RADIO.

Putaruru Press, Volume IV, Issue 155, 21 October 1926, Page 5

RADIO.

(By “ Broadcast ”) 111. THE , THREE-ELEMENT VACUUM TUBE.

Some years later Dr. Lee de Forest added another element known as the “ Grid.” Thus was invented the three-element tube or “ triode ” named by de Forest the Audion. With the addition of the Grid in the vacuum tube many things happened. The underlying theory as applied by Edison and Dr. Fleming remained fundamentally the same. The tube functioned in the same way ; but the Grid, a little wire mesh interposed between the filament and plate, made possible increased control of the current flowing from the “ B ” battery.

Let us consider the action. We will assume that the filament is lighted. We know then that current is flowing out of the “ B ” battery as it is evident that the openwork grid does not materially obstruct the stream of electrons. However, if we connect the negative-terminal of another battery to the grid lead the electrons will be repelled by the grid, so that comparatively few will manage to reach the plate. Consequently the path of the electrons is less conductive, and less current can flow from the “B ” battery. If we conmeet the positive-terminal of the “ C ” battery to the grid, however, the electrons will be attracted by it, in the same manner as they are by the plate, and the stream will be increased. In this case the grid assists the plate in the work of attracting the electrons, and as a result a greater current can. flow from the “B ” battery. The extent to which the electron stream is affected is dependent upon, first, the voltage of the “ B ” battery, and second, on the positive or negative voltage of the “ C ” batterv.

Now let us apply the audion or tliree-element valve to a radio receiving circuit, so that we may review briefly the fundamental operation. We will leave out many details, as we wish only to describe the simple functions of the three elements. We will not consider the “ C ” battery for the time being. Imagine that filament is lighted and a steady current is flowing out of the “ B ” battery. Until a voltage is impressed on the grid, there will be no variation or fluctuation of the “ B ” current, and consequently no sound in the headphones, which are connected in the plate circuit. However, as soon as the aerial system picks up energy from a passing radio wave, the energy is impressed on the grid as an alternating current; Immediately the electron stream is affected, which in turn causes the current from the “B ” battery to fluctuate. For every little change of voltage on the grid there is a large variation of the “ B ” battery current, or “ plate current,” as we will term it. Since the current induced in a radio receiving circuit, from a radio wave, is constantly changing its direction of flow, the voltage impressed on the grid is at one instant positive and at the next instant negative. Therefore, the flow of plate current is less at one instant than at the next. The flow of the plate current never changes direction ; it changes in de- | gree only as indicated ; yet it is always a faithful reproduction of the variation of the original radio wave.