Introductory tutorials on electronics
are pretty much timeless in terms of fundamentals. A few things have changes like
the assumed direction of current flow from positive to negative, to being what is
now accepted as being negative to positive. A consequence of that change is the
"-hand" rules of electric and magnetic field generation and induction have reversed
from left to right and vise versa. Atomic models have changed from the
Bohr model of planetary behavior
of discretely identified particles (electrons, protons, neutrons) to the modern
quantum mechanical model
that uses wave functions for defining the probability of a particle being at a certain
location. None of that really has a significant effect on learning Ohm's law and
how passive components like resistors, capacitors, and inductors influence voltages
and currents in a circuit. Accordingly, this 1942 "Practical Radio Course" appearing
in Radio News magazine is useful today for introducing readers to the basics
Practical Radio Course
Figure 3 - The actual direction of electron flow as contrasted
with the commonly assumed direction of current flow. According to the electron theory
this latter assumption is incorrect. However, it has become so firmly established
in popular understanding that the misconception is allowed to continue. The student
should clearly understand the important distinction, as pointed out in this article.
By Alfred A. Ghirardi
This exclusive course has been designed especially to guide the beginner in radio.
Part I - Electricity and the Electron Theory
If one is to develop a real understanding of radio he must first understand electricity.
It is not enough to consider electricity as "juice" and let it go at that. This
may be enough for the student electrician whose job it will be to wire house lighting
circuits and doorbells, but it is not sufficient for a student who is to deal understandingly
with the complex principles and practices of radio and electronics.
Actually, we do not even at this late day, know for a certainty just what electricity
is, nor do we even know precisely how electricity "flows" through a conductor. A
theory has been evolved, however, which is generally accepted because it fits in
so well with generally observed facts, although not proven to the extent that it
can be termed a "law" of science. This is known as the "electron theory" and it
is one that dovetails much that was formerly mysterious not only in electricity
but in chemistry and physics as well.
All matter, whether gaseous, liquid or solid, is made up of one or more of the
92 fundamental chemical "elements" - basic substances of the universe that cannot
be broken down into anything else by chemical means. Ninety-two elements have thus
far been isolated, from hydrogen (the lightest and most abundant) to uranium (the
heaviest and one of the rarest). An atom is the smallest particle of a given element
that still retains the characteristics of that element. If disintegrated or altered
in structure, as by cyclotron bombardment, it becomes a particle of a different
element or isotope. It is interesting to note that the outside diameter of all atom
is only about 1/100,000,000 of a centimeter - much too small to be seen, even with
the most powerful microscope. Even the new electron microscope will not reveal anything
smaller than 1/1,000,000. centimeter. Literally millions of atoms would fit comfortably
on a pin point.
In the electron theory, each atom is recognized as a miniature solar system in
itself. Its "sun" consists of a nucleus composed of protons (and sometimes neutrons)
around which planetary electrons spin in planetary orbits as illustrated in fig.
1. The energy that holds the nucleus together is what science seeks to extract by
atom-smashing. An atom, therefore. small as it is, is mostly empty space. its confines
being marked by the orbits in which the electrons most remote from the nucleus are
The difference between atoms of the different elements lies in the number of
protons and neutrons composing the nucleus, and the number of electrons revolving
around it. Thus a hydrogen atom, the simplest of all, has a single proton as a nucleus
and a single electron rotating around it. Helium, with a nucleus of two protons
and two electrons rotating around it is another simple atomic structure, Atoms of
other elements may have more complicated nuclei consisting not only of protons but
also neutrons, and up to 92 electrons spinning around these cores.
A proton is one particle of the matter-energy that constitutes an atom's nucleus.
It carries a "positive electrical charge.
A neutron is another particle of matter - energy that may comprise part of an
atom's nucleus, It carries no electrical charge, hence the name neutron.
Figure 1 - The graphic structure of two simple elements, hydrogen
(A) and helium (B), showing the orbits of electron rotation; The most complicated
of the elements, uranium, has an atomic structure which includes 92 electrons each
rotating in its own orbit around a nucleus consisting of an equivalent number oi
Figure 2 - In some substances, such as glass, some of the electrons
are less tightly bound to their nuclei than in others. If we rub glass with silk
(a substance in which the electrons are tightly bonded) the simple act of rubbing
will be sufficient to dislodge some of the electrons from their atoms in the glass
and these freed electrons will transfer to the silk. This upsets the normal electrical
balance of both substances. The glass becomes positive, the silk negative, as shown
at (A). Because of the attraction of unlike charges the two will tend to cling together.
