As you know, alternating current used for industrial and domestic purposes has 50 oscillations per second. The number of oscillations of the alternating high-frequency current reaches hundreds of thousands and millions per second.

High frequency current is characterized by the number of oscillations per second and the length of the electromagnetic wave. There is a simple relationship between the wavelength and the frequency of the current: the higher the frequency of the current, the shorter the wavelength.

By length, electromagnetic waves are divided into long - 3000 m and more, medium - from 3000 to 200 m, intermediate - from 200 to 50 m, short - from 50 to 10 m and ultrashort - less than 10 m.

High-frequency currents are obtained using special generators - spark and lamp. At the heart of any high-frequency generator is an oscillatory circuit. The oscillating circuit consists of an electrical capacitance (a capacitor, denoted by the letter C) and a self-induction coil, otherwise an inductor (denoted by L), which is a wire spiral.

If a charge is imparted to the capacitor of the oscillatory circuit, then an electric field arises between its plates (Fig. 29, 1). The capacitor begins to discharge through self-induction; when the discharge current passes through self-induction, an electromagnetic field appears around it due to the current energy (Fig. 29, 2). When the capacitor is completely discharged, the current should stop; but as the current weakens, the energy of the electromagnetic field stored in self-induction is transferred back into the current of the same direction. As a result, the capacitor will be charged again, although the sign of the charge on the capacitor plates will be reversed (Fig. 29, 3). Having received a charge, the capacitor again begins to discharge through self-induction, but the capacitor discharge current will be in the opposite direction (Fig. 29, 4). The passage of the current through self-induction will again be accompanied by the emergence of an electromagnetic field, the energy of which, as the discharge current weakens, will be converted into the energy of the induced current in the same direction. The capacitor plates will be charged again, and their charge will be of the same sign as at the beginning (Fig. 29, 5).

The energy stored now in the capacitor will be less than the initial one, since part of it was spent on overcoming the ohmic resistance of the circuit.

Going first in one direction and then in the opposite direction, the capacitor discharge current makes one oscillation.

Having received a charge again, although less than the initial one, the capacitor will again begin to discharge through self-induction. With each oscillation, the amplitude of the current will decrease. This will continue until all the energy stored in the capacitor is consumed to overcome the ohmic resistance of the circuit. A group of damped oscillations arises.

So that the oscillations in the oscillatory circuit do not stop, it is necessary to periodically supply the capacitor with a supply of energy.

A variable is a current that periodically changes in magnitude and direction. During one oscillation, the current increases to a maximum, then drops to zero, changing direction to the opposite, again increases to a maximum and again reaches zero.

The period of time (T) during which one oscillation occurs is called a period. The reciprocal of the period, that is, 1 / T, is called frequency. If the period

T is expressed in seconds, then frequency is the number of oscillations per second. The frequency corresponding to one vibration per second is taken as a unit and in honor of the physicist Herz it was named hertz (Hz).

If the oscillation is performed according to the sine law, then graphic image the oscillatory process is a sinusoid. Such vibrations are called harmonic.

When alternating current passes through a conductor around the latter, electromagnetic vibrations spreading in space in all directions; they form electromagnetic waves. Electromagnetic waves propagate in a void at the speed of light - 300,000 km / sec (3 * 10 10 cm / sec), and in different environments at a slightly lower speed.

The distance that an electromagnetic wave travels during one period is called the wavelength.

Currently, electromagnetic waves of the so-called radio frequency are divided into long - 3000 m and more, medium - from 3000 to 200 m, intermediate - from 200 to 50 m, short - from 50 to 10 m, ultrashort - less than 10 liters, and the latter into meter - from 10 to 1 m, decimeter - from 1 m to 10 cm and centimeter - from 10 to 1 cm.

Currents of any frequency, including high ones, are obtained using an oscillatory circuit, which consists of a capacitor (electrical capacitance - C) and an inductance (wire coil - L, at high-frequency currents without an iron core).

If a charge is imparted to the capacitor of the oscillatory circuit, then it begins to discharge through the inductance: in this case, a magnetic field arises around it due to the energy of the current. When the capacitor is completely discharged, the current should stop, but as the current weakens, the magnetic field energy stored in the inductor is transferred back to the current in the same direction; as a result, the capacitor will be charged again, but the sign of the charge on its plates will change to the opposite. Having received a charge, the capacitor again begins to discharge through the inductance, but its discharge current will be in the opposite direction. The passage of current through the inductance will again be accompanied by the appearance of a magnetic field, the energy of which, as the discharge current weakens, will be converted into the energy of the induced current in the same direction. The capacitor plates will be charged again, and their charge will be of the same sign as at the beginning. The energy accumulated now in the capacitor is less than the initial one, since part of it is spent on overcoming the ohmic resistance of the circuit. Going first in one direction and then in the opposite direction, the capacitor discharge current makes one oscillation.

Having received a charge again, although less than the initial one, the capacitor will again begin to discharge through the inductor. With each oscillation, the amplitude of the current will decrease. This will continue until all the energy accumulated in the capacitor is spent on overcoming the ohmic resistance of the circuit and partially on the emission of electromagnetic waves - a group of damped oscillations appears. In order for the oscillations to be low-damped or non-damped, it is necessary to periodically supply energy to the oscillatory circuit, to make up for its losses. In modern high-frequency medical devices, this is done with the help of electronic tubes used in generator circuits.

