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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
Radiotelegraph. History of invention and production
Directory / The history of technology, technology, objects around us Radio telegraph - a means for transmitting (receiving) text information by radio. The letters of the alphabet are represented by a combination of dots and dashes (Morse code). Currently, this technology is used mostly by amateur enthusiasts, as well as in the signals of various radio beacons and less often in intercoms.
Wireless radiotelegraphy is rightly considered the greatest invention of the late XNUMXth century, which opened a new era in the history of human progress. Just as the old electric telegraph laid the foundation for electrical engineering, the creation of the radiotelegraph served as the starting point for the development of radio engineering, and then electronics, the grandiose successes of which we now see everywhere. Another interesting parallel can be noted in the history of these two inventions: the creators of the telegraph, Semering and Schilling, were the first inventors who tried to use the recently discovered curiosity - electric current, for the benefit of man, and the operation of Popov and Marconi's radio telegraphs was based on the newly discovered phenomenon of electromagnetic radiation. As then, so now, communication technology was the first to demand and use the latest achievements of science. In an electric telegraph, the signal carrier is an electric current. In radiotelegraphy, this carrier is electromagnetic waves that propagate in space at great speed and do not require any wires for themselves. The discovery of electric current and the discovery of electromagnetic waves are exactly one hundred years apart, and by their example one can see what striking successes physics has achieved during this century. If the electric current, as we remember, was discovered by Galvani quite by accident, then electromagnetic waves first manifested themselves as a result of a completely purposeful experiment by Hertz, who knew perfectly well what and how to look for, because even twenty years before his remarkable discovery, the existence of electromagnetic waves with mathematical precision was predicted by the great English physicist Maxwell. To understand the principle of radiotelegraphy, let's remember what an electric field is and what a magnetic field is. Let's take a plastic ball and rub it with a woolen cloth - after that the ball will acquire the ability to attract small pieces of paper and rubbish. It, as they usually say, will become electrified, that is, it will receive a certain electric charge on its surface. In one of the previous chapters, it was already reported that this charge can be negative and positive, and two equally charged balls will repel each other with a certain force, and two balls with opposite charges will attract. Why is this happening? At one time, Faraday suggested that each ball creates around itself some kind of invisible perturbation, which he called the electric field. The field of one charged ball acts on another ball, and vice versa. At present, Faraday's hypothesis is accepted by science, although nothing is known about the nature of this field, what it is as such. In addition to the fact that the electric field exists, only two of its undoubted properties are obvious: it propagates in space around any charged body with a huge, albeit finite, speed of 300000 km / s and acts on any other electrically charged body that finds itself in this field, attracting or pushing it away with a certain force. A variation of this effect can be considered an electric current. As already mentioned, any electric current is a directed movement of charged particles. For example, in metals, this is the movement of electrons, and in electrolytes, the movement of ions. What makes these particles move ordering in one direction? The answer is known: this force is the electric field. When the circuit is closed in the conductor along its entire length from one pole of the power source to the other, an electric field arises that acts on charged particles, forcing them to move in a certain way (for example, in an electrolyte, positively charged ions are attracted to the cathode, and negatively charged ones to the anode) . Much of what has been said about the electric field can be attributed to the magnetic field. Everyone has dealt with permanent metal magnets and is aware of their property to attract and repel each other, depending on which poles - like or opposite - they are directed towards each other. The interaction of magnets is explained by the fact that a magnetic field arises around any of them, and the field of one magnet acts on another magnet, and vice versa. It has already been noted that a magnetic field arises in space around each moving charge, and any electric current (which - we repeat this again - is a directed flow of charged particles) generates a magnetic field around itself. The opposite phenomenon was also discussed - the phenomenon of electromagnetic induction, when a changing magnetic field induces an electric current in conductors. But why does this current arise, and why does it occur only when the magnetic field changes? Let's try to figure this out. Let's take the transformer already discussed above, which is two coils put on one core. By connecting the primary winding of the transformer to the network, we will get a current