That this tendency is due to electrical phenomena can be demonstrated by quickly
separating them, then connecting them together by means of a copper wire. Immediately
there will be a measurable flow of electrons (current) through this wire, as shown
at (B), due to the rush of excess electrons to reestablish the original balance.
When this flow ceases it will be found that there is no longer this tendency for
the two materials to cling together.
An electron is a particle of matter - energy that is 1/1,800th the weight of
a proton and carries a negative charge. It rotates in a planetary orbit around an
atom's nucleus, held in attraction by the opposite (positive) charge of the protons
in the nucleus. In a given normal atom the number of electrons is always equal to
the number of protons, with the result that the total negative charges of the electrons
balance the positive charges of the protons. It is possible to remove or add one
or more electrons to an atom without changing its elementary characteristics. If,
however, an electron and proton were both removed from an atom, that atom would
no longer retain the characteristics of the element. Thus if a proton and an electron
were removed from an atom of helium, that atom would become an atom of hydrogen.
When two or more elements are combined to form a compound. the individual atoms
remain unchanged but group themselves into molecules. The molecule is the smallest
particle of any given chemical combination of atoms, which evidences all the characteristics
of the compound as a whole. Thus a molecule of water (H2O) would be found
to include two hydrogen atoms and one atom of oxygen. If these were separated each
would be found to retain its separate identity as hydrogen or oxygen, yet the molecule
as a whole is definitely water. Linkage between atoms in a molecule is established
by their electrons.
The applications of these theories to chemistry are obvious but thus far the
explanation seemingly has little to do with electricity. Yet, because protons and
electrons represent electrical charges, and because it is the electrical attraction
that binds the component parts of the atom together, it becomes apparent that all
matter is basically electrical in nature. Now let us see where all this applies
to our study of electricity and radio.
It was stated that the proton is a positively charged particle of matter - energy
that constitutes all, or part, of an atom's nucleus. Since the nucleus is composed
of protons (and sometimes neutrons also) it is always electrically positive. This
being the case, this nucleus exerts an attraction for the negatively charged planetary
electrons spinning in orbits around it and thus the original structure of any stable
atom is inclined to remain unchanged. However, in the atoms of some elements, the
attraction of the nucleus for some of its outer electrons is relatively small, with
the result that one or more electrons in the outer shell of the atomic formation
may break away or may be attracted away from its nucleus by a stronger external
positive force (see Fig. 2). In that event the electrical balance of the atom is
upset. Having lost part of its negative charge its overall characteristic becomes
positive and it will therefore tend to attract other electrons to it from other
atoms to make up the deficiency. Thus there may be a certain amount of normal electron
interchange between adjacent atoms. This does not mean that the fundamental character
of the atom is changed, because, as explained above, an atom of one element cannot
change its chemical identity and become another element unless the number of protons
in its nucleus is also altered.
If this interchange of electrons can be accomplished in such manner that the
"free" electrons (those that have broken away from their atoms) tend to move in
unison (within a copper wire, for instance), we have a flow of electric current.
This is actually accomplished when we connect a battery (or some other source of
electromotive force) to the two ends of an electrical conductor. The positive terminal
of the battery sets up a strong attraction for the negatively charged electrons.
Thus one or more electrons break away from the atoms located near the positive electrode
of the battery.
These atoms then become electrically unbalanced (positive) and attract electrons
from other atoms further down the line. At the same time the negative electrode
of the battery tends to repel electrons from nearby atoms, forcing them toward the
positive end of the circuit. Thus a continuous movement or "drift" of electrons
is maintained around through every part of the continuous electrical circuit so
long as the battery is connected to the circuit.
It is difficult to find an analogy to accurately illustrate this action. The
conventional comparison with water flowing in a tube is not exact because that suggests
movement of the entire contents of the tube whereas in a solid electrical conductor
actually not any mass of material for the conductor (in other words. the nuclei
of the atoms) moves. Only some of the electrons (which are simply minute electrical
charges) move along through the conductor.
If we consider a large pipe bent into the form of a circle and filled with rocks,
each of which is covered with barnacles, and the space between the rocks filled
with water, we have perhaps a closer analogy. In this case the rocks correspond
to the stationary nuclei of the atoms, the barnacles to the associated electrons.
