed study of individual lightning strokes, and on the other—automatic recording of the values of various lightning parameters (the current amplitude in an individual pulse of a thunderstorm discharge, the polarity of lightning, the multiplicity of pulses taking place during a single lightning stroke, etc.). To investigate the course of the various stages of lightning during a single stroke, methods were used for the time sweep of the phenomenon on photographic film, in special apparatus developed and constructed at the ENIN on the model of the so-called Boys cameras, and with the aid of recording lightning pulses by a cathode oscillograph, likewise constructed specially for this purpose.

The recording of the parameters of as large a number of lightning strokes as possible was carried out with the aid of ferromagnetic recorders, based on the magnetization, by the lightning current, of rods made of ferromagnetic material, and also with the aid of klydonographs (instruments using Lichtenberg figures, both simple ones and those with the recording of the figures on a rotating drum). Together with I. S. Stekolnikov, A. A. Lamdon, V. S. Komelkov, and A. Belyakov took part in this work. Several expeditions undertaken to the mountainous regions of Azerbaijan were directed, and their results processed, by Ali-Zade. The results of all these investigations led to a considerable expansion of the previously available data on lightning, and also made it possible to establish a number of new facts. Thus, for example, it was unambiguously established that the first leader of a lightning stroke (the first pulse, which breaks down the unionized air and precedes the formation of the “main channel” of the lightning) need not at all necessarily be the so-called stepped leader, but may, in a number of cases, propagate continuously from the negatively charged cloud to the earth. Observations by Stekolnikov and Ali-Zade also showed that in the overwhelming number ...

...cases of lightning flashes passing between thunderclouds and the earth are negative lightning flashes, i.e., they propagate from a negatively charged cloud to positively charged earth.

In laboratory experiments with spark discharge, I. S. Abramson and I. S. Marshak showed that the main channel of the spark, immediately after the formation of this channel, has a definite limited cross section and only comparatively slowly increases in width. The current strength in the main channel of the spark is limited by the onset of complete ionization in it. Therefore, in the first moments after breakdown, the voltage at the ends of the spark channel assumes a value greatly exceeding the voltage between the electrodes of an arc discharge. For further investigation of these stages of spark discharge, I. S. Stekolnikov proposed and developed a new oscillographic circuit, making it possible to use a very high speed of oscillographic recording by comparatively simple methods.

The theory of spark discharge at the stage of formation and growth of the main channel was outlined by I. S. Marshak. He also gave a critique of Loeb and Meek’s theory of the initial stages of spark discharge and put forward new original assumptions concerning the course of elementary processes during these stages.

A number of Soviet physicists—I. S. Abramson, I. I. Levintov, S. M. Raiskii, V. G. Koritskii, N. N. Sobolev, and others, headed by S. L. Mandelstam—studied spark discharge as a source of radiation for spectral analysis. A particularly thorough investigation was made of the specific type of discharge they carried out, which received the name of an “activated” or “hot” arc. This type of discharge is an arc discharge with rapidly alternating spark pulses superposed on it. In the course of these investigations, as well as in the course of spectroscopic studies conducted in Leningrad (GOI) by V. K. Prokof’ev, N. S. Sventitskii, and others, and in Tomsk (Siberian Physico-Technical Institute) under the direction of N. A. Prilezhaeva (O. P. Semenova, V. I. Danilova, N. K. Rubtsova, and others), a rich body of material was accumulated on the relation between the internal and external parameters of the discharge and the brightness of the radiation of particular spectral lines, which is of substantial importance for the theory of radiation of a gas discharge.

A condensed discharge through the channel of a narrow tube, akin to the channel of a spark discharge, was investigated by A. Babushkin (also for spectral purposes). The temperature, degree of ionization, and vapor pressure in such a channel were established.

The ignition of spark discharge was experimentally investigated by M. A. Bak, A. S. Zingerman, and N. N. Nikolaevskaya. They confirmed and refined observations made in his time by V. K. Lebedinskii, and established that the lowering and raising of the spark-discharge ignition voltage under irradiation of the cathode depend on the geometry of the discharge gap and on the intensity of irradiation.

