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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
Electric lamp. History of invention and production
Directory / The history of technology, technology, objects around us Electronic lamp, radio tube - an electric vacuum device (more precisely, a vacuum electronic device) that works by controlling the intensity of the flow of electrons moving in vacuum or rarefied gas between the electrodes. Radio tubes were widely used in the XNUMXth century as active elements of electronic equipment (amplifiers, generators, detectors, switches, etc.). At present, they are almost completely replaced by semiconductor devices. Sometimes they are also used in powerful high-frequency transmitters and audio equipment.
The invention of the electron lamp is directly related to the development of lighting technology. In the early 80s of the XIX century, the famous American inventor Edison was improving the incandescent lamp. One of its disadvantages was the gradual decrease in light output due to the tarnishing of the bulb due to the appearance of a dark spot on the inside of the glass. Investigating the causes of this effect in 1883, Edison noticed that often on the tarnished glass of the cylinder in the plane of the thread loop there was a light, almost undarkened strip, and this strip always turned out to be on the side of the lamp where the positive input of the filament circuit was located. It looked as if the part of the carbon filament adjacent to the negative input was emitting the smallest material particles from itself. Flying past the positive side of the filament, they covered the inside of the glass container everywhere, except for that line on the surface of the glass, which, as it were, was obscured by the positive side of the filament. The picture of this phenomenon became more obvious when Edison introduced a small metal plate inside the glass container, placing it between the inlets of the filament. By connecting this plate through a galvanometer with the positive electrode of the thread, it was possible to observe the electric current flowing through the space inside the balloon.
Edison suggested that the flow of carbon particles emitted by the negative side of the filament makes part of the path between the filament and the plate he introduced conductive, and found that this flow is proportional to the degree of incandescence of the filament, or, in other words, the light power of the lamp itself. This, in fact, ends the study of Edison. The American inventor could not then imagine what a great scientific discovery he was on the verge of. Almost 20 years passed before the phenomenon observed by Edison received its correct comprehensive explanation. It turned out that when a lamp filament placed in a vacuum is strongly heated, it begins to emit electrons into the surrounding space. This process is called thermionic emission, and it can be considered as the evaporation of electrons from the filament material. The idea of the possibility of practical use of the "Edison effect" first occurred to the English scientist Fleming, who in 1904 created a detector based on this principle, called the "two-electrode tube", or Fleming's "diode". Fleming's lamp was an ordinary glass bottle filled with rarefied gas. A filament was placed inside the balloon together with a metal cylinder enclosing it. The heated electrode of the lamp continuously emitted electrons, which formed an "electron cloud" around it. The higher the electrode temperature, the higher the density of the electron cloud. When the electrodes of the lamp were connected to a current source, an electric field arose between them. If the positive pole of the source was connected to a cold electrode (anode), and the negative pole to a heated one (cathode), then under the action of an electric field, the electrons partially left the electron cloud and rushed to the cold electrode. Thus, an electric current was established between the cathode and anode. When the source is turned on in the opposite direction, the negatively charged anode repels electrons from itself, and the positively charged cathode attracts them. In this case, there was no electric current. That is, the Fleming diode had a pronounced one-sided conductivity.
