According to Maxwell's theory, electromagnetic oscillations arising in an oscillatory circuit can propagate in space. In his works, he showed that these waves propagate at the speed of light of 300,000 km/s. However, many scientists tried to refute Maxwell's work, one of them was Heinrich Hertz. He was skeptical of Maxwell's work and tried to conduct an experiment to disprove the propagation of the electromagnetic field.

An electromagnetic field propagating in space is called electromagnetic wave.

In an electromagnetic field, magnetic induction and electric field strength are mutually perpendicular, and from Maxwell’s theory it followed that the plane of magnetic induction and strength is at an angle of 90 0 to the direction of propagation of the electromagnetic wave (Fig. 1).

Rice. 1. Planes of location of magnetic induction and intensity ()

Heinrich Hertz tried to challenge these conclusions. In his experiments, he tried to create a device for studying electromagnetic waves. In order to obtain an emitter of electromagnetic waves, Heinrich Hertz built the so-called Hertz vibrator, now we call it a transmitting antenna (Fig. 2).

Rice. 2. Hertz vibrator ()

Let's look at how Heinrich Hertz got his radiator or transmitting antenna.

Rice. 3. Closed Hertzian oscillatory circuit ()

Having a closed oscillatory circuit (Fig. 3), Hertz began to move the plates of the capacitor in different directions and, in the end, the plates were located at an angle of 180 0, and it turned out that if oscillations occurred in this oscillatory circuit, then they enveloped this open oscillatory circuit on all sides. As a result of this, a changing electric field created an alternating magnetic field, and an alternating magnetic field created an electric one, and so on. This process came to be called an electromagnetic wave (Fig. 4).

Rice. 4. Electromagnetic wave emission ()

If a voltage source is connected to an open oscillatory circuit, then a spark will jump between the minus and plus, which is precisely an accelerating charge. Around this charge, moving with acceleration, an alternating magnetic field is formed, which creates an alternating vortex electric field, which, in turn, creates an alternating magnetic field, and so on. Thus, according to Heinrich Hertz's assumption, electromagnetic waves will be emitted. The purpose of Hertz's experiment was to observe the interaction and propagation of electromagnetic waves.

To receive electromagnetic waves, Hertz had to make a resonator (Fig. 5).

Rice. 5. Hertz resonator ()

This is an oscillatory circuit, which was a cut closed conductor equipped with two balls, and these balls were located relative to

from each other at a short distance. A spark jumped between the two resonator balls almost at the same moment when the spark jumped into the emitter (Fig. 6).

Figure 6. Emission and reception of electromagnetic waves ()

There was emission of an electromagnetic wave and, accordingly, the reception of this wave by the resonator, which was used as a receiver.

From this experience it followed that electromagnetic waves exist, they propagate, accordingly, transfer energy, and can create an electric current in a closed circuit, which is located at a sufficiently large distance from the emitter of the electromagnetic wave.

In Hertz's experiments, the distance between the open oscillatory circuit and the resonator was about three meters. This was enough to find out that an electromagnetic wave can propagate in space. Subsequently, Hertz carried out his experiments and found out how an electromagnetic wave propagates, that some materials can interfere with propagation, for example, materials that conduct electric current prevent the electromagnetic wave from passing through. Materials that do not conduct electricity allowed the electromagnetic wave to pass through.

Experiments by Heinrich Hertz showed the possibility of transmitting and receiving electromagnetic waves. Subsequently, many scientists began to work in this direction. The greatest success was achieved by the Russian scientist Alexander Popov, who was the first in the world to transmit information at a distance. This is what we now call radio; translated into Russian, “radio” means “to emit.” Wireless transmission of information using electromagnetic waves was carried out on May 7, 1895. At the University of St. Petersburg, Popov’s device was installed, which received the first radiogram; it consisted of only two words: Heinrich Hertz.

The fact is that by this time the telegraph (wired communication) and telephone already existed, and Morse code also existed, with the help of which Popov’s employee transmitted dots and dashes, which were written down and deciphered on the board in front of the commission. Popov's radio, of course, is not like the modern receivers we use (Fig. 7).

Rice. 7. Popov's radio receiver ()

Popov conducted his first studies on the reception of electromagnetic waves not with emitters of electromagnetic waves, but with a thunderstorm, receiving lightning signals, and he called his receiver a lightning marker (Fig. 8).

Rice. 8. Popov lightning detector ()

Popov's merits include the possibility of creating a receiving antenna; it was he who showed the need to create a special long antenna that could receive a sufficiently large amount of energy from an electromagnetic wave so that an alternating electric current would be induced in this antenna.

Let's consider what parts Popov's receiver consisted of. The main part of the receiver was the coherer (a glass tube filled with metal filings (Fig. 9)).

This state of iron filings has a high electrical resistance, in this state the coherer did not pass electric current, but as soon as a small spark slipped through the coherer (for this there were two contacts that were separated), the sawdust was sintered and the resistance of the coherer decreased hundreds of times.

The next part of the Popov receiver is an electric bell (Fig. 10).

Rice. 10. Electric bell in the Popov receiver ()

It was the electric bell that announced the reception of an electromagnetic wave. In addition to the electric bell, Popov's receiver had a direct current source - a battery (Fig. 7), which ensured the operation of the entire receiver. And, of course, the receiving antenna, which Popov raised in balloons (Fig. 11).

Rice. 11. Receiving antenna ()

The operation of the receiver was as follows: the battery created an electric current in the circuit in which the coherer and the bell were connected. The electric bell could not ring, since the coherer had high electrical resistance, the current did not pass, and it was necessary to select the desired resistance. When an electromagnetic wave hit the receiving antenna, an electric current was induced in it, the electric current from the antenna and the power source together was quite large - at that moment a spark jumped, the coherer sawdust sintered, and an electric current passed through the device. The bell began to ring (Fig. 12).

Rice. 12. Operating principle of the Popov receiver ()

In addition to the bell, Popov’s receiver had a striking mechanism designed in such a way that it struck the bell and the coherer simultaneously, thereby shaking the coherer. When the electromagnetic wave arrived, the bell rang, the coherer shook - the sawdust scattered, and at that moment the resistance increased again, the electric current stopped flowing through the coherer. The bell stopped ringing until the next reception of the electromagnetic wave. This is how Popov’s receiver worked.

Popov pointed out the following: the receiver can work quite well over long distances, but for this it is necessary to create a very good emitter of electromagnetic waves - this was the problem of that time.

