Чтение общенаучной литературы. Кытманова О.А. - 5 стр.

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6
potential of the grid. The grid being positively charged, it
attracts electrons and increases their flow from the
filament to the plate, for most of them pass through the
relatively wide spaces between the grid wires. On the
contrary the grid being negatively charged, it repels the
electrons and they cannot go to the plate. Consequently,
when the grid G is made alternately positive and negative
by joining the input terminals to a source of alternating potential, the electron flow
from F to P is increased and decreased accordingly, thereby varying the direct
current in the plate circuit. The grid potential might change thousands or millions
of times per second and the plate current would change accordingly. Actually, the
grid is not made positive with respect to the filament, but only more or less
negative. This is done by inserting a so-called С battery," as* shown, to "bias" the
grid negatively. When so biased there will be no current in the grid circuit. Thus,
the grid serves as a gate-valve to control the plate current while taking practically
no power itself.
The cathode of the tube Is often a thin metal sleeve coated with thorium or other
proper material having a low work function; a heating coil of tungsten wire is
mounted within but separated from the sleeve. This construction makes it possible
to heat the cathode with alternating current without introducing disturbing effects.
The Traveling-Wave Valve
In any amplifier or oscillator the function of the electron stream is to convert d. c.
energy from h. f. supply to a. c. energy at the required frequency. If a valve could
be made in which a large fraction of the electrons continuously gave up their d. c.
energy, to the h. f. circuit over a large number of periods of the latter, a high-
frequency amplifier or oscillator would result, this being to "a certain extent
successfully accomplished in the traveling-wave valve. The latter- may be used
either as an oscillator or as an amplifier. As an amplifier it is characterized by a
high gain associated "with an extremely large bandwidth. Its noise-factor, though
hot much if at all better than that of a silicon crystal is considerably less than that
of a klystron. It has therefore a certain value in some special communication and
radar circuits. Imagine a concentric line the inner conductor of which is a wire
helix, as in Fig. 10.
If a signal is applied across the terminals AB, it travels along the inner conductor
to the terminals CD at a speed being determined by the total length of the wire
forming the helix. That is, if the helix consists of N turns of diameter d, the total
length Nicd and, to a first approximation, the time taken by a signal to pass from A
to С is Nπd/c, where с is the velocity of
light.If the shortest distance between A and С
(along the axis of the helix) is I, the apparent
speed of the signal along ВС is lc/πdN. By
giving N and d suitable values, this axial speed may be made small compared with
c. Now imagine that an electron gun at E produces a beam of electrons traveling

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