Чтение общенаучной литературы. Кытманова О.А. - 4 стр.

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5
metal slipped over a filament so that heat from the latter will cause the cathode to
become; hot. The first type is called a filament
cathode; the second is known as a heater cathode, the
filament merely acting as a stove or heater. Modern
manufacturing methods dictate that most tubes
should be mounted on a base of some insulating
material such as hard rubber, bakelite or porcelain,
into which are molded metal pins through which
electrical connection is-made to the electrodes.
Suitable sockets, into which these pins are inserted,
permit easy and rapid replacement of tubes, when
necessary. Figure 8 shows the essential construction
of a diode tube. The cathode, when heated either directly or indirectly, emits
electrons which form an indivisible cloud in the area about it. This negative cloud
of electrons is called the space charge. If another electrode in the tube, such as the
plate (anode), be positively charged by an external electric source (battery or
generator), the electrons will be attracted to the plate, because, of their being
negatively charged and because the unlike fields attract. Thus there will be a
continuous flow of electricity from cathode to anode across the intervening space.
The amount of current flow will depend upon the ability of the cathode to emit
electrons and the amount of positive attraction at the plate, i. e., the positive
charge. The positive charge being increased, the rate of the flew and the speed of
the electrons will increase until a point is reached, when the cathode cannot emit
electrons at a faster rate for its present temperature. This is spoken of as the
saturation point. The emission rate can be increased by raising the temperature or
the cathode, but the life of the tube will be shortened should the temperature
exceed a fixed point. The amount of positive, charge that can be applied to the
plate depends upon the distance between the elements and the size of the plate. The
collision of the electrons with the plate results in heat. The plate must have enough
surface area to dissipate this heat by radiation. From the foregoing it is apparent
that if an alternating current (changing polarity) is applied to the plate, a current
will flow in a plate circuit whenever the plate is positive, or during every other half
cycle. Further, the frequency of these d. c. pulsations will depend upon the
frequency at which the alternating current reverses its polarity from positive to
negative. This quality in a diode is used to convert, or rectify, alternating current to
direct current.
Three-Element Electron Tube
The insertion of the so-called third electrode between the filament and the plate
has been found to make the electron tube more versatile by enabling it to serve a
number of functions, especially in telephone and radio circuits. The three element
tube and its circuits then appear as in Figure 9. The effect of the grid is like that of
a shutter which, opening and closing, controls the flow of electrons going through
it from the filament to the plate. This control is accomplished by changing the

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