The first transmission using Popov’s device took place at a distance of 25 meters, and in just a few years the distance was already more than 50 kilometers. Today, with the help of radio waves, we can transmit information throughout the globe.

Not only Popov worked in this area, the Italian scientist Marconi managed to introduce his invention into production almost all over the world. Therefore, the first radio receivers came to us from abroad. We will look at the principles of modern radio communications in the following lessons.

Bibliography

1. Tikhomirova S.A., Yavorsky B.M. Physics (basic level) - M.: Mnemosyne, 2012.
2. Gendenshtein L.E., Dick Yu.I. Physics 10th grade. - M.: Mnemosyne, 2014.
3. Kikoin I.K., Kikoin A.K. Physics-9. - M.: Education, 1990.

Homework

1. What conclusions of Maxwell did Heinrich Hertz try to challenge?
2. Give the definition of an electromagnetic wave.
3. Name the operating principle of the Popov receiver.

1. Internet portal Mirit.ru ().
2. Internet portal Ido.tsu.ru ().
3. Internet portal Reftrend.ru ().

: Germany - Go. Source: vol. VIIIa (1893): Germany - Go, p. 559-563 ( · index) Other sources: MESBE :

Hertz experiments.- The theory of electrical and magnetic phenomena, created by the works of the best mathematicians of the first half of this century and until recently accepted by almost all scientists, basically assumed the existence of special weightless electrical and magnetic fluids that have the property of acting at a distance. The principle of Newton's doctrine of universal gravitation - “actio in distans” - remained guiding in the doctrine of electricity and magnetism. But already in the 30s the brilliant Faraday, leaving without consideration the question of essence electricity and magnetism, expressed completely different thoughts regarding their external actions. The attraction and repulsion of electrified bodies, electrification through influence, the interaction of magnets and currents and, finally, the phenomena of Faraday induction do not represent manifestations directly at a distance of the properties inherent in electric and magnetic fluids, but are only consequences of special changes in the state of the medium in which there are these apparently directly influencing each other electric charges, magnets or conductors with currents. Since all such actions are equally observed in emptiness, as well as in space filled with air or other matter, then in the changes produced by the processes of electrification and magnetization on air, Faraday saw the reason for these phenomena. Thus, just as through the emergence of special vibrations of the ether and the transmission of these vibrations from particle to particle, a light source illuminates an object distant from it, and in this case only through special disturbances in the medium of the same ether and the transmission of these disturbances from the layer all electrical, magnetic and electromagnetic effects propagate in space to the layer. A similar idea was the guiding principle in all of Faraday's research; It was she who most importantly led him to all his famous discoveries. But it was not soon and not easy that Faraday’s teachings became stronger in science. For decades, during which the phenomena discovered by him managed to undergo the most thorough and detailed study, Faraday's basic ideas were either ignored or directly considered unconvincing and unproven. Only in the second half of the sixties did Faraday's talented follower, Clerk Maxwell, who died so early, appear, who interpreted and developed Faraday's theory, giving it a strictly mathematical character. Maxwell proved the necessity of the existence of a finite speed at which the transfer of the effects of electric current or magnet occurs through an intermediate medium. This speed, according to Maxwell, should be equal to the speed at which light propagates in the medium under consideration. The medium that takes part in the transmission of electrical and magnetic actions cannot be other than the same ether, which is allowed in the theory of light and radiant heat. The process of propagation of electrical and magnetic actions in space must be qualitatively the same as the process of propagation of light rays. All laws relating to light rays are fully applicable to electric rays. According to Maxwell, the phenomenon of light itself is an electrical phenomenon. A ray of light is a series of electrical disturbances, very small electrical currents, successively excited in the ether of the medium. What the change in the environment consists of under the influence of the electrification of some body, the magnetization of iron, or the formation of a current in some coil is still not known. Maxwell's theory does not yet make it possible to clearly imagine the very nature of the deformations it assumes. What is certain is that any change deformation of the medium produced in it under the influence of the electrification of bodies is accompanied by the emergence of magnetic phenomena in this environment and, conversely, any change in an environment of deformations resulting in it under the influence of some magnetic process, it is accompanied by the excitation of electrical actions. If at any point in the medium, deformed by the electrification of some body, an electric force is observed in a known direction, i.e., in this direction a very small electrified ball placed in a given place will begin to move, then with any increase or decrease in the deformation of the medium, together with an increase or decrease in the electric force at a given point, a magnetic force will appear in it in a direction perpendicular to the electric force - the magnetic pole placed here will receive a push in the direction perpendicular to the electric force. This is the consequence that follows from Maxwell's theory of electricity. Despite the enormous interest in the Faraday-Maxwell doctrine, it was met with doubt by many. Too bold generalizations flowed from this theory! The experiments of G. (Heinrich Hertz), carried out in 1888, finally confirmed the correctness of Maxwell's theory. G. managed, so to speak, to implement Maxwell’s mathematical formulas; he actually managed to prove the possibility of the existence of electric, or, correctly, electromagnetic rays. As already noted, according to Maxwell's theory, the propagation of a light beam is essentially the propagation of electrical disturbances successively formed in the ether, quickly changing their direction. The direction in which such disturbances, such as deformations, are excited, according to Maxwell, is perpendicular to the light beam itself. From here it is obvious that the direct excitation in any body of electrical currents very quickly changing in direction, i.e. excitation in a conductor of electric currents of alternating direction and of very short duration should cause corresponding electrical disturbances in the ether surrounding this conductor, rapidly changing in their direction , that is, it should cause a phenomenon qualitatively quite similar to what a ray of light represents. But it has long been known that when an electrified body or a Leyden jar is discharged, a whole series of electrical currents are formed in the conductor through which the discharge occurs, alternately in one direction or the other. A discharging body does not immediately lose its electricity; on the contrary, during the discharge it is recharged several times with one or the other electricity according to the sign. Successive charges appearing on the body decrease only little by little in magnitude. Such categories are called oscillatory. The duration of existence in a conductor of two successive flows of electricity during such a discharge, i.e., the duration electrical vibrations, or otherwise, the time interval between two moments at which a discharging body receives the largest charges appearing on it in succession, can be calculated from the shape and size of the discharging body and the conductor through which such a discharge occurs. According to theory, this duration of electrical oscillations (T) expressed by the formula:

T = 2 π L C . (\displaystyle T=2\pi (\sqrt (LC)).)