in the secondary winding. This means that the electrons in the secondary winding began to move in a direction, that is, some kind of force began to act on them. What is the nature of this force? For a long time, scientists and electrical engineers were at an impasse in front of this question. Already using transformers, they could not fully understand the processes that took place in them. It was only obvious that this phenomenon could not be explained solely by the action of a magnetic field. An interesting hypothesis explaining this and many other electrical phenomena was put forward in 1864 by the famous English physicist Maxwell. To understand it, we note that the process that occurs in the secondary winding of a transformer is very similar to that observed in any conductor of a closed electrical circuit - both here and there the electrons begin to move in a directed manner. But in the conductor of the circuit, this happens under the influence of an electric field. Perhaps an electric field also arises in the secondary winding of the transformer? But where does it come from? In a closed circuit, an electric field appears due to the inclusion of a current source (battery or generator) in it. But in the secondary circuit of the transformer, as you know, there are no external sources of current. Maxwell suggested that the electric field arises here under the influence of a changing magnetic field. He went further and began to assert that these two fields are closely related to each other, that any changing magnetic field generates an electric field, and any changing electric field generates a magnetic one, and that they cannot exist without each other at all, representing, as it were, a single electromagnetic field. Maxwell's theory can be explained by the following simple example. Imagine that a charged ball is suspended from a spring. If we pull it down and then let it go, the ball will begin to oscillate around some point of equilibrium. Suppose that these oscillations occur with a very high frequency (that is, the ball has time to rise and fall several hundred or even thousands of times in one second). Now we will measure the magnitude of the electric field strength at some point near the ball. Obviously, it is not a constant value: when the ball approaches, the tension will increase, when it moves away, it will decrease. The period of these changes will obviously be equal to the period of oscillations of the ball. In other words, an alternating electric field arises at this point. Following Maxwell's hypothesis, we must assume that this changing electric field will generate around itself a magnetic field changing with the same periodicity, and the latter will cause the appearance of an alternating electric field already at a greater distance from the charge, and so on. Thus, a system of periodically changing electric and magnetic fields will appear in the space surrounding the ball. A so-called electromagnetic wave is formed, running in all directions from an oscillating charge at a speed of 300000 km / s. With each new oscillation of the ball, another electromagnetic wave is radiated into space. How many vibrations, so many waves. But no matter how many waves are emitted per unit time, the speed of their propagation is strictly constant. If we assume that the ball makes one oscillation per second, then during this time the "head" part of the wave will be at a distance of 300000 km from the radiation source. If the frequency is 1000000 oscillations per second, then all these waves will fill space in 1 second, counting in a straight line away from the radiation source 300000 km. The share of each individual wave will have a path of 300 m. Thus, the length of each wave is directly related to the oscillation frequency of the system that generated it. Note that this wave, as it were, has all the conditions for its propagation in itself. Although each dense medium weakens its strength to one degree or another, an electromagnetic wave can, in principle, propagate in air and water, pass through wood, glass, human flesh. However, the best medium for it is vacuum. Now let's see what happens if there is a conductor in the path of propagation of an electromagnetic wave. Obviously, the electric field of the wave will act on the electrons of the conductor, which, as a result, will begin to move in a directed manner, that is, an alternating electric current will appear in the conductor, having the same oscillation period and the same frequency as the electric field that generated it. Thus, it is possible to give an explanation for the phenomenon of electromagnetic induction discovered by Faraday. It is clear that our example is somewhat ideal. In real conditions, the electromagnetic field emitted by an oscillating charged ball will be very weak, and its intensity at a large distance is practically zero. The current induced in the secondary conductor will be so small that no devices will register it. For this reason, during the life of Maxwell, his theory did not receive experimental confirmation. Many scientists shared his views and were looking for a way to help detect electromagnetic waves. Experiments in this direction became the starting point for the development of radio engineering. Only in 1886 did the German physicist Hertz conduct an experiment that confirmed Maxwell's theory. To excite electromagnetic waves, Hertz used a device that he called a vibrator, and for detection - another device - a resonator.