The simplest oscillator tube is a triode. It has 3 electrodes: cathode, control grid and anode. When heated, the cathode releases electrons. If a positive potential is applied to the anode and negative to the cathode, then an electric field arises between the anode and the cathode, under the influence of which negatively charged electrons are attracted to the anode, which has a positive potential. Penetrating between the turns of the control grid located between the cathode and the anode, electrons reach the anode, as a result of which current flows in the anode circuit. The control grid is located closer to the cathode and has a stronger effect on the electrons than the anode. When there is a positive potential on the control grid, the movement of electrons is accelerated - per unit time, more of them fall on the anode, the current increases; when there is a negative potential on the grid, it repels electrons, not allowing them to pass to the anode - the anode current becomes weaker.

The triode has a number of disadvantages, and this forced the transition to more advanced lamps - tetrodes, beam tetrodes, pentodes, etc. These lamps are used in medical high-frequency generators operating on self-excitation with feedback.

The anode current flowing in the generator lamp circuit charges the capacitor of the oscillating circuit, which leads to the occurrence of electrical oscillations in the anode oscillating circuit. Current fluctuations create an alternating magnetic field in the inductance coil of the oscillating circuit, the lines of force of which intersect the turns of the adjacent inductance coil of the control grid, inducing alternating potentials on it. As a result of this, the oscillatory circuit in the anode circuit, through the connection with the lamp grid, begins to control the anode current supplying it. This relationship is called feedback. In the presence of feedback(if you turn on the power to the generator) oscillations occur in the anode oscillatory circuit, the generator is self-excited. This is the principle of operation of the generator on self-excitation.

In practice, in devices of high and ultra-high frequency, the structure of the oscillatory circuit is much more complicated. In high-frequency devices, oscillations initially occur in a low-power master oscillator. The oscillations arising in it are usually transmitted inductively to an intermediate amplifier, and then to an output amplifier assembled on more powerful tubes. The principle of amplification is that oscillations from the previous circuit are fed to the control grids of more powerful lamps of the subsequent circuit, which leads to an increase in the oscillation power.

The therapy circuit, which is used for carrying out the treatment procedure, is connected to the previous circuit, which is usually an output amplifier only inductively, in order to protect the patient from high voltage under which the previous contours are located.

All circuits must be tuned to resonance, that is, to the same frequency. In this case, the transfer of energy from one circuit to another is carried out most completely.

Previously, spark generators were used to obtain high-frequency currents. They are currently discontinued because they do not generate a stable frequency, which creates radio interference.

Any electric current, including high-frequency, has a thermal effect. This heat arises inside the tissues, and therefore received the name endogenous as opposed to exogenous, when heat penetrates into the tissues from the outside, as it happens when exposed to therapeutic mud, paraffin, heating pad.

In order to understand the reason for the appearance of heat inside tissues at high frequency currents, it is necessary to disassemble the mechanism of their passage through the tissues. In tissue fluids and inside cells there are ions, mainly sodium and chlorine, into which the basic salt contained in the body, sodium chloride, dissociates. In addition to sodium and chlorine ions, other ions (calcium, magnesium, phosphorus, etc.) are present in a smaller amount in the body, as well as protein molecules that carry an electric charge.

In addition to charged particles, the tissues of the body contain polar molecules (dipoles), in which electric charges inside the molecule are displaced and two poles can be distinguished - positive and negative. Dipole molecules (dipoles) include, in particular, water molecules.

When a high-frequency voltage is applied to the tissues of the body, a high-frequency electric field arises in the space between the electrodes. Under its influence, all charged particles are set in motion: negative ones are directed to the positive, positive - to the negative pole. Dipole molecules begin to rotate along the field so that the negative pole is directed towards the positively charged, positive - towards the negatively charged electrode.

As soon as ions and other charged particles have time to move, the direction of the electric field changes, which forces them to reverse the direction of motion. With each period of high-frequency current, this process will be repeated. The charged particles will begin to oscillate with a very small amplitude around the middle position at the frequency of the high-frequency current. Such a current, at which the movement of charged particles arises, in this case oscillatory, is called the conduction current.

During their oscillatory movements, charged particles encounter resistance both when they collide with each other and with the surrounding tissue particles, which is accompanied by the formation of heat. The rotation of the dipole molecules also encounters resistance from the surrounding particles and is accompanied by the release of heat (the so-called dielectric losses). The turn in a high-frequency electric field of dipoles carrying charges at their ends is called the displacement current (polarization). Human body tissues have electrical capacitance and ohmic resistance, connected in parallel, which is schematically shown in Fig. 40. There is practically no inductive resistance in tissues.

Cell membranes are dielectrics, although imperfect, and interstitial fluids and protoplasm of cells have ionic conductivity. The result is microscopic capacitors (two conductors separated by a dielectric layer). The total capacity of the human body is quite significant and amounts to 0.01-0.02 microfarads.