VI. RADIATION OF A GAS DISCHARGE. APPLICATION OF ELECTRIC DISCHARGES IN GASES TO THE CREATION OF NEW LIGHT SOURCES

The plasma—the region of the discharge—represents the greatest interest in the development of new light sources. A quantitative solution of questions connected with the radiation of a gas discharge in general and, in particular, with the radiation of the plasma became possible only after the elementary processes in the discharge had been deciphered and a number of quantitative relations between the parameters of the plasma had been established for individual concrete cases of discharge. Such material was accumulated under clean experimental conditions by B. N. Klyarfeld. Together with E. S. Plokhotskii he also carried out measurements of the luminous efficacy of the positive column in mercury vapor under various conditions, as well as simultaneous measurements of the potential gradient and the radiation intensity of the positive column. All this served, in the hands of V. A. Fabrikant, as the starting point for the creation of a theory that makes it possible to calculate not only the relative but also the absolute intensity of individual spectral lines in the radiation of a gas discharge. Beginning with the simplest case, when all elementary processes reduce exclusively to the direct excitation of atoms in inelastic collisions with electrons, V. A. Fabrikant derived formulas relating the radiation intensity, on the one hand, to the plasma parameters (the concentration \(N_e\) and the electron temperature \(T_e\)), and, on the other, to atomic constants—the initial excitation potential of the gas atom \(V_a\) and the optimal excitation potential \(V_m\) (the energy of the electron colliding with the atom that is optimal for excitation). These relations make it possible: 1) to calculate the intensity of the radiation of a given spectral line, knowing \(T_e\), \(N_e\), \(V_a\), and \(V_m\); 2) to determine \(V_m\), having measured \(N_e\) and \(T_e\) by means of the probe method; and, finally, 3) to determine \(T_e\) and \(N_e\), having measured the radiation intensity and knowing \(V_a\) and \(V_m\). The last operation is an example of the application of optical methods to the investigation of a gas discharge. These methods become more and more possible as the theory of discharge radiation develops and as quantitative data on the atomic constants of various gases are accumulated. Comparison of Fabrikant’s theory with experiment gave good results when applied to a discharge in sodium vapor at low pressures, and also to the relative intensity of the resonance lines of mercury, 2537 Å and 1850 Å. Gradually complicating the problem, V. A. Fabrikant proceeded to take into account secondary processes—stepwise ionization, quenching collisions of the second kind, etc. Taking diffusion of radiation into account enabled him to draw a general picture of the excitation of atoms in a low-pressure discharge. He carried out an interesting investigation of radiation diffusion with the aid of luminescent probes. He was also able to analyze in greater detail the processes hav—

which occur in luminescent lamps. At high gas pressures, methods that make it possible to calculate the intensities of individual spectral lines on the basis of individual elementary processes become no longer applicable. At the same time, the whole plasma system (electrons, ions, neutral atoms, photons) approaches a state of thermal equilibrium. This provides new possibilities and new relations for the theory of radiation from a gas discharge. In this field V. A. Fabrikant also made a number of interesting conclusions and proposed a number of experimental works; in particular, the form of the lines in the radiation of a discharge in mercury vapor at high and ultrahigh pressure was studied and explained.

The development of new light sources using one or another type of electric discharge in gases proceeded in parallel at the VEI (A. M. Shemaev) and at the Moscow Electric-Lamp Plant. At the latter, glow-discharge lamps using so-called glow luminescence were developed: neon signal lamps of various types, receiving lamps for television with mechanical scanning by means of a Nipkow disk, and point lamps for phototelegraphy. High-voltage low-pressure lamps were also developed there, using the luminescence of the positive column of a glow discharge and employed as luminous letters and tubes for signboards, advertising inscriptions, and silhouettes. S. V. Borisov, I. D. Natonek, N. I. Dvukraev, M. I. Kalugin, and others took part in these works. D. A. Goukhberg, together with S. V. Borisov, Khodkevich, and V. P. Sassarov, developed and brought to mass production an original type of high-pressure mercury lamps using a corded arc discharge (the “Igar” lamps for light-copying purposes), as well as several types of medical mercury lamps in a quartz envelope. A distinctive feature of these and other lamps, distinguishing them both from foreign lamps and from VEI lamps, is the specially treated oxide cathodes on a tungsten core, made by a method developed by D. A. Goukhberg. A further stage in the search for a light source of high efficiency and good color quality was the mercury spherical ultrahigh-pressure lamp. These lamps are distinguished by the extraordinarily high brightness of the luminous discharge cord and are used as light sources for projectors and other projection installations. Recently D. A. Goukhberg has constructed ultrahigh-pressure lamps giving light very close to white and having a continuous spectrum with intensity varying slowly from one end of it to the other. These lamps are especially suitable for absorption spectroscopy and are many times superior, in their qualities and service life, to hydrogen lamps previously used in this field. The development of all these lamps in the laboratory of the Moscow Electric-Lamp Plant was accompanied there by a series of experimental investigations of the discharge at ultrahigh pressures. These investigations established

...relations were established between the current strength, the potential gradient, the pressure, the luminous flux, and the luminous efficacy of the discharge.

At the same plant, pilot production was mastered of luminescent lamps whose operation is based on the radiation of a low-pressure discharge in argon containing mercury vapor, and on the phosphorescence of “luminophores” deposited on the inner wall of the lamp tube under the action of the resonance radiation of mercury (the lines 2537 Å and 1850 Å). The basis for developing methods of preparing the corresponding luminophores was provided by the work of Academician S. I. Vavilov’s laboratory at the P. N. Lebedev Physical Institute of the Academy of Sciences of the USSR.