Here WITH stands for electrical capacity discharging body and L - self-induction coefficient conductor through which the discharge occurs (see). Both quantities are expressed according to the same system of absolute units. When using an ordinary Leyden jar, discharged through a wire connecting its two plates, the duration of electrical oscillations, i.e. T, determined in 100 and even 10 thousandths of a second. In his first experiments, G. electrified two metal balls (30 cm in diameter) differently and allowed them to discharge through a short and rather thick copper rod, cut in the middle, where an electric spark was formed between the two balls, which were mounted facing each other the ends of the two halves of the rod. Fig. 1 depicts a diagram of G.'s experiments (rod diameter 0.5 cm, ball diameter b And b′ 3 cm, the gap between these balls is about 0.75 cm and the distance between the centers of the balls S V S′ equals 1 m). Subsequently, instead of balls, G. used square metal sheets (40 cm on each side), which he placed in one plane. Charging of such balls or sheets was carried out using a functioning Ruhmkorff coil. The balls or sheets were charged many times per second from the coil and then discharged through a copper rod located between them, creating an electric spark in the gap between the two balls b And b′. The duration of the electrical oscillations excited in the copper rod exceeded a little one 100-thousandth of a second. In his further experiments, using, instead of sheets with halves of a copper rod attached to them, short thick cylinders with spherical ends, between which a spark jumped, G. received electrical vibrations, the duration of which was only about a thousand-millionth of a second. Such a pair of balls, sheets or cylinders, such vibrator, as G. calls it, from the point of view of Maxwellian theory, it is a center that propagates electromagnetic rays in space, that is, it excites electromagnetic waves in the ether, just like any light source that excites light waves around itself. But such electromagnetic rays or electromagnetic waves are not able to have an effect on the human eye. Only in the case when the duration of each electric train. the oscillation would reach only one 392-billionth of a second, the observer's eye would be impressed by these oscillations and the observer would see an electromagnetic beam. But to achieve such rapidity of electrical oscillations it is necessary vibrator, in size corresponding to physical particles. So, to detect electromagnetic rays, special means are needed; in the apt expression of V. Thomson (now Lord Kelvin), a special “electric eye” is needed. Such an “electric eye” was arranged by G in the simplest way. Let us imagine that at some distance from the vibrator there is another conductor. Disturbances in the ether excited by the vibrator should affect the state of this conductor. This conductor will be subject to a successive series of impulses, tending to excite in it something similar to what caused such disturbances in the ether, i.e., tending to form electric currents in it, changing in direction according to the speed of electrical oscillations in the vibrator itself. But impulses, successively alternating, are only able to contribute to each other when they are completely rhythmic with the electrical movements they actually cause in such a conductor. After all, only a tuned string in unison is able to vibrate noticeably from the sound emitted by another string, and is thus able to become an independent sound source. So, the conductor must, so to speak, electrically resonate with the vibrator. Just as a string of a given length and tension is capable of oscillations known in terms of speed when struck, so in each conductor an electric impulse can produce electrical oscillations of only well-defined periods. Having bent copper wire of the appropriate dimensions in the form of a circle or rectangle, leaving only a small gap between the ends of the wire with small balls stolen on them (Fig. 2), of which one, by means of a screw, could approach or move away from the other, G. received, as he did named resonator to his vibrator (in most of his experiments, when the above-mentioned balls or sheets served as the vibrator, G. used copper wire 0.2 cm in diameter, bent in the form of a circle with a diameter of 35 cm, as a resonator). For a vibrator made of short thick cylinders, the resonator was a similar circle of wire, 0.1 cm thick and 7.5 cm in diameter. For the same vibrator, in his later experiments, G. built a resonator of a slightly different shape. Two straight wires, 0.5 cm dia. and 50 cm in length, located one on top of the other with a distance between their ends of 5 cm; from both ends of these wires facing each other, two other parallel wires of 0.1 cm in diameter are drawn perpendicular to the direction of the wires. and 15 cm in length, which are attached to the spark meter balls. No matter how weak the individual impulses themselves are from disturbances occurring in the ether under the influence of a vibrator, they, nevertheless, promoting each other in action, are able to excite already noticeable electrical currents in the resonator, manifested in the formation of a spark between the balls of the resonator. These sparks are very small (they reached 0.001 cm), but are quite sufficient to be a criterion for the excitation of electrical oscillations in the resonator and, by their size, serve as an indicator of the degree of electrical disturbance of both the resonator and the ether surrounding it. By observing the sparks appearing in such a resonator, Hertz examined the space around the vibrator at different distances and in different directions. Leaving aside these experiments of G. and the results that were obtained by him, let us move on to research that confirmed the existence ultimate speed of propagation of electrical actions. A large screen made of zinc sheets was attached to one of the walls of the room in which the experiments were carried out. This screen was connected to the ground. At a distance of 13 meters from the screen, a vibrator made of plates was placed so that the planes of its plates were parallel to the plane of the screen and the middle between the vibrator balls was opposite the middle of the screen. If, during its operation, a vibrator periodically excites electrical disturbances in the surrounding ether and if these disturbances propagate in the medium not instantly, but at a certain speed, then, having reached the screen and reflected back from the latter, like sound and light disturbances, these disturbances, together with those which are sent to the screen by a vibrator, form in the ether, in the space between the screen and the vibrator, a state similar to that which occurs under similar conditions due to the interference of counterpropagating waves, i.e. in this space the disturbances will take on the character "standing waves"(see Waves). The state of the air in places corresponding to "nodes" And "antinodes" of such waves, obviously, should differ significantly. Placing his resonator with its plane parallel to the screen and so that its center was on a line drawn from the middle between the vibrator balls normal to the plane of the screen, G. observed at different distances of the resonator from the screen, the sparks in it are very different in length. Near the screen itself, almost no sparks appear in the resonator, also at distances equal to 4.1 and 8.5 m. On the contrary, sparkles are greatest when the resonator is placed at distances from the screen equal to 1.72 m, 6.3 m and 10.8 m. G. concluded from his experiments that on average 4.5 m separate from each other those positions of the resonator in which the phenomena observed in it, i.e., sparks, turn out to be closely identical. G. obtained exactly the same thing with a different position of the resonator plane, when this plane was perpendicular to the screen and passed through a normal line drawn to the screen from the middle between the vibrator balls and when axis of symmetry the resonator (i.e., its diameter passing through the middle between its balls) was parallel to this normal. Only with this position of the resonator plane maxima sparks in it were obtained where, in the previous position of the resonator, minima, and back. So 4.5 m corresponds to the length "standing electromagnetic waves" arising between the screen and the vibrator in a space filled with air (the opposite phenomena observed in the resonator in its two positions, i.e., maxima sparks in one position and minima in the other, are fully explained by the fact that in one position of the resonator electrical oscillations are excited in it electrical forces, so-called electrical deformations in the ether; in another position they are caused as a consequence of the occurrence magnetic forces, i.e. they get excited magnetic deformations).