At relatively low frequencies (for high-frequency currents up to several million hertz per second), ionic conductivity prevails, a conduction current arises, while at high frequencies (several tens of millions of hertz), the polarization current increases. At ultrahigh frequencies exceeding 1 billion Hz, the polarization current increases even more, phenomena that are attributed to the oscillatory (oscillatory) action of high frequency currents become more pronounced; these include physicochemical shifts, in particular, an increase in the dispersion of proteins. The ionic composition and the number of polar molecules in different tissues differ from each other, therefore, at the same frequency, and hence the wavelength, an unequal amount of heat will arise in the tissues. In fact, all tissues will be heated, although the one for which the wavelength is closer to the selective (selective) one will be somewhat larger. According to N.N. Malov, the wavelength of 2.1 m is selective for muscles, 2.6 m for blood, 6 m for skin, 5.5 m for liver, 11 m for brain, and 35 m for fat. It should be noted that the frequency and, accordingly, the wavelength of the oscillations generated by modern medical devices of high frequency are not sufficiently selective for the tissues of the human body. Despite this, the difference in tissue heating is manifested to one degree or another. Due to the very small shift of ions from the middle position during oscillatory movements, there is no pronounced change in the concentration of ions at the border of cell membranes, both outside and inside the cell; this can explain the absence of the irritating effect of high-frequency current on the tissue.

Pain sensitivity under the action of high-frequency currents decreases, which basically does not depend on the generated heat, but is the result of the oscillatory oscillatory effect of high-frequency currents. It is possible that in this case the connection between the elements of the nerve ending that perceives pain is disrupted, which leads to a decrease in its excitability; the higher the frequency of the current, the more pronounced its analgesic effect.

Darsonval- the method of electrotherapy, in which the impact is impulsive alternating currents high frequency and voltage, but low power (frequency 110-400 kHz, voltage 20 kV, current up to 100-200 mA). The method is named after the French physiologist Darsonval, who developed the basic principles of its application in medical practice. Darsonval has been used in the treatment of a wide range of diseases since 1891.

Darsonvalization is subdivided into local and general.

Local darsonvalization is carried out using a vacuum electrode, through which a current of various voltages is supplied. As the voltage rises, the ionization of the air and the magnitude of the spark discharge increase. For general darsonvalization, the patient is placed in a coil of an oscillating circuit called a "Darsonval cage".

The active factor in local darsonvalization is a pulsed high-frequency current and an electric discharge between the electrode and the patient's body, which have an effect directly in the affected area; with general darsonvalization - eddy high-frequency currents, induced in tissues according to the principles of electromagnetic induction, and changing the parameters of the activity of the central nervous system, vascular and immune systems.

Diathermic current. Unlike currents d "Arsonval, the diathermic current has up to 2 million polarity changes per second, and the current decreases to 500 mA. The current intensity, however, increases to 1-5 A. The electrodes are metal, lead or steel, without gaskets. directly on the skin.

The action of local diathermy is reduced to inducing a rush of blood in the exposed tissues. In addition, the relatively deep penetration of heat affects the condition of the underlying tissues. At the place where the electrodes are applied, a feeling of warmth is created due to the resistance exerted by the current from the tissues with different conductivity.

In dermatological practice, focal diathermy is used to treat sluggish atonic tissues that have lost tension, elasticity, scleroderma, scars, ulcers from frostbite, X-ray ulcers, with chills, with red, cold, sweaty hands, etc.

You can use segmental diathermy of the cervical and thoracic sympathetic nodes. In this case, a 6 X 8 cm metal electrode is placed on the area located between the VI cervical and II thoracic vertebrae. The second electrode of a slightly larger size (8 X 14 cm) is placed on the substrate area. The strength of the current is given in 2-3 A, the duration of the session is up to 20 minutes. A total of 15-20 sessions are carried out. This segmental diathermy is successfully used for hyperhidrosis of the feet and palms, for skin atrophy, for scleroderma, etc.

In dermatological practice, surgical diathermy is also used. For the latter, electrodes with a very small effective surface are used, as a result of which tissue coagulation is obtained at the site of their application.

Three types of surgical diathermy are used:

• 1) electrocoagulation,
• 2) electrotomy (electric cutting)
• 3) electrodissection.

The simplest is electrocoagulation. For dermatological purposes, an active electrode is applied to the area to be removed, or a needle-shaped electrode is injected into the tissue at the desired depth. When a current of 0.5-2 A is passed, the command quickly sets in, tissue coagulation, and necrosis is formed. Under the influence of a protective dressing for 2-3 weeks, the necrotic area falls off and a pink scar remains, which gradually takes on the color of normal skin and evens out with the surface of the surrounding skin. If large areas of tissue are destroyed, then in these cases the scar is cosmetically good enough. However, when the wound heals, it is necessary to carefully protect it from any injury, protecting it with bandages.

Electrocoagulation is used to destroy angiomas, birthmarks, warts, xanthelasma, tattoos, telangiectasias. In case of hypertrichosis, for the purpose of hair removal, the use of electrocoagulation is more expedient than electrolysis, since it gives an effect in 3-5 seconds. However, the use of electrocautery for hair removal requires skill and experience on the part of the staff in order not to cause necrosis on the surface of the skin at the hair root and thus scar formation.