The Hertz vibrator consisted of two rods of the same length, which were attached to the clamps of the secondary winding of the induction coil. On the ends of the rods facing each other, small metal balls were strengthened. When the inductive current passed through the secondary winding of the coil, a spark jumped between the balls, and electromagnetic waves were emitted into the surrounding space. Hertz's resonator consisted of a wire bent into a ring, at both ends of which metal balls were also strengthened. Under the action of an alternating magnetic field of an electromagnetic wave, an alternating electric current was induced in the resonator, as a result of which a discharge occurred between the balls. Thus, during the discharge in the vibrator, a spark jump between the resonator balls was observed. This phenomenon could be explained only on the basis of Maxwell's theory, so that thanks to Hertz's experience, the existence of electromagnetic waves was clearly proved. Hertz was the first person to consciously control electromagnetic waves, but he never set himself the task of creating a device that would allow wireless radio communication. However, Hertz's experiments, the description of which appeared in 1888, interested physicists around the world. Many scientists began to look for ways to improve the emitter and receiver of electromagnetic waves. The Hertzian resonator was a device of very low sensitivity and therefore could capture the electromagnetic waves emitted by the vibrator only within the room. First, Hertz managed to transmit over a distance of 5, and then - 18 m. In 1891, the French physicist Edouard Branly discovered that metal filings placed in a glass tube, when an electric current is passed through them, do not always show the same resistance. When electromagnetic waves appeared near the tube, for example, from a spark obtained by means of a Ruhmkorff coil, the resistance of the sawdust quickly fell and was restored only after they were slightly shaken. Branly pointed out that this property of theirs can be used to detect electromagnetic waves.
In 1894, the English physicist Lodge first used the Branly tube, which he called a "coherer" (from the Latin coheare - to link, to bind) in order to register the passage of electromagnetic waves. This made it possible to increase the reception range to several tens of meters. To restore the sensitivity of the coherer after the passage of electromagnetic waves, Lodge installed a continuously operating clockwork that constantly shook it. In fact, Lodge only had to take a step to create a radio receiver, but he did not take this step. For the first time, the idea of the possibility of using electromagnetic waves for communication needs was presented by the Russian engineer Popov. He pointed out that the transmitted signals can be given a certain duration (for example, some signals can be made longer, others shorter) and, using Morse code, dispatches can be transmitted without wires. However, this device made sense only if it was possible to achieve a stable radio transmission over a long distance. Having studied the tubes of Branly and Lodge, Popov set about developing an even more sensitive coherer. In the end, he managed to create a very sensitive coherer with platinum electrodes filled with iron filings.
The next problem was to improve the process of shaking the sawdust after sticking together caused by the passage of an electromagnetic wave. The clock mechanism used by Lodge to restore the sensitivity of the coherer did not provide reliable operation of the circuit: such shaking was erratic and could lead to missed signals. Popov was looking for an automatic method that would allow the sensitivity of the coherer to be restored only after the signal was received. Having done many experiments, Popov invented a method for periodically shaking the coherer with the help of an electric bell hammer and used an electric relay to turn on the circuit of this bell. The scheme developed by Popov was highly sensitive, and already in 1894 he was able to use it to receive signals at a distance of several tens of meters. During these experiments, Popov drew attention to the fact that the range of the receiver noticeably increases if a vertical wire is connected to the coherer. So the receiving antenna was invented, using which Popov made significant improvements to the operating conditions of the receiver. By 1895 he had created what was the first radio receiver in history. This radio receiver was arranged as follows. The sensitive tube with metal filings (coherer) was strengthened in a horizontal position; a piece of wire, which was a receiving antenna, was attached to one end of the tube, and a grounded wire was attached to the other end. The electric circuit of the battery was closed through a coherer and an electromagnetic relay: due to the high resistance of the sawdust in the tube (up to 100000 ohms), the current in the battery circuit was insufficient to attract the relay armature. But as soon as the tube was exposed to electromagnetic waves, the sawdust stuck together, and the resistance of the tube was significantly reduced. The current in the circuit increased, and the armature of the relay was attracted. In this case, the second circuit was closed, and the current was directed through the windings of the bell relay, as a result of which the bell came into action. The hammer struck the bell, and the chain opened. The hammer returned to its original position under the action of a spring and hit the tube, shaking the sawdust. Thus, the tube was again made sensitive to electromagnetic waves.
