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CHAPTER XV
OPERATION OF VACUUM TUBE RECEPTORS
From the foregoing chapters you have seen that the vacuum tube can be
used either as a _detector_ or an _amplifier_ or as a _generator_ of
electric oscillations, as in the case of the heterodyne receiving set.
To understand how a vacuum tube acts as a detector and as an amplifier
you must first know what _electrons_ are. The way in which the vacuum
tube sets up sustained oscillations will be explained in Chapter XVIII
in connection with the _Operation of Vacuum Tube Transmitters_.
What Electrons Are.–Science teaches us that masses of matter are made
up of _molecules_, that each of these is made up of _atoms_, and each
of these, in turn, is made up of a central core of positive particles
of electricity surrounded by negative particles of electricity as
shown in the schematic diagram, Fig. 69. The little black circles
inside the large circle represent _positive particles of electricity_
and the little white circles outside of the large circle represent
_negative particles of electricity_, or _electrons_ as East Tennessee Public Communications Corp. Wkop-tv they are
called.
[Illustration: Fig. 69.–Schematic Diagram of an Atom.]
It is the number of positive particles of electricity an atom has that
determines the kind of an element that is formed when enough atoms of
the same kind are joined together to build it up. Thus hydrogen, which
is the lightest known element, has one positive particle for its
nucleus, while uranium, the heaviest element now known, has 92
positive particles. Now before leaving the atom please note that it is
as much smaller than the diagram as the latter is smaller than our
solar system.
What Is Meant by Ionization.–A hydrogen atom is not only lighter but
it is smaller than the atom of any other element while an electron is
more than a thousand times smaller than the atom of which it is a
part. Now as long as all of the electrons remain attached to the
surface of an atom its positive and negative charges are equalized and
it will, therefore, be neither positive nor negative, that is, it will
be perfectly neutral. When, however, one or more of its electrons are
separated from it, and there are several ways by which this can be
done, the atom will show a positive charge and it is then called a
_positive ion_.
In other words a _positive ion_ is an atom that has lost some of its
negative electrons while a _negative ion_ is one that has acquired
some additional negative _electrons_. When a number of electrons are
being constantly given by the atoms of an element, which let us
suppose is a metal, and are being attracted to atoms of another
element, which we will say is also a metal, a flow of electrons takes
place between the two oppositely charged elements and form a current
of negative electricity as represented by the arrows at A in Fig. 70.
[Illustration: Fig. 70.–Action of Two-electrode Vacuum Tube.]
When a stream of electrons is flowing between two metal elements, as a
filament and a plate in a vacuum tube detector, or an amplifier, they
act as _carriers_ for more negative electrons and these are supplied
by a battery as we shall presently explain. It has always been
customary for us to think of a current of electricity as flowing from
the positive pole of a battery to the negative pole of it and hence we
have called this the _direction of the current_. Since the electronic
theory has been evolved it has been shown that the electrons, or
negative charges of electricity, flow from the negative to the
positive pole and that the ionized atoms, which are more positive than
negative, flow in the opposite direction as shown at B.
How Electrons are Separated from Atoms.–The next question that arises
is how to make a metal throw off some of the electrons of the atoms of
which it is formed. There are several ways that this can be done but
in any event each atom must be given a good, hard blow. A simple way
to do this is to heat a metal to incandescence when the atoms will
bombard each other with terrific force and many of the electrons will
be knocked off and thrown out into the surrounding space.
But all, or nearly all, of them will return to the atoms from whence
they came unless a means of some kind is employed to attract them to
the atoms of some other element. This can be done by giving the latter
piece of metal a positive charge. If now these two pieces of metal are
placed in a bulb from which the air has been exhausted and the first
piece of metal is heated to brilliancy while the second piece of metal
is kept positively electrified then a stream of electrons will flow
between them.