The second type of use of surgical diathermy is electrotomy. It is produced using the so-called diathermic scalpel. At the same time, tissue coagulates around the incision, which protects the body from the appearance of metastases or the introduction of microbes into the tissue. Healing by primary intention is rare; usually healing occurs by secondary intention.

The third use of surgical diathermy is dissection or electrodissection. In this case, the skipping spark achieves complete charring of the tissue to be destroyed. The scar obtained after coagulation is very good in cosmetic terms. However, even in these cases, it is necessary to protect the lesion from injury and secondary infection before healing.

High and ultra high frequency currents... For therapeutic purposes, high-frequency currents are used, namely from 10,000,000 to 300,000,000 and more periods per 1 second. This frequency corresponds to electromagnetic waves with a length of 30 to 1 m. Frequencies corresponding to a wavelength of 10 to 1 m are called ultra high frequency (UHF). The source of the UHF current, as it is customary to say, the generator of ultrashort waves (VHF), is the equipment, in principle, similar to the diathermic one.

As electrodes, various sizes and shapes of metal plates are used, covered with an insulating substance (wood, rubber, glass, ebonite).

The electrodes are located at some distance from the skin surface. The closer the electrode is to the skin surface, the more superficial the effect of UHF. So, if necessary, to act on the skin (impetigo, folliculitis, boils, acne, small abscesses, etc.), the electronic plate is placed very close to the affected area of ​​the skin.

The duration of a session for local inflammatory and nagoitelny processes is about 5-10 minutes. With a wavelength of 12 m, using five-minute sessions, very good results are obtained in the treatment of neurodermatitis, eczema, and toxic skin diseases. Sessions are made daily.

To establish the appearance of an electric field between the electrodes, a neon lamp attached to the apparatus is introduced into the electric field. When the device is working properly, the neon light starts to glow.

Submerge the stick in the pond. The water level should rise. But this increase is so negligible that it is difficult to detect it. And if you alternately immerse a stick in water and pull it out, then waves will run through the water. They are noticeable at a considerable distance from the place of origin. This mechanical movement of water can be compared to electromagnetic phenomena. A constant electromagnetic field is generated around a constant current conductor. It is difficult to find it far from the current-carrying conductor.

But if an alternating electric current is passed through the conductor, then the electromagnetic forces around the conductor will change all the time, that is, the electromagnetic field around it will be agitated. Electromagnetic waves run from an alternating current conductor.

The distance between the two nearest wave crests on a pond is the wavelength. It is denoted by a Greek letter λ (lambda). The time during which any part of the waving water surface rises, falls and returns to its initial position again - this is the period of oscillation - T... The reciprocal is called the vibration frequency and is denoted by the letter f... The vibration frequency is measured in periods per second. The unit for measuring the frequency of oscillations, corresponding to one period per second, is named hertz (hertz) - in honor of Heinrich Rudolf Hertz (1857 - 1894), the famous researcher of oscillations and waves (1,000 hertz = 1 kilohertz, 1 million hertz = 1 megahertz) ...

Wave speed ( with) is the distance the waves propagate in one second. During one period T, the wave motion has time to propagate exactly to the length of one wave X. For wave motion, the following relations are valid:

with T = λ; s / f = λ

These relationships between vibration frequency, wavelength and wave speed are true not only for waves on water, but also for any vibrations and waves.

It is necessary to immediately emphasize one property of electromagnetic oscillations. When they propagate in empty space, then, whatever their frequency, whatever the wavelength, the speed of their propagation is always the same -300 thousand km / sec. Visible light is one of the types of electromagnetic oscillations (with a wavelength of 0.4 to 0.7 nanometers and a frequency of 10 14 - 10 15 Hz). The speed of propagation of electromagnetic waves is the speed of light (3 10 10 cm / sec).

In air and in other gases, the speed of propagation of electromagnetic waves is only slightly less than in void. And in various liquid and solid media, it can be several times less than in a void; moreover, here it depends on the vibration frequency.

Radiation and emitters

We live in a world of electromagnetic waves. And sunlight, and the mysterious streams of cosmic rays falling on the Earth from interstellar space, and the heat emitted by a hotly heated furnace, and the electric current circulating in power networks - all these are electromagnetic oscillations. All of them propagate in the form of waves, in the form of rays.

Any object, any body that generates waves is called an emitter. The stick that is dangled in the pond is the emitter of water waves. Water resists its movement. It takes power to move a stick. This power transmitted to water is numerically equal to the product of the square of the speed of movement of the stick by the resistance to movement. Part of this power turns into heat - it goes to heating the water, and partly goes to the formation of waves.

We can say that the total resistance experienced by a stick is the sum of two resistances: one of them is the resistance to heat generation, and the other is the resistance to wave formation - the resistance to radiation, as it is commonly called.

The same patterns and electromagnetic phenomena. The power that the electric current consumes in a conductor is equal to the product of the resistance of the conductor by the square of the current in it. If we take the current in amperes, and the resistance in ohms, then the power will be obtained in watts.