VII. ELEMENTARY PROCESSES OF ELECTRIC DISCHARGES IN GASES

It was indicated above what a great role in the phenomena of gas discharge is played by inelastic collisions of the second kind. The experimental existence of inelastic collisions of the second kind between excited mercury atoms and free electrons was directly proved by the experiments of A. I. Leipunsky and G. D. Latyshev. The scheme of these experiments recalls the scheme of the well-known experiments of Franck and Hertz on the study of excitation and ionization of atoms. Between the anode and the second grid of the apparatus there was applied a retarding field for the electrons of 4.7 volts, which corresponded to the lowest excited state of mercury atoms (the metastable level) \(6\,p^3P_0\). Free electrons were emitted by a heated cathode and were accelerated by the field of the first grid (potential \(V_1\)). For \(V_1 > 4.7\,\text{V}\), the electrons possessed energy sufficient to overcome the retarding field, and a current arose in the anode circuit. For \(V_1 < 4.7\,\text{V}\), current in the anode circuit arose only when the discharge tube was illuminated by mercury radiation. Upon absorption of this radiation in the space between the grids, excited mercury atoms arose. By means of collisions of the second kind their energy was transferred to free electrons and replenished the supply of energy necessary for the electrons to overcome the retarding potential between the second grid and the anode.

Another elementary process studied in detail by Soviet physicists is the ionization of sodium atoms and other metals upon collision with a heated tungsten filament.

The necessity for the existence of such a process does not follow directly from simple energy considerations, since the ionization work of the sodium atom is greater than the work function of an electron from metallic tungsten. The process becomes conceivable in the light of quantum-mechanical considerations on the finite probability of transition of an electron from a sodium atom into the conduction-electron band of metallic tungsten. Therefore, the experimental study of this phenomenon, pro-

carried out by N. D. Morgulis at the Physical Institute of the Academy of Sciences of the Ukrainian SSR over a wide temperature range, is of undoubted theoretical interest. An analogous work has recently been carried out by G. A. Morozov on the surface ionization of barium on tungsten. Phenomena of the same kind also include the scattering of potassium ions on the surface of tantalum, investigated by M. A. Eremeev and M. V. Zubchaninov.

G. V. Spivak, A. A. Zaitsev, and G. Zakharyin carried out a study of the elementary process of accommodation of mercury atoms, as well as atoms of noble gases, when they collide with the surface of a metal, both clean and covered with gas films of various kinds. In this work the temperature dependence of the accommodation coefficient was established, and also a connection was established between this coefficient, the molecular weight of the gas whose particles undergo energy exchange with the metallic surface, and the nature of the gas adsorbed on this surface.

S. D. Gvozdover carried out work on the study of the secondary emission of electrons from a metallic surface under the action of positive ions—one of the basic elementary processes of gas discharge at the cathode. On the basis of the results of this investigation, S. D. Gvozdover developed a theory of the cathode potential drop in a discharge with a hot cathode, in good agreement with experimental data, and subsequently, together with V. L. Bulatov, constructed a theory of a low-pressure gas discharge with a hot cathode situated under conditions of a “free” regime (i.e., at a current strength less than the saturation current of emission from the cathode, and therefore not requiring intensified bombardment of the cathode by positive ions).

S. D. Gvozdover also gave a new and original solution to the problem of calculating the velocity of the drift motion of electrons in the positive column under the action of a longitudinal field gradient. In his solution Gvozdover proceeds from the fact that the concept of the free path of an electron has no precise physical meaning without specifying what kind of interaction of the electron with the gas particles is meant. He showed that, in calculating the mobility of an electron in the positive column, it is necessary to use the concept of the free path referring to the process of transfer by electrons of mechanical momentum to gas particles. Gvozdover arrived at an expression for the mobility which has received universal recognition.

Of essential importance for the development of the theory of gas discharge is the development of the quantum-mechanical theory of elementary atomic processes on the basis of wave mechanics. With regard to stepwise excitation processes, a theory of this kind has recently been developed by B. M. Yavorskii.

A large number of works by Soviet physicists on elementary processes in gas discharge belong to the field of so-called

“electronics,” to the development of which in the USSR the above-mentioned article by S. Yu. Lukyanov is devoted. Therefore we shall not touch upon these works here.

Several theoretical works by Ya. I. Frenkel are devoted to individual phenomena of the gas discharge.

In a work devoted to the nature of ball lightning, Ya. I. Frenkel considers the latter as a special semistable colloidal charged system. In the work “On the Emission of Electrons under Bombardment of the Cathode by Positive Ions,” a new, distinctive interpretation of this phenomenon is given. Ya. I. Frenkel also analyzed the question of coupled oscillations of plasma and of a vacuum resonator, of the autofocusing of an electron beam when such a beam passes through a gas, and a number of others.