On May 7, 1895, Popov demonstrated the operation of his radio receiver during a report at a meeting of the Russian Physical and Chemical Society. The source of electromagnetic oscillations in his experiments was Hertz's transmitting vibrator, only in Popov's transmitter was the spark gap switched on between the antenna and the ground. In January 1896, an article by Popov describing his successor was published in the journal of this society. Then Popov attached a Morse telegraph apparatus to his scheme and entered the recording on tape. The result was the world's first radiotelegraph - a transmitter and receiver with the recording of signals in Morse code.
Let's take a closer look at his device. A Morse telegraph key was connected between the battery and the primary winding of the Ruhmkorff coil. When this key was closed, the direct current of the battery went through the turns of the winding. The breaker with a high frequency closed and opened the circuit, as a result of which (see the chapter "Transformer") the direct current was converted into alternating current. Due to electromagnetic induction, a high-voltage alternating current was induced in the secondary winding of the Ruhmkorff coil. This winding was closed to a spark gap. Thus, each closing of the telegraph key generated streams of sparks in the spark gap. Short or longer circuits produced short and long streams of sparks that corresponded to the dots and dashes of Morse code. One pole of the arrester was grounded, and the other was connected to an antenna, which radiated the electromagnetic waves generated by the arrester into the surrounding space. Some of these waves hit the receiver antenna and induced a weak alternating current in it. Moreover, the duration of each received current pulse exactly corresponded to the duration of the spark gap signal. The device of the receiver was almost the same as in the previous model: the coherer was connected to a battery and an electromagnet, the relay of which, using a local battery, actuated a Morse writing apparatus included in the circuit instead of a bell. As long as the coherer was not exposed to electromagnetic waves, its resistance was so great that no current flowed in the coherer circuit. When electromagnetic waves acted on the coherer, its resistance greatly decreased, and the current in the circuit increased so much that the electromagnet attracted its armature, turning on the telegraph circuit. This attraction did not stop as long as the electromagnetic waves acted on the coherer. Simultaneously with the closing of the circuit, a hammer came into action, which struck the coherer. The resistance of the latter increased. However, if the waves continued to act, then the resistance immediately decreased again and the state of small resistance continued despite the shaking. All this time the telegraph apparatus drew a line on the tape. And only when the influence of electromagnetic waves ceased, the shaking effect was manifested, and the resistance increased to the previous value - the apparatus was switched off until a new wave appeared. Thus, dots and dashes were drawn on the telegraph tape, corresponding to the signals of the dispatch being sent. On March 24, 1896, Popov demonstrated his equipment at a meeting of the Russian Physical and Chemical Society and transmitted signals over a distance of 250 m. The world's first radiogram consisted of two words "Heinrich Hertz". Simultaneously with Popov, the young Italian Guglielmo Marconi created his radiotelegraph installation. Since childhood, he was passionately interested in electricity, and then became interested in the idea of a wireless telegraph. In 1896, he assembled a transmitter and receiver, very similar in design to those invented by Popov. In the same year, Marconi brought his invention to England. His mother was an Englishwoman, and thanks to her connections he was well received in the British Isles. In 1896, Marconi received an English patent for his radiotelegraph (this was the first patent taken for telegraphy without wires; thus, from a formal point of view, Marconi is quite rightly considered the inventor of the radio, since he was the first to patent his invention). In June 1897, a joint-stock company was organized to apply Marconi's invention. At 23, he showed amazing ingenuity and enterprise. From the very first steps, his enterprise received a solid financial basis. Whenever possible, Marconi tried to demonstrate the benefits of a new means of wireless communication. So, in June 1898, traditional sailing races were to take place in the Dublin area. These races have always attracted everyone's attention. Marconi went to Dublin and agreed with one of the major Irish newspapers that he would transmit to her by radio from a steamer in the racing area, all information that the public might be interested in for publication in special editions of the newspaper. The experience was a complete success. For several hours, Marconi led the transfer, which was accepted by the editors. The information obtained in this way was ahead of all others, and the newspaper significantly increased its circulation. For Marconi, this was also a great success: in a short time, the share capital of his company doubled, reaching 200 thousand pounds sterling. This gave him the opportunity to quickly improve his radiotelegraph. A few years later, he was already significantly ahead of Popov in his developments. One of the main elements of the first radio receivers was the coherer. It is natural, therefore, that the main efforts of inventors who sought to increase the sensitivity of receiving apparatuses were directed precisely at its improvement. Marconi was the first to draw attention to an important property of a coherer, namely, to the dependence of its action on the magnitude of the high-frequency oscillation voltage applied to it. In order to fully collect the energy of the magnetic field created by the negligibly small current induced in the antenna, it was necessary to amplify it. Marconi found a simple and ingenious way to solve this problem. In 1898, he included in his radio jigger (which means "sorter") - a high-frequency transformer, the primary winding of which was connected to the same circuit as the antenna, and the secondary winding was connected to the coherer. In the same year, Marconi took out a patent for this scheme.