V electrical resistance For any conductor (as in the mechanical resistance of water to the movement of a stick), two components can be distinguished: resistance to heat generation - ohmic resistance and resistance to radiation - resistance caused by the formation of electromagnetic waves around the conductor that carry away energy.

Take, for example, an electric hotplate with an ohmic resistance of 20 ohms and a current of 5 amps. The power converted into heat in this tile will be equal to 500 W (0.5 kW). To calculate the power of the waves traveling from the emitter, you need to multiply the square of the current in the conductor by the radiation resistance of this conductor.

Radiation resistance is complexly dependent on the shape of the conductor, on its size, on the length of the emitted electromagnetic wave. But for a single rectilinear conductor, at all points of which there is a current of the same direction and the same strength, the radiation resistance (in ohms) is expressed by a relatively simple formula:

R rad = 3200 (l / λ) 2

Here l is the length of the conductor, and λ is the length of the electromagnetic wave (this formula is valid when l significantly less than λ ).

For rough estimates, this formula can be used for any electrical structures, any machines and devices, for example, for a heating plate, in which the wire is not straight, but coiled into a spiral laid in a zigzag. But as l it is necessary to substitute not the full length of the conductor in the formula for the radiation resistance, but one of the given dimensions of the structure under consideration. For hot plate l approximately equal to the tile diameter.

Central power plants generate alternating current with a frequency of 50 Hz. This current corresponds to an electromagnetic wave 6 thousand km long. Not only an electric stove, but also the largest electrical machines and apparatus and even long-distance power lines are sized l many times smaller than the length of this electromagnetic wave. The radiation resistance of the largest electrical machines and apparatus for a current with a frequency of 50 Hz is measured in negligible fractions of an ohm. Even at currents of thousands of amperes, powers of less than one watt are emitted.

Therefore, in practice, when using an industrial current with a frequency of 50 Hz, it is not necessary to take into account its wave properties. The energy of this current is firmly "tied" to the wires. To connect the consumer (lamps, furnaces, motors, etc.), direct contact with the current-carrying wires is required.

With an increase in the frequency of the current, the length of the electromagnetic wave decreases. For example, for a current with a frequency of 50 MHz, it is equal to 3 m.With such a wave, even a conductor small size can have significant resistance to radiation and, at relatively low currents, emit significant amounts of energy.

According to refined calculations, a conductor with a length of half a wave (l = λ / 2) has radiation resistance R ex. about 73 ohms. With a current of, say, 10 A, the radiated power would be 7.3 kW. A conductor capable of emitting electromagnetic energy is called an antenna. This term was borrowed by electricians at the end of the last century from entomology - an antenna is called an antenna-tentacle in insects.

At the origins of radio engineering

Electromagnetic vibrations occurring at a frequency of a million billion hertz, our vision perceives as light. A thousand times slower vibrations can be felt by the skin as heat rays.

Electromagnetic vibrations, the frequency of which ranges from several kilohertz to thousands of megahertz, are not perceived by the senses, but they are of great importance in our life. These vibrations are capable of propagating, like light and heat, in the form of rays. In Latin, the word "ray" is "radius". From this root the word "radio waves" is formed. These are oscillations generated by high frequency currents. Their main, most important application is wireless telegraph and telephone communications... For the first time in the world, the wireless transmission of signals by radio waves was practically carried out by the Russian scientist Alexander Stepanovich Popov. On May 7 (April 25), 1895, at a meeting of the physics department of the Russian Physicochemical Society, he demonstrated the reception of radio waves.

Nowadays, with the help of radio, you can establish a wireless connection between any parts of the world. New branches of high-frequency technology have emerged - radar, television. Radio engineering began to be used in various industries.

It is right to start the review of high-frequency technology with methods of obtaining alternating currents of high frequency.

The oldest and simplest way to produce high-frequency electromagnetic oscillations is to discharge a capacitor through a spark. The first radio transmitters of A.S. Popov had spark generators with such simple spark gaps in the form of two balls separated by an air gap.

Machine generator of high frequency current.

At the beginning of this century, improved spark gaps appeared, which gave high-frequency oscillations with a power of up to 100 kW. But there was a great loss of energy in them. Currently, there are more advanced sources of high frequency currents (HFC).

To obtain currents with a frequency of up to several kilohertz, machine generators are usually used. Such a generator consists of two main parts - a stationary stator and a rotating rotor. The surfaces of the rotor and stator facing each other are toothed. When the rotor rotates, the mutual movement of these teeth causes a pulsation of the magnetic flux. In the working winding of the generator, laid on the stator, a variable electromotive force (emf) arises. The frequency of the current is equal to the product of the number of rotor teeth and the number of its revolutions per second. For example, with 50 teeth on the rotor and its rotation speed of 50 rpm, a current frequency of 2500 Hz is obtained.

At present, machine generators of HDTV are produced with a capacity of up to several hundred kilowatts. They give frequencies from a few hundred hertz to 10 khz.

One of the most common modern ways receiving HDTV is the use of oscillatory circuits connected to electrically controlled valves.