According to the length of the “standing wave” (l) and by time (T), corresponding to one complete electrical oscillation in the vibrator, based on the theory of the formation of periodic (wave-like) disturbances, it is easy to determine the speed (v), with which such disturbances are transmitted in the air. This speed v = 2 l T . (\displaystyle v=(\frac (2l)(T)).) In G.'s experiments: l= 4.5 m, T= 0.000000028″. From here v= 320,000 (approximately) km per second, i.e. very close to the speed of light propagating in the air. G. studied the propagation of electrical vibrations in conductors, that is, in wires. For this purpose, an insulated copper plate of the same type was placed parallel to one vibrator plate, from which came a long wire stretched horizontally (Fig. 3). In this wire, due to the reflection of electrical vibrations from its insulated end, “standing waves” were also formed, the distribution of “nodes” and “antinodes” of which along the wire G. found using a resonator. G. derived from these observations for the speed of propagation of electrical vibrations in a wire a value equal to 200,000 km per second. But this definition is not correct. According to Maxwell's theory, in this case the speed should be the same as for air, i.e. it should be equal to the speed of light in air. (300,000 km per second). Experiments carried out after G. by other observers confirmed the position of Maxwell's theory.

Having a source of electromagnetic waves, a vibrator, and a means of detecting such waves, a resonator, G. proved that such waves, like light waves, are subject to reflections and refractions and that electrical disturbances in these waves are perpendicular to the direction of their propagation, i.e., he discovered polarization in electric rays. For this purpose, he placed a vibrator that produces very fast electrical oscillations (a vibrator made of two short cylinders) in the focal line of a parabolic cylindrical mirror made of zinc; in the focal line of another similar mirror he placed a resonator, as described above, made of two straight wires . By directing electromagnetic waves from the first mirror to some flat metal screen, G., with the help of another mirror, was able to determine the laws of reflection of electric waves, and by forcing these waves to pass through a large prism made of asphalt, he also determined their refraction. The laws of reflection and refraction turned out to be the same as for light waves. Using these same mirrors, G. proved that electric rays polarized, when the axes of two mirrors placed opposite each other were parallel under the action of a vibrator, sparks were observed in the resonator. When one of the mirrors was rotated about the direction of the rays by 90°, i.e., the axes of the mirrors made a right angle with each other, any trace of sparks in the resonator disappeared.

In this way, G.'s experiments proved the correctness of Maxwell's position. The G. vibrator, like a light source, emits energy into the surrounding space, which, through electromagnetic rays, is transmitted to everything that is able to absorb it, transforming this energy into another form accessible to our senses. Electromagnetic rays are quite similar in quality to rays of heat or light. Their difference from the latter lies only in the lengths of the corresponding waves. The length of light waves is measured in ten thousandths of a millimeter, while the length of electromagnetic waves excited by vibrators is expressed in meters. The phenomena discovered by G. later served as the subject of research by many physicists. In general, G.'s conclusions are fully confirmed by these studies. Now we know, moreover, that the speed of propagation of electromagnetic waves, as follows from Maxwell’s theory, changes along with changes in the medium in which such waves propagate. This speed is inversely proportional K , (\displaystyle (\sqrt (K)),) Where K the so-called dielectric constant of a given medium. We know that when electromagnetic waves propagate along conductors, electrical vibrations are “damped”; that when electric rays are reflected, their “voltage” follows the laws given by Fresnel for light rays, etc. G.’s articles concerning the phenomenon under consideration, collected together, now published under the title: H. Hertz, “Untersuchungen über die Ausbreitung der elektrischen Kraft” (Lpts., 1892).

Hertz created charge oscillations in an electric vibrator circuit and observed how sparks jumped in a nearby resonator circuit and electromagnetic oscillations arose.

Hertz's amazing experiments were then successfully repeated in many countries and laboratories around the world. As we know, the remarkable research of Alexander Stepanovich Popov began with reflection on Hertz’s experiments, which then led to the invention of radio communications.

Hertz named the vibrations he recorded rays of electric force.

Portrait of Heinrich Hertz

He discovered that electric rays interfered and were refracted in a prism made of asphalt, just as light rays were refracted in a glass or quartz prism or lens. These rays differ only in their oscillation frequency or wavelength: for Hertz's rays, the wavelength ranged from 60 centimeters to several meters, while the wavelength of light rays ranged from 0.4 to 0.75 microns.

Heinrich Hertz wrote: “...it seems very probable that the experiments described prove the identity of light, heat rays and electromagnetic wave motion.”

Hertz's experiments forced scientists to increasingly recall Maxwell's bold theory, which united all light and electrical phenomena into a single whole.

Calculations have shown that the speed of Hertz's electromagnetic waves is equal to the speed of light!

More and more scientific facts in favor of Maxwell's theory accumulated.

The relationship derived by Maxwell was confirmed, according to which the refractive index of any substance is equal to the square root of the product of its dielectric and magnetic permeability. Thus, a clear and obvious connection was established between the electrical and optical properties of the substance...

A photograph of the small setup that allowed him to discover that one oscillating circuit in a radio circuit could pick up electromagnetic waves sent by another circuit.

They found a simple explanation for the discovery of Bartolin and Malus: in a light beam containing transverse electromagnetic waves of various orientations, when reflected from dielectrics or passing through anisotropic crystals, waves remain whose oscillations lie in a strictly defined plane - polarized waves.

In 1879, the English physicist John Kerr discovered that it is possible to observe the phenomenon of double refraction in any homogeneous substance, such as a liquid or gas, under the influence of a strong electric and magnetic field.

Another confirmation of the close connection between the optical and electrical properties of matter and at the same time evidence that a gas or liquid under certain conditions becomes similar to anisotropic crystals!

How close this is to the scientific miracles of the 20th century in converting some substances into others...

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Ministry of Higher and Secondary Education of the Republic of Uzbekistan

National University of the Republic of Uzbekistan named after. Mirzo Ulugbek

Faculty of Physics

Report

Discipline: "Optics"

On the topic: “Experiments of Heinrich Hertz”

Prepared by:

2nd year student

Nebesny Andrey Anatolievich

Supervisor:

Doctor of Physical and Mathematical Sciences prof.

