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This picture shows one of the devices now in use for the production of continuous radio-frequency
waves. At the top and on each side of the 200-kilowatt generator are the oscillation transformers.
At the extreme right is the motor which drives the generator



At the extreme left is a bank of large vacuum tubes; in the center is a spiral oscillation transformer, to the right of which is a huge power tube. At the extreme right is the control panel, while beneath the table are the condensers. The transmitter is in another room -the Studio Room

491. The vacuum tube. There are several devices by which the voice waves may modulate, or vary the amplitude of, the carrier waves, the most important being the highly exhausted vacuum tube" (see Fig. 465, the halftone opposite p. 441, and the drawing and legend opposite p. 33).


In attempting to reach an understanding of an "audion amplifier or other form of vacuum tube, it is well to remember


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FIG. 465. A popular form of vacuum tube used in radio receiving

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that a current of electricity is a stream of negative electrons which, when passing through a vacuum, move with enormous velocity (thousands of miles per second (§ 498)), but when passing along a wire (ordinary conduction) move quite slowly (a few centimeters per second). Now we found in studying the tungar rectifier (§ 374) that these negative electrons escape freely from an incandescent filament under certain conditions. When the battery B (Fig. 466) has its + terminal connected to the plate P of the vacuum tube and

its terminal to the filament F, no current can flow across the vacuum so long as the filament is cold. When, however, the filament is maintained at incandescence by a battery A, the negative electrons escape from it and are drawn in a steady stream across the vacuum by the attraction of the plate P. This flow of electrons from filament to plate constitutes what is considered by convention to be a current of electricity flowing the opposite way, namely, from plate to filament. We now see how battery A, by keeping the filament in a state of incandescence, merely establishes and maintains one of the conditions under which battery B may discharge a steady current through the vacuum. No electronic flow from the cold plate to the filament is ever possible, because cold bodies do not, except in rare instances (see pp. 441 ff.) eject electrons from themselves. The vacuum tube can therefore be utilized as a vacuum valve, or rectifier, for evidently, if a source of alternating current be substituted for the direct current source (battery B), the vacuum valve would transmit current in one direction only, half of each cycle being held in check.

If a screen of fine wire G, known as a "grid," be introduced between the filament and the plate of Fig. 466 (see Fig. 467) and the grid be maintained at a sufficiently high potential by a battery C, the electrons are repelled back into the incandescent filament and cannot escape from it, and thus the electronic flow is

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FIG. 466. A two-electrode vacuum valve


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FIG. 467. A three-electrode vacuum tube

completely checked; that is, no current flows across the vacuum. If now the potential of the grid be varied, say, from zero to the amount required to stop the electronic flow, the current from battery B through the vacuum is thereby varied from the possible maximum in Fig. 466 to zero. Variation of the grid potential, therefore, affords us a means of controlling and of varying the flow

of current through a

vacuum tube. Indeed,

it is found that slight D. C. generator
changes in the grid volt-
age produce
produce surpris-

ingly great changes in
the current through the (1)
tube; that is, the tube
is an amplifier.

492. Transfer of energy through a condenser. In A. C. generator Fig. 468, (1), the E. M.F. of the direct-current dynamo causes a rush of (2) electrons out of one side of the condenser while electrons to an equal extent rush into the other side. The sides of the condenser are thus charged + and - and they remain so as long as the dynamo runs. It is evident that under these conditions there is no flow of current and that consequently the lamp does not burn. If, however, an alternating-current dynamo is used (Fig. 468, (2)), the alternating E. M. F. causes an alternating rush of electrons which charges the condenser first one way and then the opposite way. It is clear, then, that with an alternating-current dynamo, lamp, and condenser thus arranged we may have an alternating current through the



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The lamp burns

FIG. 468. Energy transferred through a condenser

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