Nikola Tesla. The first Russian biography Rzhonsnitsky Boris Nikolaevich

Chapter Six High frequency currents. Resonance transformer. Is the electric current safe? Tesla's lecture on high frequency currents

Chapter six

High frequency currents. Resonance transformer. Is the electric current safe? Tesla's lecture on high frequency currents

According to Tesla, the year he spent in Pittsburgh was lost for research works in the field of multiphase currents. It is possible that this statement is close to the truth, but it is also possible that this very year was the beginning of the inventor's further creative successes. The discussion with the engineers at the Westinghouse plant did not pass without leaving a trace. Justification of the alternating current frequency proposed by him in 60 periods required a more thorough analysis of the economic efficiency of using both lower and higher frequencies. Tesla's scientific conscientiousness did not allow him to leave this issue without careful examination.

Returning from Europe in 1889, he set about designing a high-frequency alternator and soon created a machine with a stator consisting of 348 magnetic poles. This generator made it possible to receive alternating current with a frequency of 10 thousand periods per second. Soon he managed to create an even higher-frequency generator and began to study various phenomena at a frequency of 20 thousand periods per second.

Studies have shown that as the frequency of the alternating current increases, the volume of iron in electromagnetic motors can be significantly reduced, and starting from a certain frequency, it is possible to create electromagnets consisting of windings alone, with no iron at all in the coils. Motors constructed from such iron-free electromagnets would be extremely light, but in many other respects uneconomical, and the reduction in metal costs would not pay off due to the significant increase in electricity consumption.

Exploring wide range frequencies of alternating current initially within the limits that could be applied in a polyphase system (25-200 periods per second), Tesla soon moved on to studying the properties and possibilities of practical use of currents of increased (10-20 thousand periods per second) and high (20- 100 thousand periods per second) frequencies. To obtain a significantly larger number of periods and significantly higher voltages than could be achieved by the high-frequency current generators created by him, it was necessary to find and rely on other principles.

Well acquainted with the world literature on electrical physics and electrical engineering, Tesla studied the work of the famous American physicist Joseph Henry, who, back in 1842, suggested that some electrical discharges (including the discharge of the Leyden jar) contain not only “main discharges”, but also counter, and each subsequent one is somewhat weaker than the previous one. This was the first time the existence of a damped two-way electric discharge was noticed.

Tesla also knew that eleven years after Henry, the English physicist Lord Kelvin experimentally proved that the electrical discharge of a capacitor is a two-way process, continuing until its energy is expended to overcome the resistance of the medium. The frequency of this two-way process reaches 100 million vibrations per second. The seemingly homogeneous spark between the spark gap balls actually consists of several million sparks passing in both directions in a short period of time.

Kelvin gave a mathematical expression for the process of a two-way discharge of a capacitor. Later Fedderson, Schiller, Kirchhoff, Helmholtz and other researchers not only verified the correctness of this mathematical expression, but also significantly supplemented the theory of electric discharge.

Tesla was also familiar with the works of Anton Oberbank, who observed the phenomenon of electrical resonance, that is, the process of a sharp increase in the amplitude (range) of oscillations when the frequency of the external oscillation approaches the frequency of the natural internal oscillations of the system.

He was also well aware of the experiments of Hertz and Lodge, who were studying electromagnetic waves. Tesla was especially impressed by the experiments of Heinrich Hertz, which confirmed the theoretical assumptions of James K. Maxwell about the wave nature of electromagnetic phenomena. It should be noted that in the works of Hertz Tesla first found an indication of the phenomenon of the so-called "standing electromagnetic waves", that is, waves superimposed on one another so that in some places they reinforce each other, creating "antinodes", and in others they reduce to scratch, creating "nodes".

Knowing all this, Nikola Tesla in 1891 completed the design of the device, which played an exceptional role in the further development of various branches of electrical engineering, and especially radio engineering. To create currents of high frequency and high voltage, he decided to use the well-known property of resonance, that is, the phenomenon of a sharp increase in the amplitude natural vibrations any system (mechanical or electrical) when superimposed on them by external vibrations with the same frequency. Based on this famous phenomenon, Tesla created his own resonance transformer.

The action of a resonance transformer is based on tuning into resonance of its primary and secondary circuits. The primary circuit, containing both a capacitor and an induction coil, produces very high voltage alternating currents with frequencies of several million periods per second. The spark between the balls of the arrester causes rapid changes in the magnetic field around the primary coil of the vibrator. These changes in the magnetic field cause the appearance of a corresponding high voltage in the winding of the secondary coil, which consists of a large number of turns of thin wire, and the frequency of the alternating current in it, corresponding to the number of spark discharges, reaches several million changes per second.

The frequency reaches its highest value at the moment when the periods of the primary and secondary circuits coincide, that is, when the phenomenon of resonance is observed in these circuits.

Tesla has developed a very simple methods automatic charging of a capacitor from a current source low voltage and discharging it through an air-core transformer. The inventor's theoretical calculations showed that even with the smallest values ​​of capacitance and induction in the resonance transformer created by him, with the appropriate tuning, very high voltages and frequencies can be obtained by resonance.