Valiev Uygun Vakhidovich

Tashkent 2015

Introduction

1. Statement of the problem

2. Interesting phenomenon

3. Hertz vibrator

4. Ruhmkorff coil

5. Experimenting with a vibrator

Afterword

Literature

Introduction

Heinrich Hertz was born in 1857 in Hamburg (Germany) into the family of a lawyer. Since childhood, he had an excellent memory and excellent abilities in drawing, languages, technical creativity and showed interest in the exact sciences. In 1880, at the age of 23, he graduated from the University of Berlin, brilliantly defending his doctoral dissertation on theoretical electrodynamics. Hertz's scientific supervisor was the famous European physicist G. Helmholtz, for whom Hertz worked as an assistant for the next three years.

Helmholtz, who dealt with many problems in physics, developed his own version of theoretical electrodynamics. His theory competed with the previously presented theories of W. Weber and J. C. Maxwell. These were the main three theories of electromagnetism at that time. However, experimental confirmation was required.

1. Statement of the problem

In 1879, the Berlin Academy of Sciences, on the initiative of Helmholtz, put forward a competitive task: “To establish experimentally whether there is a connection between electrodynamic forces and dielectric polarization.” The solution to this problem, i.e. experimental confirmation was supposed to answer which of the theories is correct. Helmholtz suggested that Hertz take on this task. Hertz tried to solve the problem using electrical oscillations that occur during the discharge of capacitors and inductors. However, he soon encountered a problem - much higher-frequency oscillations were required than they were able to obtain at that time.

High frequency oscillations, significantly higher than the frequency of industrial current (50 Hz), can be obtained using an oscillatory circuit. The oscillation frequency u=1/v(LC) will be greater, the lower the inductance and capacitance of the circuit.

A simple calculation shows that to create the frequencies that Hertz subsequently managed to obtain (500 MHz), a capacitor with a capacity of 2 nF and an inductor of 2 nH are needed. However, the industrial progress of that time had not yet reached the possibility of creating such small capacitances and inductances.

2. Interesting phenomenon

Having failed to solve this problem, he retained the hope of finding the answer. Since then, everything related to electrical vibrations has invariably interested him.

Later, in the fall of 1886, while debugging lecture equipment, namely, checking induction coils with a finely adjustable spark gap between the metal balls at the ends of the windings using a micrometric screw, Hertz discovered an interesting phenomenon: to excite a spark in one of the coils, it is not necessary to connect a powerful one to the second battery, the main thing is that a spark slips through the spark gap of the primary coil.

He conducted a series of experiments to confirm his observation.

3. Hertz vibrator

In his experiments, Hertz used a simple device, now called a Hertz vibrator, to produce electromagnetic waves.

This device is an open oscillatory circuit (picture on the right). The usual oscillatory circuit shown in the figure on the left (it can be called closed) is not suitable for emitting electromagnetic waves. The fact is that the alternating electric field is concentrated mainly in a very small region of space between the plates of the capacitor, and the magnetic field is concentrated inside the coil. In order for the radiation of electromagnetic waves to be sufficiently intense, the region of the alternating electromagnetic field must be large and not surrounded by metal plates. There is a similarity here with the emission of sound waves. An oscillating string or tuning fork without a resonator box emit almost no radiation, since in this case air vibrations are excited in a very small region of space directly adjacent to the string or branches of the tuning fork.

The area in which an alternating electric field is created increases if the capacitor plates are moved apart. The capacity decreases. Simultaneous reduction of the plate area will further reduce the capacity. Reducing the capacitance will increase the natural frequency of this oscillatory circuit. To increase the frequency even more, you need to replace the coil with a straight wire without turns. The inductance of the straight wire is much less than the inductance of the coil. Continuing to move the plates apart and simultaneously reducing their sizes, we will arrive at an open oscillatory circuit. It's just a straight wire. In an open circuit, the charges are not concentrated at the ends, but are distributed throughout the conductor. The current at a given moment in time in all sections of the conductor is directed in the same direction, but the current strength is not the same in different sections of the conductor. At the ends it is zero, and in the middle it reaches a maximum.

To excite oscillations in such a circuit, you need to cut the wire in the middle so that there remains a small air gap, called a spark gap. Thanks to this gap, it is possible to charge both conductors to a high potential difference.

When sufficiently large opposite charges were imparted to the balls, an electrical discharge occurred between them and free electrical oscillations appeared in the electrical circuit. After each recharge of the balls, a spark again jumps between them, and the process was repeated many times. Having placed at some distance from this circuit a coil of wire with two balls at the ends - a resonator - Hertz discovered that when a spark jumps between the balls of the vibrator, a small spark also appears between the balls of the resonator. Consequently, during electrical oscillations in an electrical circuit, a vortex alternating electromagnetic field appears in the space around it. This field creates an electric current in the secondary circuit (resonator).

Due to the small capacitance and inductance, the oscillation frequency is very high. The oscillations, of course, will be damped for two reasons: firstly, due to the presence of active resistance in the vibrator, which is especially high in the spark gap; secondly, due to the fact that the vibrator emits electromagnetic waves and loses energy. After the oscillations stop, the source again charges both conductors until breakdown of the spark gap occurs and everything repeats all over again. The figure below shows a Hertz vibrator connected in a series circuit with a galvanic battery and a Ruhmkorff coil.

In one of the first vibrators assembled by the scientist, movable tin balls with a diameter of 0.3 m were mounted on the ends of a copper wire 2.6 m long and 5 mm in diameter equipped with a spark gap in the middle as resonating ones. Subsequently, Hertz removed these balls to increase the frequency.

4. Ruhmkorff coil

The Ruhmkorff coil, which Heinrich Hertz used in his experiments, named after the German physicist Heinrich Ruhmkorff, consists of a cylindrical part with a central iron rod inside, on which a primary winding of thick wire is wound. Several thousand turns of a secondary winding made of very thin wire are wound on top of the primary winding. The primary winding is connected to a battery of chemical cells and a capacitor. A breaker (buzzer) and a switch are inserted into the same circuit. The purpose of the breaker is to quickly alternately close and open the circuit. The result of this is that with each short circuit and opening in the primary circuit, strong instantaneous currents appear in the secondary winding: when interrupted - forward (in the same direction as the current of the primary winding) and when closed - reverse. When the primary winding is closed, an increasing current flows through it. The Ruhmkorff coil stores energy in the core in the form of a magnetic field. The magnetic field energy is:

C - magnetic flux,

L -- inductance of a coil or turn with current.