The general theory of plasma developed by A. A. Vlasov in recent years, as a statistical ensemble of a large number of charged particles interacting not only at short but also at long distances, provides points of departure for deciphering a number of phenomena in the plasma of a gas discharge. Thus, this theory sheds light on the causes of the very long relaxation time and very large relaxation distance in gas-discharge plasma with respect to violations of the Maxwellian velocity distribution. Further, it leads to the necessary presence in plasma of “vibrational properties,” and thereby generalizes the question of the occurrence in plasma of electromagnetic high-frequency oscillations; finally, in it are indicated ways of explaining the disintegration of plasma into separate luminous strata (the so-called striated positive column of a discharge).

VIII. TECHNICAL APPLICATIONS OF ELECTRIC DISCHARGES IN GASES

We have here the opportunity only briefly to mention the extensive work on the application of electric discharges carried out in the USSR, both in the field of developing electrovacuum (gas-discharge) devices and in the field of studying the specific processes that take place in these devices. This includes the development, carried out by Vologdin, of mercury rectifiers for very high voltages and very large powers.

In the field of questions connected with the operation of mercury rectifiers, I. L. Kaganov, D. Uait, Babat, I. Kenin, E. Petukhov, and others worked. The regulation of the rectified voltage has been investigated, the problem of controlling current with the aid of grids has been solved, the extinction of the arc when the circuit is broken has been studied, as well as the behavior of controlled rectifiers in cascade and multiphase circuits and the dependence of the reverse current on various conditions, etc.

A special type of ionic converters, based on the action of a magnetic field on the discharge, was proposed and investigated by I. M. Sitnikov.

Questions connected with the development and study of the operating conditions of gazotrons and thyratrons of various power ratings, as well as a number of other electron-tube devices, were investigated in the laboratories of the “Svetlana” plant. The result of these works was the equipping of our industry and our radio stations with electron-tube devices, whose availability made it possible to solve a number of essential practical problems.

Another area of the technical use of electric discharges in gases that has found broad development in the USSR is the methods of electric welding by means of an electric arc, first indicated in the last century by the Russian electrical engineers N. G. Slavyanov and N. N. Benardos. At the present time arc welding is, in the USSR, the basic technological process in the manufacture of metal structures.

Work on the study of the properties of the welding arc and of the processes occurring in it, and the development of new, improved methods of electric welding, is headed in the USSR by Academician V. P. Nikitin.

A further development of N. G. Slavyanov’s method was V. P. Nikitin’s proposed separation of the thermal processes of preparing the base metal and the so-called “filler” metal. In this method various combinations of the electric arc and other sources of heat may be used, such as a gas flame and high-frequency currents. Among the physical studies connected with arc welding, we shall point to the investigation, carried out by V. B. Nikitin and then by V. P. Nikitin and I. Ya. Rabinovich, of the physical properties of the electric arc under welding conditions, which made it possible to establish the dependence of the stability of combustion of the welding arc on the electrical parameters of the current source. G. M. Tikhodeev conducted a study of physicochemical processes in the welding arc. N. N. Rykalin gave a theory of heat propagation in electric-arc welding; K. K. Khrenov developed methods of electric welding and cutting under water; E. O. Paton developed high-speed automatic welding under a layer of flux. To these should also be added numerous studies of special electric machines for welding, as well as of a number of technological details of the welding process.

In recent times, in the technique of processing metal products, methods of spark cutting and drilling of metals, proposed by B. R. Lazarenko and N. I. Lazarenko, have acquired great importance. These methods permit no less precise geometrical machining than ordinary mechanical methods of cutting, milling, and drilling, and differ from them in a number of advantages. These methods are based on the phenomenon of “erosion,” which is a specific destruction of the anode during a spark discharge, still little studied from the theoretical standpoint.

Special mention should be made of methods for using electric discharges in gases to activate chemical reactions—the work of S. S. Vasil’ev, N. I. Kobozev, E. N. Eremin, A. S. Prevoditelev, N. N. Nechaeva, Zalogina, N. N. Nekrasov, L. B. Shekhter, A. G. Belyankin, and others—as well as of applications of electric discharges in gases, already mentioned in other sections of this article, for gas purification and separation of materials and for the creation of new light sources.

Summing up the achievements of Soviet physics in the field of electrical phenomena in gases, we see that over the past thirty years very much has been done in this field in the USSR, both in the development of scientific knowledge and in the practical application of the results obtained.

The favorable conditions now created in the USSR for the development of creative scientific work will undoubtedly lead in the near future to still greater successes in the study of electrical phenomena in gases and to an even deeper and more fruitful introduction of the results achieved here into the advanced technology of our country.