Wireless Telegraphy — A State of the Art in 1935
(source SFR)THREE-ELECTRODE TUBES
Principle of the Triode tube
The three-electrode tube truly revolutionized all of radioelectric technology. It is, in particular, what made the realization of broadcasting and television possible.
To understand its operation, we must allow ourselves a brief foray into the recently acquired notions on the intimate constitution of matter.
It is known that the atom is no longer what was imagined about fifty years ago: the smallest possible grain of matter. Today the atom has become very complex: its structure is comparable to that of a planetary system comprising a central nucleus of positive charge or "proton" around which move grains charged with negative electricity called "electrons."
It has recently been discovered that the nucleus itself is composite, and even that it was necessary to abandon the simple and magnificent idea of finding in the study of the infinitely small the transposition of the results acquired in astronomy. Nature is rich, varied, difficult to penetrate.
Without allowing ourselves to be drawn towards this immense and topical subject, let us note the existence of negative electrons at the base of the constitution of matter, electrons enrolled as satellites of an atomic nucleus or free electrons, traversing atomic systems at speeds increasing with the temperature of the body in which they move.
Nothing is easier, moreover, than to get them to separate from this body, to project themselves outwards: it suffices for this to increase their speed by caloric input and to offer them in sufficient proximity the attractive refuge of a plate charged with positive electricity.
This is what is achieved in every Wireless Telegraphy tube. The three-electrode tube contains, in a vacuum bulb, three elements: the filament, the grid, and the plate. Let us pass a current through the filament: it heats up, which has the effect of causing an emission of free electrons which are launched out of the filament through the tube. These electrons, which are grains of negative electricity, escape to be collected on a plate that we have taken care to charge with positive electricity so that it properly plays its role as a receptacle for electrons (fig. 3). There will therefore occur, through the tube, a current of electrons going from the filament to the plate. These electrons will then borrow the conductor connected to the plate and, if this conductor is, by its other end, connected to the filament, they will thus return to the filament. This amounts to saying that an electric current circulates between the filament and the plate, closing through the tube.
What is the role of the third electrode that we have not yet spoken of: the grid? This is arranged in such a way that the electrons going from the filament to the plate are forced to pass through it. Let us connect this grid to an electrical circuit such that we place it, at will, under variable voltage, negative, then positive, for example. The more positive the voltage of our grid, the more it will favor the movement of electrons from the grid to the plate (figs. 4 and 5). In fact, a very slight increase in grid voltage very strongly favors the movement of electrons, that is to say, the intensity of the plate circuit current. There is therefore an amplification effect.
This is what is achieved in every Wireless Telegraphy tube. The three-electrode tube contains, in a vacuum bulb, three elements: the filament, the grid, and the plate. Let us pass a current through the filament: it heats up, which has the effect of causing an emission of free electrons which are launched out of the filament through the tube. These electrons, which are grains of negative electricity, escape to be collected on a plate that we have taken care to charge with positive electricity so that it properly plays its role as a receptacle for electrons (fig. 3). There will therefore occur, through the tube, a current of electrons going from the filament to the plate. These electrons will then borrow the conductor connected to the plate and, if this conductor is, by its other end, connected to the filament, they will thus return to the filament. This amounts to saying that an electric current circulates between the filament and the plate, closing through the tube.
What is the role of the third electrode that we have not yet spoken of: the grid? This is arranged in such a way that the electrons going from the filament to the plate are forced to pass through it. Let us connect this grid to an electrical circuit such that we place it, at will, under variable voltage, negative, then positive, for example. The more positive the voltage of our grid, the more it will favor the movement of electrons from the grid to the plate (figs. 4 and 5). In fact, a very slight increase in grid voltage very strongly favors the movement of electrons, that is to say, the intensity of the plate circuit current. There is therefore an amplification effect.
In fact, a very slight increase in grid voltage very strongly favors the movement of electrons, that is to say, the intensity of the plate circuit current. There is therefore an amplification effect.
If this grid is connected to a receiving antenna, it is subjected to the electrical oscillations that the waves imprint on the antenna and, consequently, it is the site of an alternately increasing and decreasing electrical voltage. This grid voltage will act sometimes in the same direction as that of the battery connected to the plate, sometimes in the opposite direction. In the first case, it will reinforce the attraction that the plate exerts on the electrons. Consequently, the electric current in the plate circuit will increase. In the opposite case, the grid will more or less oppose the passage of electrons, which will strike the plate in smaller numbers. The current in the plate circuit will therefore decrease. The detecting property of the three-electrode tube thus appears. Finally, let us connect the grid and the plate of the tube by appropriate electrical circuits. The influx of electrons onto the plate causes, in these circuits, electrical impulses, analogous to those we spoke of in connection with Hertz's spark gap. But here, these impulses are transmitted to the grid of the tube which reacts on the flow of electrons, sometimes to increase it, sometimes to reduce it, this action taking place at the same rhythm, or at the same frequency, as the initial impulses: The grid has the effect of periodically compensating for the tendency of the oscillations to damp. Thus, the production, in the circuit, of sustained electrical oscillations is observed, which can determine the radiation of a transmitting antenna.
If this grid is connected to a receiving antenna, it is subjected to the electrical oscillations that the waves imprint on the antenna and, consequently, it is the site of an alternately increasing and decreasing electrical voltage. This grid voltage will act sometimes in the same direction as that of the battery connected to the plate, sometimes in the opposite direction. In the first case, it will reinforce the attraction that the plate exerts on the electrons. Consequently, the electric current in the plate circuit will increase. In the opposite case, the grid will more or less oppose the passage of electrons, which will strike the plate in smaller numbers. The current in the plate circuit will therefore decrease. The detecting property of the three-electrode tube thus appears. Finally, let us connect the grid and the plate of the tube by appropriate electrical circuits. The influx of electrons onto the plate causes, in these circuits, electrical impulses, analogous to those we spoke of in connection with Hertz's spark gap. But here, these impulses are transmitted to the grid of the tube which reacts on the flow of electrons, sometimes to increase it, sometimes to reduce it, this action taking place at the same rhythm, or at the same frequency, as the initial impulses: The grid has the effect of periodically compensating for the tendency of the oscillations to damp. Thus, the production, in the circuit, of sustained electrical oscillations is observed, which can determine the radiation of a transmitting antenna.
Sources and references
[1] Société Française de Radioélectrique, "VINGT-ANNÉES DE TSF", 1935