When the magnetic field reaches a certain value, the armature is attracted and the circuit opens. When the circuit is opened, a voltage surge (back EMF) occurs in both windings, directly proportional to the number of turns of the windings, large in value even in the primary winding, and even larger in the secondary winding, the high voltage of which breaks the air gap between the terminals of the secondary winding (air breakdown voltage is approximately equal to 3 kV by 1mm). The back EMF in the primary winding, through the low resistance of the battery of chemical cells, charges the capacitor C.

5. Experimenting with vibratorum

Heinrich Hertz experience

Hertz received electromagnetic waves by exciting a series of pulses of rapidly alternating current in a vibrator using a high voltage source. Oscillations of electric charges in a vibrator create an electromagnetic wave. Only the oscillations in the vibrator are performed not by one charged particle, but by a huge number of electrons moving in concert.

In an electromagnetic wave, the vectors E? and B? are perpendicular to each other, and the vector E? lies in the plane passing through the vibrator, and vector B? perpendicular to this plane.

The figure shows the electric and magnetic field strength lines around the vibrator at a fixed point in time: the magnetic field induction lines are located in the horizontal plane, and the electric field strength lines are located in the vertical plane. The waves are emitted with maximum intensity in the direction perpendicular to the vibrator axis. No radiation occurs along the axis.

Hertz was not able to discover this immediately. For his experiments, he darkened his room. And he walked around with a resonator, sometimes even through a magnifying glass, observing where in the room, relative to the generator, a spark would appear.

While experimenting with his vibrator, the scientist noticed that the seemingly completely natural pattern of weakening of the spark in the resonator with increasing distance to the source of oscillations is disrupted when the resonator is near walls or next to an iron stove.

After much thought, Hertz realized that the problem was the reflection of waves, and the strange behavior of the spark in the resonator near the walls was nothing more than interference. To confirm this, he attached a grounded metal sheet to the wall and installed a vibrator opposite it. With the resonator in his hands, he began to slowly move in a direction perpendicular to the wall. It turned out that periodically, at regular intervals, the resonator fell into dead zones in which there was no spark. These were zones in which the direct wave of the vibrator met a reflected wave of the opposite phase and was extinguished, which fully confirmed the presence of interference processes.

This caused genuine delight throughout the scientific world. He then easily demonstrated the linear propagation of radiation. When the path from the vibrator to the resonator was blocked with a metal screen, the sparks in the resonator completely disappeared. At the same time, it turned out that insulators (dielectrics) are transparent to electromagnetic waves. A complete analogy with the laws of light reflection was just as easily demonstrated - for this, a vibrator and a resonator were installed on one side of a grounded metal sheet, which played the role of a mirror, and the equality of the angles of incidence and reflection was checked.

The most demonstrative experiment was the demonstration of the possibility of refraction of electromagnetic radiation. For this, a prism made of asphalt weighing over a ton was used. The prism had the shape of an isosceles triangle with a side of 1.2 meters and an apex angle of 300. By directing the “electric beam” to an asphalt prism, Hertz recorded its deviation by 320, which corresponded to an acceptable refractive index value of 1.69.

In his experiments, Hertz not only experimentally proved the existence of electromagnetic waves, but also studied all the phenomena typical of any waves: reflection from metal surfaces, refraction in a large dielectric prism, interference of a traveling wave with a wave reflected from a metal mirror, etc. Experimentally, it was also possible to measure the speed of electromagnetic waves, which turned out to be equal to the speed of light in a vacuum. These results provide one of the strongest proofs of the correctness of Maxwell's electromagnetic theory, according to which light is an electromagnetic wave.

Afterword

Just seven years after Hertz, electromagnetic waves found application in wireless communications. It is significant that the Russian inventor of radio Alexander Stepanovich Popov in his first radiogram in 1896 transmitted two words: “Heinrich Hertz”.