The conductors a and b here designate the antenna circuit into which the primary winding of the jigger c has been included. As a result of the transformation, the voltage of the weak antenna current in the secondary circuit increased significantly. From the jigger d, the signal went to the coherer j, to which the battery b' was connected and the relay K, which turned on the telegraph apparatus, as was the case in the previous circuits. This simple innovation made it possible to increase the sensitivity of the first radio receiving stations several times over. The transmission range immediately increased from 30 to 85 miles. In the same year, Marconi made a transfer across the English Channel. Another extremely important step towards increasing the sensitivity of the receiver was made in 1899 by Rybkin, Popov's closest assistant. In one of the experiments conducted by him, it turned out that due to the distance, the instruments did not work. Not being sure of their complete serviceability, Rybkin tried to include an ordinary telephone receiver in the coherer circuit instead of a relay and a telegraph apparatus and found out that each discharge at the station causes a slight crackle in the telephone, so that any dispatch could be easily received by ear. The most striking thing here was that the coherer, with this inclusion, did not require shaking. This phenomenon, at that time not entirely understood, was explained only a few years later. The fact is that if the coherer usually worked as a variable resistance, which, as a result of sintering of metal grains, changed almost from infinity to a relatively small value, then in this scheme it acted on a completely different basis and was nothing more than a detector in the modern sense of this word, that is, a device that passed current in only one direction, had one-sided conductivity and turned (rectified) alternating current into a pulsating direct current. The negligible antenna currents rectified by the detector were completely insufficient to actuate the telegraph relay, but on the other hand they were able to act on a very sensitive device - the telephone receiver membrane, generating weak sound waves in the same way as it was in an ordinary telephone. Putting the phone to your ear, you could hear long and short crackles, corresponding to the dots and dashes of Morse code. The receiving device with the transition to the phone has been greatly simplified. There was no mechanism for recording telegraph signs, the battery decreased, and the need for constant shaking of metal powder disappeared. If in the previous receiver, which worked for a recording apparatus, interference from lightning discharges often led to false trips of the relay and distorted the records, then aural reception with a known telegraphist's skill made it more possible to isolate correctly alternating telegraph characters against the background of a chaotic crackle of interference. But the most significant advantage of the new receiver was its greater sensitivity. The next step in the improvement of radio receivers was associated with an increase in their selectivity, since the very first attempts to move from experiments to the practical use of electromagnetic waves for transmitting signals over a distance showed with all their sharpness that the further development of this new type of communication and its widespread use would be possible only in in the event that effective methods are found that allow several transmitting stations to operate simultaneously on the air. For the case with a wired connection, this problem was then solved very simply. It was enough to connect each of the receiving devices located at some point with their individual wires to the corresponding transmitting installation. But what should have been done in the case of wireless transmission? The experiments of the first stations of Popov and Marconi immediately revealed all the imperfections in this respect of the equipment used at that time. Reception of signals in the coverage area of two simultaneously operating stations turned out to be completely impossible due to mutual interference. A way out was found in the transmission of radiotelegraph signals by waves of various lengths, using the phenomenon of resonance to isolate them in the receiving device.
