Power Engineering. Соколова Е.В. - 21 стр.

        5 When the electron is released from the filament it goes shooting out into the
void in which the elements are situated. When it leaves the filament, it takes with it a
v' charge, and thereby leaves the filament positively charged. If there is no body at a
positive potential within the bulb other than the filament, the electron will eventual-
lyv?ts way back to the source whence it came. If, howxever,a plate is within the tube
and is more positive than the filament, the electron will be attracted to it. Even when
the plate is at the proper positive potential to attract many electrons, some go back to
the filament, and others congregate somewhere between the filament and the plate
and constitute what is called the "space charge".
        6 Every electron which hits the plate constitutes a minute electric current and
when enough electrons arrive per second a measurable current is attained. It is this
current by the electron from the filament which constitutes the tubes plate current
which is used in so many ways. The symbol for plate current is I; the plate current is
usually measured and expressed in milliamperes.
        7. The source of the electrons is usually called the filament although some
modern tubes get their supply of electrons in another manner. This filament is heated
by a battery, called an A-battery or by a small step-down transformer from the a.-c.
lighting circuit. A-battery inserted between part of the filament system and the plate
maintains the plate positive with respect to the filament. It is called the B-battery.
        8. When the filament is heated to a proper temperature a copious stream of
electrons is emitted. Some of the electrons are attracted to the positive part of the
filament, that is, to the side of the filament, a few electrons will get through the fog
called the space charge but if it is at a higher potential than the filament it attracts
many more electrons. It is usually so maintained by means of the B-battery and in
some cases may be as high as 10 000 volts above the potential of the filament. This
B-battery may be attached to the filament in several ways. Its negative end may be
connected to the negative end of the A-battery or to the positive end of the A-battery.
It is standard practice in the telephone plant to connect A-plus and B-minus together;
in other places it is common practice to connect the two negative leads together. The
most negative part of the filament in the case of d.-c. tubes, or the center of the fila-
ment in the case of tubes run from a.c., is considered as the point to which all other
voltage are referred.
        9. The experiment above shows that the effect of increasing the positive poten-
tial of the plate is increasing the flow of electrons. The filament temperature, too, has
an important effect upon the flow of electrons. The hotter the filament, the more elec-
trons per second will be released into the space surrounding the heated element. If,
however, the voltage on the plate is low, there will soon be reached a definite plate
current which cannot be exceeded no matter how hot the filament becomes. In other
words the plate is taking all the electrons it can get through the space charge. It is true
that more electrons leave the filament at higher temperatures but they simply add to
the space charge or return to the filament. If the plate buttery voltage is increased, a
greater plate current will flow. But again a point will be reached where passing more
current the filament ceases to increase the plate current

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