Lliterature

1. Library "Quantum", No. 1, 1988

2. Landsberg G.S., Optics - M.: FIZMATLIT, 2003, 848p.

3. Kaliteevsky N.I., “Wave optics”, M.: Vyssh. school, 1978, 383s

4. http://www.physbook.ru/

5. https://ru.wikipedia.org

6. http://ido.tsu.ru

7. http://alexandr4784.narod.ru

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The theory of electrical and magnetic phenomena, created by the works of the best mathematicians of the first half of this century and until recently accepted by almost all scientists, basically assumed the existence of special weightless electrical and magnetic fluids that have the property of acting at a distance. The principle of Newton's doctrine of universal gravitation - "actio in distans" - remained guiding in the doctrine of electricity and magnetism. But already in the 30s the brilliant Faraday, leaving without consideration the question of essence electricity and magnetism, expressed completely different thoughts regarding their external actions. The attraction and repulsion of electrified bodies, electrification through influence, the interaction of magnets and currents and, finally, the phenomena of Faraday induction do not represent manifestations directly at a distance of the properties inherent in electric and magnetic fluids, but are only consequences of special changes in the state of the medium in which there are these apparently directly influencing each other electric charges, magnets or conductors with currents. Since all such actions are equally observed in emptiness, as well as in space filled with air or other matter, then in the changes produced by the processes of electrification and magnetization on air, Faraday saw the reason for these phenomena. Thus, just as through the emergence of special vibrations of the ether and the transmission of these vibrations from particle to particle, a light source illuminates an object distant from it, and in this case only through special disturbances in the medium of the same ether and the transmission of these disturbances from the layer all electrical, magnetic and electromagnetic effects propagate in space to the layer. A similar idea was the guiding principle in all of Faraday's research; It was she who most importantly led him to all his famous discoveries. But it was not soon and not easy that Faraday’s teachings became stronger in science. For decades, during which the phenomena discovered by him managed to undergo the most thorough and detailed study, Faraday's basic ideas were either ignored or directly considered unconvincing and unproven. Only in the second half of the sixties did Faraday's talented follower, Clerk Maxwell, who died so early, appear, who interpreted and developed Faraday's theory, giving it a strictly mathematical character. Maxwell proved the necessity of the existence of a finite speed at which the transfer of the effects of electric current or magnet occurs through an intermediate medium. This speed, according to Maxwell, should be equal to the speed at which light propagates in the medium under consideration. The medium that takes part in the transmission of electrical and magnetic actions cannot be other than the same ether, which is allowed in the theory of light and radiant heat. The process of propagation of electrical and magnetic actions in space must be qualitatively the same as the process of propagation of light rays. All laws relating to light rays are fully applicable to electric rays. According to Maxwell, the phenomenon of light itself is an electrical phenomenon. A ray of light is a series of electrical disturbances, very small electrical currents, successively excited in the ether of the medium. What the change in the environment consists of under the influence of the electrification of some body, the magnetization of iron, or the formation of a current in some coil is still not known. Maxwell's theory does not yet make it possible to clearly imagine the very nature of the deformations it assumes. What is certain is that any change deformation of the medium produced in it under the influence of the electrification of bodies is accompanied by the emergence of magnetic phenomena in this environment and, conversely, any change in an environment of deformations resulting in it under the influence of some magnetic process, it is accompanied by the excitation of electrical actions. If at any point in the medium, deformed by the electrification of some body, an electric force is observed in a known direction, i.e., in this direction a very small electrified ball placed in a given place will begin to move, then with any increase or decrease in the deformation of the medium, together with an increase or decrease in the electric force at a given point, a magnetic force will appear in it in a direction perpendicular to the electric force - the magnetic pole placed here will receive a push in the direction perpendicular to the electric force. This is the consequence that follows from Maxwell's theory of electricity. Despite the enormous interest in the Faraday-Maxwell doctrine, it was met with doubt by many. Too bold generalizations flowed from this theory! The experiments of G. (Heinrich Hertz), carried out in 1888, finally confirmed the correctness of Maxwell's theory. G. managed, so to speak, to implement Maxwell’s mathematical formulas; he actually managed to prove the possibility of the existence of electric, or, correctly, electromagnetic rays. As already noted, according to Maxwell's theory, the propagation of a light beam is essentially the propagation of electrical disturbances successively formed in the ether, quickly changing their direction. The direction in which such disturbances, such as deformations, are excited, according to Maxwell, is perpendicular to the light beam itself. From here it is obvious that the direct excitation in any body of electrical currents very quickly changing in direction, i.e. excitation in a conductor of electric currents of alternating direction and of very short duration should cause corresponding electrical disturbances in the ether surrounding this conductor, rapidly changing in their direction , that is, it should cause a phenomenon qualitatively quite similar to what a ray of light represents. But it has long been known that when an electrified body or a Leyden jar is discharged, a whole series of electrical currents are formed in the conductor through which the discharge occurs, alternately in one direction or the other. A discharging body does not immediately lose its electricity; on the contrary, during the discharge it is recharged several times with one or the other electricity according to the sign. Successive charges appearing on the body decrease only little by little in magnitude. Such categories are called oscillatory. The duration of existence in a conductor of two successive flows of electricity during such a discharge, i.e., the duration electrical vibrations, or otherwise, the time interval between two moments at which a discharging body receives the largest charges appearing on it in succession, can be calculated from the shape and size of the discharging body and the conductor through which such a discharge occurs. According to theory, this duration of electrical oscillations (T) expressed by the formula:

T = 2π√(LC).

Here WITH stands for electrical capacity discharging body and L - self-induction coefficient conductor through which the discharge occurs (see). Both quantities are expressed according to the same system of absolute units. When using an ordinary Leyden jar, discharged through a wire connecting its two plates, the duration of electrical oscillations, i.e. T, determined in 100 and even 10 thousandths of a second. In his first experiments, G. electrified two metal balls (30 cm in diameter) differently and allowed them to discharge through a short and rather thick copper rod, cut in the middle, where an electric spark was formed between the two balls, which were mounted facing each other the ends of the two halves of the rod. Fig. 1 depicts a diagram of G.'s experiments (rod diameter 0.5 cm, ball diameter b And b" 3 cm, the gap between these balls is about 0.75 cm and the distance between the centers of the balls S V S" equals 1 m).

Subsequently, instead of balls, G. used square metal sheets (40 cm on each side), which he placed in one plane. Charging of such balls or sheets was carried out using a functioning Ruhmkorff coil. The balls or sheets were charged many times per second from the coil and then discharged through a copper rod located between them, creating an electric spark in the gap between the two balls b And b". The duration of the electrical oscillations excited in the copper rod exceeded a little one 100-thousandth of a second. In his further experiments, using, instead of sheets with halves of a copper rod attached to them, short thick cylinders with spherical ends, between which a spark jumped, G. received electrical vibrations, the duration of which was only about a thousand-millionth of a second. Such a pair of balls, sheets or cylinders, such vibrator, as G. calls it, from the point of view of Maxwellian theory, it is a center that propagates electromagnetic rays in space, that is, it excites electromagnetic waves in the ether, just like any light source that excites light waves around itself. But such electromagnetic rays or electromagnetic waves are not able to have an effect on the human eye. Only in the case when the duration of each electric train. the oscillation would reach only one 392-billionth of a second, the observer's eye would be impressed by these oscillations and the observer would see an electromagnetic beam. But to achieve such rapidity of electrical oscillations it is necessary vibrator, in size corresponding to physical particles. So, to detect electromagnetic rays, special means are needed; in the apt expression of V. Thomson (now Lord Kelvin), a special “electric eye” is needed. Such an “electric eye” was arranged by G in the simplest way. Let us imagine that at some distance from the vibrator there is another conductor. Disturbances in the ether excited by the vibrator should affect the state of this conductor. This conductor will be subject to a successive series of impulses, tending to excite in it something similar to what caused such disturbances in the ether, i.e., tending to form electric currents in it, changing in direction according to the speed of electrical oscillations in the vibrator itself. But impulses, successively alternating, are only able to contribute to each other when they are completely rhythmic with the electrical movements they actually cause in such a conductor. After all, only a tuned string in unison is able to vibrate noticeably from the sound emitted by another string, and is thus able to become an independent sound source. So, the conductor must, so to speak, electrically resonate with the vibrator. Just as a string of a given length and tension is capable of oscillations known in terms of speed when struck, so in each conductor an electric impulse can produce electrical oscillations of only well-defined periods. Having bent copper wire of the appropriate dimensions in the form of a circle or rectangle, leaving only a small gap between the ends of the wire with small balls stolen on them (Fig. 2), of which one, by means of a screw, could approach or move away from the other, G. received, as he did named resonator to his vibrator (in most of his experiments, when the above-mentioned balls or sheets served as the vibrator, G. used copper wire 0.2 cm in diameter, bent in the form of a circle with a diameter of 35 cm, as a resonator).

For a vibrator made of short thick cylinders, the resonator was a similar circle of wire, 0.1 cm thick and 7.5 cm in diameter. For the same vibrator, in his later experiments, G. built a resonator of a slightly different shape. Two straight wires, 0.5 cm dia. and 50 cm in length, located one on top of the other with a distance between their ends of 5 cm; from both ends of these wires facing each other, two other parallel wires of 0.1 cm in diameter are drawn perpendicular to the direction of the wires. and 15 cm in length, which are attached to the spark meter balls. No matter how weak the individual impulses themselves are from disturbances occurring in the ether under the influence of a vibrator, they, nevertheless, promoting each other in action, are able to excite already noticeable electrical currents in the resonator, manifested in the formation of a spark between the balls of the resonator. These sparks are very small (they reached 0.001 cm), but are quite sufficient to be a criterion for the excitation of electrical oscillations in the resonator and, by their size, serve as an indicator of the degree of electrical disturbance of both the resonator and the ether surrounding it.

