Radio mirror (Jan-Oct 1923)

K A tMU DIGEST I I.HS TR ATED 11 A. B. C. Lessons for Radio Beginners Chapter IX — The Three Electrode Vacuum Tube By Arthur G. Mohaupt THE three-electrode racuum robe has played u greater part thuu any other Hive 01 apparatus in furthering the progress of the Radio art. In fact. Uadio telegraphy and telephony would not be at its present high stage of development if u were not for the introduction and perfection of the three-electrode vacuum tube. The vacuum tube has now been developed to such an extent that many results are accomplished with it which were formerly considered impossible. Prior to the advent of the vacuum tube Radio made slow progress. Numerous detectors and other de\ lcea were tried out. n v trtrtrtr *F G P Figure 3S but none proved exactly satisfactory. However, in the few years that the vacuum tube has been available, progress has been made In so many ways and in so many directions that it was almost impossible for the average man to keep pace with it. Electrons from Heated Objects In order to fully understand the details of operation of the three-electrode vacuum tube, it will first be necessary to spend a little time on the so-called electron theory. According to this now generally accepted theory, the atom as known by the chemist is no longer the smallest particle of matter; but the atom itself is said to be composed of a large number of still smaller quantities known as electrons. At the center of the atom is a nucleus bearing a positive charge of electricity, while surrounding the central charge are a large number of small negative charges of electricity called electrons. These electrons are in a constant state of motion or vibration, the degree of activity depending upon the temperature of the object. The higher the temperature, the more active are the electrons; and when the temperature reaches a certain degree, the activity becomes so great that some of the electrons actually leave the metal (are flipped off, we might say) and travel outward into space with great velocity. Some metals are capable of emitting (sending off) more electrons than others. Also, the electronic emission can he greatly increased by coating the metals with various metallic oxides, such as thorium oxide for example. Charges Space Negatively It is interesting to note at this point that according to the electron theory matter is really a form of energy, electrical energy, and that it manifests its presence only by the effects it produces on our physical senses. Since the emitted electrons are tiny negative charges of electricity, if the emission is permitted to continue for a short time, the surrounding space will soon become negatively charged. The result is that it is rendered a conductor for electric current. It is this fact which forms the basis of operation of the vacuum tube as used in Radio receiving and transmitting apparatus. Emission Controlled by Filament Heat Another important factor to be considered is that the rate at which the electrons are given off can be readily con Q <-^VWVV A Figure 36 trolled by regulating the temperature of the hot metal. The presence of a nearby electrically charged object also influences the rate of emission, for a positive charge will attract the electrons and thus accelerate their flow, while a negative charge will repel them and thus retard their flow. Let us now see how these principles are employed in the vacuum tube construction and operation. Construction of the Three-Electrode Tub* The three-electrode vacuum tube consists of a glass tube out of which the three elements or electrodes are mounted and hermetically sealed (air tight). One of these elements is known as the filament, another as the plate, and the third as the grid. Erich of these plays its individual and important function, as we shall presently see. The general appearance of i ho ihivt'-i-lectrode vacuum tube is illustrated in Kigure 35. The filament forms the unit which is heated to ineandescence and from which the negatively charged electrons are emitted. The filament is heated by means of an electric current generally supplied by a 6-volt storage battery, although there is also a tube on the market which is so designed that the filament can be heated by means of a 1%-volt dry cell. It is evident that a tube of the latter kind is very desirable in that the expensive and troublesome storage battery is not needed. The filament is generally in the form of an inverted V. Filament Circuit Connections The storage battery for supplying the current to the filament is commonly known as the A battery. The temperature of the filament is controlled by the current strength supplied to it, and the current in turn is regulated by means of a rheostat connected as is illustrated in Figure 36. As is shown, the negative terminal of the A battery is connected directly to one terminal of the filament, while the positive terminal of the battery is connected to the rheostat. The other terminal of the rheostat is connected to the free terminal of the filament. As the position of the rheostat R is changed, the current, and hence also the filament temperature is regulated accordingly. The entire circuit including the filament, A battery and rheostat is known as the filament circuit. VhB Flate and B Battery The filament, when heated to incandescence, sends out the negatively charged electrons, and soon the space in the tube becomes so densely charged negatively that no more electrons can