Being included in the receiving circuit, the lamp acted like a rectifier, passing current in one direction and not passing it in the opposite direction, and could thus serve as a waveguide - detector. To slightly increase the sensitivity of the lamp, an appropriately selected positive potential was applied. In principle, the receiving circuit with a Fleming lamp was almost no different from other radio circuits of that time. It was inferior in sensitivity to the scheme with a magnetic type detector, but it had incomparably greater reliability. A further outstanding achievement in the field of improvement and technical application of the vacuum tube was the invention in 1907 by the American engineer De Forest of a lamp containing an additional third electrode. This third electrode was called by the inventor "grid", and the lamp itself - "audin", but in practice another name was assigned to it - "triode". The third electrode, as can be seen from its name, was not continuous and could pass electrons flying from the cathode to the anode. When a voltage source was turned on between the grid and the cathode, an electric field arose between these electrodes, which strongly influenced the number of electrons reaching the anode, that is, the strength of the current flowing through the lamp (the strength of the anode current). With a decrease in the voltage applied to the grid, the strength of the anode current decreased, with an increase it increased. If a negative voltage was applied to the grid, the anode current stopped altogether - the lamp turned out to be "locked". A remarkable property of the triode was that the control current could be many times less than the main one - insignificant voltage changes between the grid and the cathode caused quite significant changes in the anode current. The latter circumstance made it possible to use the lamp to amplify small alternating voltages and opened up unusually wide possibilities for its practical application. The appearance of a three-electrode lamp led to the rapid evolution of radio receiving circuits, since it became possible to amplify the received signal by tens and hundreds of times. The sensitivity of the receivers has increased many times over. One of the early tube receiver circuits was proposed already in 1907 by the same De Forest.
An LC circuit is connected here between the antenna and ground, at the terminals of which a high-frequency alternating voltage occurs, formed under the action of energy received from the antenna. This voltage was applied to the grid of the lamp and controlled the fluctuations of the anode current. Thus, in the anode circuit, amplified oscillations of the received signal were obtained, which could set in motion the membrane of a telephone included in the same circuit. De Forest's first three-electrode Audin lamp had many drawbacks. The location of the electrodes in it was such that most of the electron flow fell not on the anode, but on a glass container. The control effect of the grid turned out to be insufficient. The lamp was poorly evacuated and contained a significant number of gas molecules. They ionized and continuously bombarded the filament, having a devastating effect on it. In 1910, the German engineer Lieben created an improved triode vacuum tube, in which the grid was made in the form of a perforated sheet of aluminum and placed in the center of the cylinder, dividing it into two parts. At the bottom of the lamp was the filament, at the top - the anode. Such an arrangement of the grid made it possible to enhance its control action, since the entire electron flow passed through it. The anode in this lamp had the form of a twig or a spiral of aluminum wire, and a platinum filament served as the cathode. Lieben paid special attention to the increase in the emission properties of the lamp. For this purpose, it was first proposed to coat the filament with a thin layer of calcium or barium oxide. In addition, mercury vapor was introduced into the balloon, which created additional ionization and thereby increased the cathode current.
So, the vacuum tube first came into use as a detector, then as an amplifier. But it won the leading place in radio engineering only after the possibility of using it to generate undamped electrical oscillations was discovered. The very first tube generator was created in 1913 by the remarkable German radio engineer Meissner. Based on the Lieben triode, he also built the world's first radiotelephone transmitter and in June 1913 made a radiotelephone connection between Nauen and Berlin at a distance of 36 km.
The tube generator contained an oscillatory circuit consisting of an inductor L and a capacitor C. If such a capacitor is charged, then damped oscillations appear in the circuit. In order for the oscillations not to die out, it is necessary to compensate for the energy losses for each period. Therefore, energy from a constant voltage source must periodically enter the circuit. For this purpose, a tube triode was included in the electrical circuit of the oscillatory circuit, so that oscillations from the circuit were fed to its grid. The anode circuit of the lamp included a coil Lc, inductively coupled with the coil L of the oscillatory circuit. At the moment the circuit is turned on, the current from the battery, gradually increasing, moves through the triode and coil Lc. In this case, according to the law of electromagnetic induction, there will be an electric current in the coil L, which charges the capacitor C. The voltage from the capacitor plates, as can be seen from the diagram, is supplied to the cathode and the grid. When turned on, the positively charged capacitor plate is connected to the grid, that is, it charges