The principles of electrical tuning of the resonance transformer and the ability to change the capacitance to change the wavelength of electromagnetic oscillations created by the transformer, discovered by him in 1890, became one of the most important foundations of modern radio engineering, and Tesla's thoughts about the huge role of the capacitor and, in general, capacity and self-induction in the development of electrical engineering came true ...

Tesla's resonance transformer: E - battery or other current source. J is an induction coil. BB is a spark gap. SS - battery of Leyden jars. L1 is the primary coil of the transformer. L2 - secondary coil of the transformer. K - mechanical breaker. In the bottom picture, coils L1 and L2 are immersed in oil.

When creating a resonance transformer, one more practical problem had to be solved: to find insulation for EHV coils. Tesla took up the problems of the theory of insulation breakdown and, on the basis of this theory, found The best way insulate the turns of the coils - immerse them in paraffin, linseed or mineral oil, now called transformer oil. Later, Tesla once again returned to the development of electrical insulation issues and made very important conclusions from his theory.

As soon as he began experiments with high-frequency currents, Nikola Tesla clearly imagined the enormous prospects that opened up before mankind with the widespread use of high-frequency currents. It would be a significant exaggeration to say that even then he saw all the special cases of their application in the form in which it is now, but the very direction of Tesla's work testifies to the unusually versatile conclusions that he drew from his discovery.

First of all, he came to the conviction that electromagnetic waves play an extremely important role in most natural phenomena. Interacting with each other, they either strengthen or weaken, or cause new phenomena, the origin of which we sometimes attribute to completely different reasons. But not only electromagnetic radiation play a huge role in a wide variety of natural phenomena. Tesla, by the intuition of a great scientist, understood the significance of various radiations even before the remarkable discoveries of radioactive elements. When later, in 1896, Henri Becquerel, and then Pierre and Marie Curie discovered this phenomenon, Tesla found in this confirmation of his predictions expressed by him back in 1890.

The enormous importance of alternating currents in the development of industry, which finally received the electric motor it needed, became clear to Nikola Tesla at the first acquaintance with the advantages of three-phase current, which requires only three wires to transmit it. For Tesla, already at that time it was undoubted that a way of transmitting electricity and completely without wires, using electromagnetic waves, should be discovered. This problem attracted Tesla's attention and became the subject of his studies at the end of 1889.

but practical use high-frequency currents for a wide variety of purposes required the study of at first glance the most diverse, unrelated issues. It was these experiments on a large scale that Nikola Tesla began to carry out in his laboratory.

Having begun systematic experiments with high-frequency and high-voltage currents, Tesla had to first of all develop measures to protect against the danger of electric shock. This particular, auxiliary, but very important task led him to discoveries that laid the foundation for electrotherapy - an extensive field of modern medicine.

Nikola Tesla's train of thought was extremely original. It is known, he reasoned, that D.C. low voltage (up to 36 volts) has no harmful effects on humans. As the voltage rises, the potential for injury increases rapidly. With an increase in voltage, since the resistance of the human body is practically unchanged, the current strength also increases and reaches a threatening value at 120 volts. Higher voltages become hazardous to human health and life.

Alternating current is a different matter. For him, the limit of dangerous voltage is much higher than for DC, and this limit is shifted with increasing frequency. It is known that electromagnetic waves of very high frequency do not have any painful effect on a person. An example of this is light perceived at normal brightness by a healthy eye without any painful sensations. Within what frequencies and voltages is alternating current dangerous? Where does the safe current zone begin?

Step by step Tesla investigated the effect of alternating electric current on a person at different frequencies and voltages. He conducted experiments on himself. First, through the fingers of one hand, then through both hands, finally, through the whole body, he passed currents of high voltage and high frequency. Studies have shown that the effect of electric current on the human body consists of two components: the effect of the current on tissues and cells by heating and the direct effect of the current on nerve cells.

It turned out that heating does not always cause destructive and painful consequences, and the effect of the current on nerve cells stops at a frequency of over 700 periods, similar to how a person's hearing does not respond to vibrations exceeding 2 thousand per second, and the eye does not respond to vibrations beyond the visible colors of the spectrum.

Thus, the safety of high frequency currents was established even at high voltages. Moreover, the thermal effects of these currents could be used in medicine, and this discovery by Nikola Tesla found wide application; diathermy, UHF treatment and other methods of electrotherapy are a direct consequence of his research. Tesla himself developed a number of electrothermal apparatus and devices for medicine, which have become widespread both in the United States and in Europe. His discovery was then further developed by other eminent electricians and physicians.

Once, while experimenting with high-frequency currents and bringing their voltage to 2 million volts, Tesla accidentally brought a black-painted copper disk closer to the equipment. At the same instant, a thick black cloud enveloped the disc, and immediately rose upward, and the disc itself shone, as if an invisible hand had scraped off all the paint and polished it.

Surprised, Tesla repeated the experiment, and again the paint disappeared, and the disc shone, teasing the scientist. Having repeated dozens of experiments with different metals, Tesla realized that he had discovered a way to clean them with high-frequency currents.

“It’s curious,” he thought, “but will these currents also affect the human skin, if they will not be able to remove various paints that are difficult to remove from it”.