possibly escape from the filament. In order to avoid this condition and to make possible a continuous stream of electrons, the plate is inserted and sealed into the tube. The plate is generally in the form of a rectangular or circular cylinder surrounding the filament. In order to bring the plate into action. another battery is employed known as the B battery, Figure 37. Tbis battery is hooked into the circuit so that its positive terminal is connected to the plate and its negative terminal to the point N in the filament circuit. The point N is commonly known as the neutral or zero potential point. What Lets Current Flow in Tube The result of the B battery in the circuit is that the plate will alwaysbe at a positive potential with respect to the filament. Consequently, since opposite electrical charges attract each other, the electrons (negative) emitted from the filament (negative) will be attracted by the plate (positive), and thus form a continuous stream between these two elements. The intervening space is by this action rendered a fairly good conductor of electricity, and current can flow through the plate circuit as is indicated by the arrows. Leaving the B battery at the positive terminal, the current flows through the milliammeter MA to the plate P, within the tube it flows from the plate to the filament, and then returns to the negative terminal of the B battery. The milliammeter is inserted to measure the strength of the current flow in the plate circuit. It is a peculiar condition, and is a little difficult to comprehend at first, that the current in the plate circuit travels in a direction opposite to that in which the electrons within the tube travel. We must satisfy ourselves by assuming that the stream of electrons provides a conducting path over which the current can flow. Cold Filament Stops Flow The circuit between the plate and filament, including the B battery, is generally called the output circuit of the tube, for it is into this circuit, we will learn later on, that the telephone receivers are connected when the tube acts as a detector. As long as the filament is cold, no electrons are being emitted, and hence the space between the fialment and plate is an insulator and no current can flow in the plate circuit. Under these conditions the B battery can be left in the circuit indefinitely without being discharged. As soon as the flalment Ik heated, the electrons close the circuit and current can flow. Control of B Voltage The strength of the current flowing in the plate circuit depends upon the temperature of the filament and the 10. M. F. (voltage) of the B battery. If the B battery is provided with taps for varying the potential of the plate, the plate circuit current can be varied accordingly. However, when the potential has reached a certain value, any further increase will not affect the strength of the current, for under this condition all of the electrons coming from the filament are taken up by the plate as fast as they can be furnished. The saturation point is thus said to be reached. However, if the temperature of the filament is raised slightly by increasing the current flow through it, a high plate poten Figure 38 tial will again cause a further increase in the plate circuit current, for more electrons are being emitted at the higher temperature. Every vacuum tube has a particular plate potential and filament temperature at which it functions best, and these values can be found only by experiment. The Grid and Its Functions The grid, or third element of the tube, is in the form of a wire network or perforated plate, and is placed between the filament and plate of the tube, as is illustrated in Figure 38. The grid acts as a sort of control valve or regulator for the current flowing in the plate circuit. If the grid is charged positively, it will assist the plate and cause an increase in the electron emission, and hence also in the plate current (low. However, if the grid is charged negatively, U will repel .some of the electrons, thereby opposing the plate action and deci • . ., the plate current flow. Tuning' Apparatus in Circuit To bring the grid into play, it is connected to the secondary of the coupler or tuning coil, and is thus affected by the electrical waves as they are received by the antenna and enter the receiving sei. Under normal conditions the grid is thus ata negative potential with respect to the filament. As the incoming electrical oscillations pass through the primary of the coupler, they induce in the secondary an alternating potential which passes from zero to a positive maximum and then to a corresponding negative maximum value. Signals Control Flow Via Grid Since the coupler secondary is connected across the filament and grid, the alternating voltage pulsations from the received signals will be impressed on the grid. As the grid becomes less negative, the electron flow will be greater and the plate circuit current will be increased; but as the grid becomes more negative, the electron flow will be retarded and the plate current flow decreased. Pulsations will thus be set up in the plate current which correspond in every detail to the potential pulsations induced in the coupler secondcontinued on page 12) MULTIPOINT (Patent Pending) A Synthetic CRYSTAL DETECTOR sensitive over its entire surface Jiliminatos aJl detector troubles. Extraordinary clearness and volume. Endorsed by Badlo experts and press. 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