By observing the sparks appearing in such a resonator, Hertz examined the space around the vibrator at different distances and in different directions. Leaving aside these experiments of G. and the results that were obtained by him, let us move on to research that confirmed the existence ultimate speed of propagation of electrical actions. A large screen made of zinc sheets was attached to one of the walls of the room in which the experiments were carried out. This screen was connected to the ground. At a distance of 13 meters from the screen, a vibrator made of plates was placed so that the planes of its plates were parallel to the plane of the screen and the middle between the vibrator balls was opposite the middle of the screen. If, during its operation, a vibrator periodically excites electrical disturbances in the surrounding ether and if these disturbances propagate in the medium not instantly, but at a certain speed, then, having reached the screen and reflected back from the latter, like sound and light disturbances, these disturbances, together with those which are sent to the screen by a vibrator, form in the ether, in the space between the screen and the vibrator, a state similar to that which occurs under similar conditions due to the interference of counterpropagating waves, i.e. in this space the disturbances will take on the character "standing waves"(see Waves). The state of the air in places corresponding to "nodes" And "antinodes" of such waves, obviously, should differ significantly. Placing his resonator with its plane parallel to the screen and so that its center was on a line drawn from the middle between the vibrator balls normal to the plane of the screen, G. observed at different distances of the resonator from the screen, the sparks in it are very different in length. Near the screen itself, almost no sparks appear in the resonator, also at distances equal to 4.1 and 8.5 m. On the contrary, sparkles are greatest when the resonator is placed at distances from the screen equal to 1.72 m, 6.3 m and 10.8 m. G. concluded from his experiments that on average 4.5 m separate from each other those positions of the resonator in which the phenomena observed in it, i.e., sparks, turn out to be closely identical. G. obtained exactly the same thing with a different position of the resonator plane, when this plane was perpendicular to the screen and passed through a normal line drawn to the screen from the middle between the vibrator balls and when axis of symmetry the resonator (i.e., its diameter passing through the middle between its balls) was parallel to this normal. Only with this position of the resonator plane maxima sparks in it were obtained where, in the previous position of the resonator, minima, and back. So 4.5 m corresponds to the length "standing electromagnetic waves" arising between the screen and the vibrator in a space filled with air (the opposite phenomena observed in the resonator in its two positions, i.e., maxima sparks in one position and minima in the other, are fully explained by the fact that in one position of the resonator electrical oscillations are excited in it electrical forces, so-called electrical deformations in the ether; in another position they are caused as a consequence of the occurrence magnetic forces, i.e. they get excited magnetic deformations).

Along the length of the "standing wave" (l) and by time (T), corresponding to one complete electrical oscillation in the vibrator, based on the theory of the formation of periodic (wave-like) disturbances, it is easy to determine the speed (v), with which such disturbances are transmitted in the air. This speed

v = (2l)/T.

In G.'s experiments: l= 4.5 m, T= 0.000000028". From here v= 320,000 (approximately) km per second, i.e. very close to the speed of light propagating in the air. G. studied the propagation of electrical vibrations in conductors, that is, in wires. For this purpose, an insulated copper plate of the same type was placed parallel to one vibrator plate, from which came a long wire stretched horizontally (Fig. 3).

In this wire, due to the reflection of electrical vibrations from its insulated end, “standing waves” were also formed, the distribution of “nodes” and “antinodes” of which along the wire G. found using a resonator. G. derived from these observations for the speed of propagation of electrical vibrations in a wire a value equal to 200,000 km per second. But this definition is not correct. According to Maxwell's theory, in this case the speed should be the same as for air, i.e. it should be equal to the speed of light in air. (300,000 km per second). Experiments carried out after G. by other observers confirmed the position of Maxwell's theory.

Having a source of electromagnetic waves, a vibrator, and a means of detecting such waves, a resonator, G. proved that such waves, like light waves, are subject to reflections and refractions and that electrical disturbances in these waves are perpendicular to the direction of their propagation, i.e., he discovered polarization in electric rays. For this purpose, he placed a vibrator that produces very fast electrical oscillations (a vibrator made of two short cylinders) in the focal line of a parabolic cylindrical mirror made of zinc; in the focal line of another similar mirror he placed a resonator, as described above, made of two straight wires . By directing electromagnetic waves from the first mirror to some flat metal screen, G., with the help of another mirror, was able to determine the laws of reflection of electric waves, and by forcing these waves to pass through a large prism made of asphalt, he also determined their refraction. The laws of reflection and refraction turned out to be the same as for light waves. Using these same mirrors, G. proved that electric rays polarized, when the axes of two mirrors placed opposite each other were parallel under the action of a vibrator, sparks were observed in the resonator. When one of the mirrors was rotated about the direction of the rays by 90°, i.e., the axes of the mirrors made a right angle with each other, any trace of sparks in the resonator disappeared.

In this way, G.'s experiments proved the correctness of Maxwell's position. The G. vibrator, like a light source, emits energy into the surrounding space, which, through electromagnetic rays, is transmitted to everything that is able to absorb it, transforming this energy into another form accessible to our senses. Electromagnetic rays are quite similar in quality to rays of heat or light. Their difference from the latter lies only in the lengths of the corresponding waves. The length of light waves is measured in ten thousandths of a millimeter, while the length of electromagnetic waves excited by vibrators is expressed in meters. The phenomena discovered by G. later served as the subject of research by many physicists. In general, G.'s conclusions are fully confirmed by these studies. Now we know, moreover, that the speed of propagation of electromagnetic waves, as follows from Maxwell’s theory, changes along with changes in the medium in which such waves propagate. This speed is inversely proportional √K, Where TO the so-called dielectric constant of a given medium. We know that when electromagnetic waves propagate along conductors, electrical vibrations are “damped,” that when electric rays are reflected, their “voltage” follows the laws given by Fresnel for light rays, etc.

G.'s articles concerning the phenomenon under consideration, collected together, are now published under the title: H. Hertz, “Untersuchungen über die Ausbreitung der elektrischen Kraft” (Lpts., 1892).

AND. Borgman.

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