And this experience was a success. The skin of the hand, painted with paint, instantly became clean as soon as Tesla introduced it into the field of high-frequency currents. It turned out that these currents can remove small rashes from the skin of the face, cleanse pores, kill microbes that always cover the surface of the human body in abundance.

Tesla believed that his lamps have a special beneficial effect not only on the retina of the eye, but also on the entire human nervous system. In addition, Tesla's lamps cause ozonation of the air, which can also be used in the treatment of many diseases. Continuing to engage in electrotherapy, Tesla in 1898 made a detailed report on his work in this area at the next congress of the American Electrotherapy Association in Buffalo.

In the laboratory, Tesla passed currents of 1 million volts through his body at a frequency of 100 thousand periods per second (the current reached 0.8 amperes). But, operating with currents of high frequency and high voltage, Tesla was very careful and demanded that his assistants comply with all the safety rules he had developed. So, when working with a voltage of 110-50 thousand volts at a frequency of 60-200 periods, he taught them to work with one hand in order to prevent the possibility of current flowing through the heart. Many other rules pioneered by Tesla have become firmly established in modern high voltage safety technology.

Having created a variety of equipment for the production of experiments, Tesla in his laboratory began researching a huge range of issues related to a completely new field of science, in which he was most interested in the possibilities of practical use of high-frequency and high-voltage currents. His works covered all the variety of phenomena, ranging from the generation (creation) of high-frequency currents and ending with a detailed study of various possibilities of their practical use. With each new discovery, more and more new problems arose.

As one of his particular problems, Tesla was interested in the opportunity to use the discovery by Maxwell and Hertz of the electromagnetic nature of light. He had a thought: if light is electromagnetic oscillations with a certain wavelength, could it be artificially obtained not by heating the filament of an electric incandescent lamp (which makes it possible to use only 5 percent of the energy that turns into a luminous flux), but by creating such oscillations, which would cause the appearance of light waves? This problem became the subject of research in Tesla's laboratory at the beginning of 1890.

Soon he accumulated a huge amount of facts that allowed him to move on to generalizations. However, Tesla's caution made him check each of his statements dozens and hundreds of times. He repeated each experiment hundreds of times before drawing any conclusions from it.

The unusualness of all the discoveries of Nikola Tesla and his enormous authority attracted the attention of the leaders of the American Institute of Electrical Engineers, who again, like three years ago, invited Tesla to give a lecture on his work. Tesla chose the topic: "Experiments with alternating currents of a very high frequency and their use for artificial lighting."

According to the tradition established from the first years of the institute's existence, a limited number of invitations were sent out only to the most outstanding electrical engineers. Before such a select audience on May 20, 1892, Tesla gave one of his most inspired lectures and demonstrated the experiments he had already carried out in his laboratory.

There is nothing that more could attract the attention of man and deserve to be a subject of study than nature. To understand its huge mechanism, to discover its creative forces and to know the laws governing it is the greatest goal of the human mind, - with these words, Tesla began his speech.

And now he is already demonstrating to the audience the results of his research in a new, still unexplored area of ​​high-frequency currents.

Scattering electromagnetic energy in the space surrounding the source of high-frequency currents, it allows the use of this energy for a variety of purposes, the scientist says with conviction and immediately shows a wonderful experience. He puts forward an ingenious proposition about the possibility of transmitting electricity without wires and, as proof, forces both conventional lamps incandescent and specially created lamps without filaments inside glow, introducing them into an alternating electromagnetic field of high frequency. - Lighting with lamps of this kind, says Tesla, where light does not arise under the influence of heating the filaments by the flowing current, but due to special vibrations of molecules and atoms of gas, will be easier than lighting with modern incandescent lamps. Illumination of the future, the scientist emphasized, is illumination with high-frequency currents.

Tesla dwelt in particular detail on the description of his resonance transformer as a source of waves of very high frequency and again emphasized the importance of the discharge of a capacitor in creating such oscillations. Tesla correctly assessed the great future of this the most important detail modern radio equipment. He expressed this thought in the following words:

I think that the discharge of the capacitor will play an important role in the future, since it will not only provide an opportunity to receive light more in a simple way in the sense that the theory I have outlined indicates, but will prove to be important in many other respects as well.

After detailing the results of experiments with high-frequency currents obtained with the help of a resonance transformer, Tesla concluded the lecture with words indicating his clear understanding of the importance of further study of the phenomena on which his work barely opened the veil of secrecy:

We pass with incomprehensible speed through infinite space; everything around us is in motion, and energy is everywhere. There must be a more direct way to utilize this energy than is currently known. And when light comes out of the environment around us, and when in the same way without effort all forms of energy from their inexhaustible source will be received, humanity will go forward with giant strides.

The mere contemplation of this magnificent perspective lifts our spirits, strengthens our hope and fills our hearts with the greatest joy.

To thunderous applause, Tesla finished his wonderful speech. The unusualness of everything shown and especially the bold conclusions of the scientist, who saw the revolutionary consequences of his discoveries, amazed the audience, although not everyone understood the content of the lecture as deeply as Nikola Tesla would have liked.