it positively, which contributes to an increase in the current passing through the Lc coil. This will continue until the anode current reaches its maximum (after all, the current in the lamp is determined by the number of electrons evaporated from the cathode, and their number cannot be unlimited - increasing to some maximum, this current no longer increases with an increase in the grid tension). When this happens, a constant current will flow through the coil Lc. Since inductive coupling occurs only with alternating current, there will be no current in the coil L. As a result, the capacitor will begin to discharge. The positive charge of the grid, therefore, will decrease, and this will immediately affect the magnitude of the anode current - it will also decrease. Consequently, the current through the coil Lc will also be decreasing, which will create a current in the opposite direction in the coil L. Therefore, when the capacitor C is discharged, the decreasing current through Lc will still induce a current in the coil L, whereby the plates of the capacitor will be charged, but in the opposite direction, so that a negative charge will accumulate on the plate connected to the grid. This will eventually cause a complete cessation of the anode current - the flow of current through the coil L will stop again, and the capacitor will begin to discharge. As a result, the negative charge on the grid will be less and less, the anode current will again appear, which will increase. So the whole process will be repeated from the beginning. From this description it can be seen that an alternating current will flow through the grid of the lamp, the frequency of which is equal to the natural frequency of the LC oscillating circuit. But these oscillations will not be damped, but constant, since they are maintained by the constant addition of energy from the battery through the coil Lc inductively coupled to the coil L. The invention of the tube generator made it possible to make an important step in radio communication technology - in addition to the transmission of telegraph signals consisting of short and longer pulses, reliable and high-quality radiotelephone communication became possible - that is, the transmission of human speech and music using electromagnetic waves. It may seem that radiotelephone communication has nothing complicated in it. In fact, sound vibrations are easily converted into electrical vibrations with the help of a microphone. Why, by amplifying them and feeding them into the antenna, not to transmit speech and music over a distance in the same way as the Morse code was transmitted before? However, in reality, this method of transmission is not feasible, since only powerful high-frequency oscillations are well radiated through the antenna. And slow vibrations of sound frequency excite electromagnetic waves in space so weak that there is no way to receive them. Therefore, before the creation of tube generators that produce high-frequency oscillations, radiotelephone communications seemed to be an extremely difficult task. To transmit sound, these vibrations are changed or, as they say, modulated with low (sound) frequency vibrations. The essence of modulation lies in the fact that the high-frequency oscillations of the generator and the low-frequency oscillations from the microphone are superimposed on each other and thus fed into the antenna.
Modulation can occur in a variety of ways. For example, a microphone is included in the antenna circuit. Since the impedance of a microphone changes with sound waves, the current in the antenna will in turn change; in other words, instead of oscillations with a constant amplitude, we will have oscillations with a changing amplitude - a modulated current of high frequency. The modulated high-frequency signal received by the receiver, even after amplification, is not capable of causing oscillations of the telephone membrane or the horn of the loudspeaker with an audio frequency. It can only cause high-frequency vibrations that are not perceived by our ear. Therefore, it is necessary to carry out the reverse process in the receiver - to select an audio frequency signal from high-frequency modulated oscillations - or, in other words, to detect the signal. Detection was carried out using a vacuum diode. The diode, as already mentioned, passed current in only one direction, turning the alternating current into a pulsating one. This pulsating current was smoothed out with a filter. The simplest filter could be a capacitor connected in parallel with the handset.
The filter worked like this. At that moment in time, when the diode passed current, part of it branched into a capacitor and charged it. In the intervals between pulses, when the diode was blocked, the capacitor was discharged onto the tube. Therefore, in the interval between pulses, the current flowed through the tube in the same direction as the pulse itself. Each subsequent pulse recharged the capacitor. Due to this, an audio-frequency current flowed through the tube, the shape of which almost completely reproduced the shape of the low-frequency signal at the transmitting station. After amplification, low-frequency electrical vibrations turned into sound; The simplest detector receiver consists of an oscillatory circuit connected to an antenna and a circuit connected to the circuit, consisting of a detector and a telephone. The first vacuum tubes were still very imperfect. But in 1915, Langmuir and Guede proposed an efficient way to pump out lamps to very low pressures, due to which vacuum lamps replaced ion lamps. This took electronic technology to a much higher level. Author: Ryzhov K.V.
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