To understand the essence of this method, let us consider in more detail the properties of an inductive coil and a capacitor. Imagine a coil with a large number of turns, through which an alternating current passes. A changing electric current, as mentioned before, generates a changing magnetic field in the surrounding space, which in turn creates a changing electric field. This electric field induces an electric current in the turns of the coil, directed towards the main one - a phenomenon called self-induction occurs. Outwardly, this effect is manifested, in particular, in the fact that when the circuit is closed, the current in any coil does not reach its maximum value immediately, but with some delay compared, for example, with a conventional straight conductor. When the network is opened, the changing electric field induces a current in the coil that coincides in direction with the main one, and therefore the current in the coil remains for some time after the power is turned off. This property of the coil to delay and, as it were, retain the current in itself for some time without any external influence is characterized by a special value called inductance. Each coil has its own inductance, the value of which depends on the size of the conductor and its shape, but does not depend on the current flowing. As for the capacitor, it usually consists of two plates located very close opposite each other, but separated by a dielectric, that is, a substance that does not transmit electric current. The plates of a capacitor are called its plates. If you connect the capacitor plates to the poles of a DC source (for example, to an electric battery), then an electric charge will accumulate on them, which will remain even after the battery is disconnected. The ability of a capacitor to store a charge is determined by its electrical capacitance. Each capacitor has its own capacitance, and its value depends on the area of the plates, on the distance between them and on the properties of the dielectric separating them. If the capacitor plates are connected with a piece of wire, then its rapid discharge will occur - the electrons from the plate where they were in excess will flow to another where they were not enough, after which the charge of each of the plates will be equal to zero. Well, what if the capacitor is not discharged on itself, but through an induction coil? In this case, a very interesting phenomenon is observed. Imagine a charged capacitor with a coil attached to its plates. Obviously, the capacitor will begin to discharge, and an electric current will appear in the circuit, but its strength will not immediately reach its maximum value, but will increase gradually due to the phenomenon of self-induction in the coil. At the moment when the capacitor is completely discharged, the current in the coil will reach its maximum value. What will happen? Despite the fact that both plates of the capacitor will already have a zero charge, the flow of current through the coil will continue, since, due to the same self-induction, the current in the coil cannot stop instantly. It is as if the coil will turn for a few moments into a current source and will charge the capacitor in the same way as an electric battery did. Only now the charges of the plates are reversed - the one that was negatively charged before becomes positive, and vice versa. As a result, when the current in the coil is zero, the capacitor will be charged again. However, at the same moment it will again begin to discharge through the coil, and the whole process will be repeated in the opposite direction. If there were no inevitable losses of electricity, such a recharge could take an arbitrarily long time. The described phenomenon is called electrical oscillations, and the capacitor-coil system in which these oscillations occur is called an oscillatory circuit. Depending on how many times in one second the capacitor has time to recharge, they talk about one or another oscillation frequency. The oscillation frequency is directly related to the properties of the oscillatory circuit, primarily the inductance of the coil and the capacitance of the capacitor. It is noticed that the smaller these values, the greater the frequency of oscillations in the circuit, that is, the capacitor has time to recharge more times in one second. Like any oscillations (for example, oscillations of a pendulum), oscillations in the capacitor-coil system, if they are not supported from the outside, will eventually stop, since the initial energy will be spent on heating the wires and electromagnetic radiation. This means that with each oscillation, the maximum current in the coil and the maximum voltage on the capacitor plates will be less and less. However, just like the oscillation of a pendulum in a mechanical clock, electrical oscillations can be maintained by, for example, connecting a capacitor to an external alternating current source. But alternating current, as we remember, also changes its value with a certain frequency, or, in other words, has its own oscillation frequency. Any oscillatory circuit is not indifferent to what oscillation frequency the current feeding it has. If, for example, this current has an oscillation frequency that is too large or too small compared to the oscillation frequency of the circuit itself, then the current strength and its voltage in the oscillatory circuit will never be large (since this external influence will interfere with its own oscillations more than help them). However, in cases where the frequency of oscillations of the external current is close to the natural frequency of oscillations of the circuit, the current strength and voltage of the circuit current begin to increase and reach their maximum when these frequencies coincide completely. In this case, the oscillatory circuit is said to be in resonance. Resonance is especially pronounced in circuits with low resistance. In this case, the voltage across the capacitor and the coil can be many times greater than the external supply voltage. There is a kind of surge or surge of voltage. The phenomenon of electrical resonance was used to implement selective radio communications. Marconi was one of the first to tune the oscillatory circuits of the transmitting and receiving stations to the same frequency. For this, in particular, he used his jigger, including a capacitor in parallel with its secondary winding and thus obtaining an oscillatory circuit. The circuit of the transmitters was also changed by including inductive coils and capacitors in the antenna circuit, so that each transmitting station could transmit signals with a certain wave oscillation frequency. Since now several radio stations were transmitting messages, each with its own frequency, the waves they emitted excited alternating currents of various frequencies in the receiving antenna. But the receiver chose only those signals whose frequency coincided with the natural frequency of oscillation of its oscillatory circuit, because only in this case was the resonance phenomenon observed. The jigger in this circuit worked as a filter and did not amplify any antenna current (as it was before), but singled out among them the current of the frequency to which the given receiver was tuned. Since that time, resonant circuits have become an integral part of both receiving and transmitting devices.
At the beginning of the 1901th century, several dozen scientists in many countries were enthusiastically engaged in wireless telegraphy. However, the greatest successes were still associated with the name of Marconi, who, undoubtedly, was one of the most prominent radio engineers of this time. After a series of experiments on transmission over long distances, Marconi made a striking discovery - it turned out that the bulge of the globe does not in the least interfere with the movement of electromagnetic waves. This prompted him to experiment with telegraphy across the ocean. Already in 1800, the first transatlantic radio transmission in history took place, during which Marconi's assistant, Fleming, transmitted the letter "S" from the English station in Poldu in Morse code, and Marconi, who was on the other side of the Atlantic Ocean, on the island of Newfoundland, received it at a distance XNUMX miles. The next important point in the improvement of receivers was the creation of new wave traps (detectors). Branly's coherer played an important role in the early years of radio communications. However, he was too capricious and difficult to handle. In addition, it had to be constantly shaken to restore the ability to respond to the next radio signal. One of the central tasks was the creation of a "self-adjusting" coherer. The first attempt in this direction was made in 1899 by Popov with a telephone. The second is Marconi, who designed his magnetic detector at the beginning of the XNUMXth century.
The principle of operation of the magnetic detector was based on the phenomenon of the so-called hysteresis. The fact is that usually iron is magnetized with some delay in time. However, the magnetization can be enhanced if, at the moment of exposure to an external magnetic field, a noticeable shaking of the iron molecules is caused. This can be done by mechanical shock or a short pulse of another magnetic field. This phenomenon was used by Marconi. In his magnetic detector, an endless belt of soft iron wire was stretched over two roller disks, moving at a speed of five inches per second and passing under the poles of two permanent magnets inside a small glass tube. The primary and secondary windings were wound on this tube, and the primary winding was connected to the antenna circuit, and the secondary was connected to the telephone. Passing under the poles of the magnet, the iron tape was magnetized first in one and then in the opposite direction. The magnetization reversal itself occurred under the middle double poles of the same name, but not immediately at the moment the tape passed under them, but somewhat delayed (due to the property of iron mentioned above). The picture of the magnetic lines emanating from the poles and closed in the iron wire was distorted, and the magnetic lines seemed to be carried along by the wire in the direction of motion. The high-frequency magnetic field formed inside the primary winding during the passage of the received radio signal instantly weakened the hysteresis phenomenon in the iron wire and produced shock remagnetization in it. The configuration of the lines of force changed dramatically, and they were installed in the position that is characteristic of them when the wire is stationary. This sudden displacement of the lines of force created an instantaneous current in the secondary winding, which caused a sound in the telephone. The device did not require shaking and was always ready to receive the next signal. In the same years, other types of detectors were proposed by other radio engineers. Since that time, the rapid development of radio engineering began. In 1902, using his magnetic detector, Marconi conducted a series of remarkable experiments on the Italian war cruiser Carlo Alberto. During the voyage from Italy to England and Russia, he was completely free to receive at a distance of 2000 km from Poldu, where the transmitting station was located. In November of the same year, 1902, official radio communications were established between the United States and England. President Roosevelt and King Edward VIII exchanged radiograms of greeting. And in October 1907, the Marconi firm opened to the general public the first radiotelegraph station in history, transmitting messages from Europe to America. The interest in this novelty turned out to be huge - 14 thousand words were transmitted on the first day. Author: Ryzhov K.V.
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