If we break this pipe and insert a pump (corresponding to the battery or other source
of electromotive force) in this break the water will be drawn from one end of the
tube and forced into the other by the action of the pump. Its flow will correspond
to the attraction by the positive terminal of the battery and repulsion by the negative
The movement of the water will dislodge some barnacles which are loosely attached
to the rocks. These barnacles, looking for a new foot-hold will strike other rocks
and in so doing will dislodge other barnacles, some of them gaining footholds in
spots thus vacated. Even though no one barnacle is carried continuously around the
complete course of the tube, there will nevertheless be some always in motion and
the overall result will be a constant flow or "drift" of barnacles throughout the
complete conducting" path offered by the tube. Immediately the pump is turned off
each barnacle will again associate itself with some rock and movement will cease
except for small, normal, local movements of the barnacles.
In this analogy we assume some spaces between rocks and this is done advisedly.
According to the atomic theory of matter many things which we normally think of
as dense and solid are in reality far, far from it. It is the belief that if we
had microscopes powerful enough to enable us to see the actual structure of a copper
wire, for instance, or even the hardest steel, we would find it to be exceedingly
porous in structure.
The amount of attraction which the nucleus of an atom exerts on the individual
electrons associated with it varies according to a definite plan embraced by the
electron theory. In effect, the electrons spin in "Shell-like" orbital paths concentrically
disposed around the nucleus (Fig. 1). There may be up to seven of these concentric
orbits in the make-up of the more complicated atoms, such as those of the elements
thorium, radium and uranium. We need not go into a detailed discussion of how the
electrons are disposed among these orbital shells beyond saying that if an atom
has more than 2 electrons the surplus up to 8 will rotate in a second orbital shell
outside the first, if there are more than 10 a third orbital shell will accommodate
the surplus up to an additional 18, etc. If any shell is unfilled with its normal
complement of electrons, the electrons in that shell are less tightly attracted
to the nucleus. Moreover, the attraction for electrons in outer shells will be less
than for those in the inner shells.
It is important to visualize this structure clearly because it helps one to understand
why in an element or compound in which all electrons are strongly attracted to their
nuclei it is difficult to establish electrical current flow. The reason is that
there will be few "free" electrons. Such a compound will therefore be a poor electrical
conductor. On the other hand, where there are electrons in the atomic structure
which are less closely bound to the nucleus, by virtue of their occupancy of an
unfilled shell, that material will be a better electrical conductor. If the material
is one in which the atoms include numerous electrons (and therefore several shells)
and also has insufficient electrons to fill its outer shell, these "orphan" electrons
will be liberated more freely and the resulting electrical conductance of that material
will be still better.
Because the precise atomic structure of every known element has been accurately
diagrammed and tabulated it is apparent that the electron theory is extremely helpful
to the scientist and engineer. It enables him to predict many of the electrical,
chemical and physical characteristics of compounds made up of two or more elements,
without the necessity for "cut and try" tests. He may plan to combine two elements,
perhaps obtaining a compound totally different from either of the elements, yet
will be able to determine many of the characteristics of this compound before he
even attempts to make it. Thus much experimentation is avoided and new developments
are speeded up.
He may even plan to transmute the atoms of one element into those of another
element - for example lithium atoms into helium atoms - simply by rearranging their
constituent particles of matter - energy by some external means such as bombardment
with the nuclei of lighter atoms by means of a Cyclotion or other suitable device.
In. this conversion, for example, a small amount of the weight vanishes (passes
from the matter state into energy), with a discharge of 17,000,000 electron volts.
This atomic energy would be very useful if it could be extracted and harnessed to
do useful work.
Such possibilities are mentioned to indicate the vast scope and importance of
this electron theory. The radio student can scarcely hope to acquire a complete
under-standing of all its ramifications but he should at least understand the fundamentals
as set forth in the present articles. With such a foundation he will be better equipped
to understand electrical and radio principles to be discussed in future articles
of this series.
An important and fundamental supposition of the electron theory is that electrons
are nothing but negative electrical charges, or, in other words they are electricity.
When they flow or "drift" through a wire or other electrical conductor they constitute
what we have accustomed ourselves to calling a flow of electrical energy, or current
- that is, such a flow of electrons causes around the conductor all of various effects
which we have learned to associate with the flow of an electric current - magnetism,
heating, chemical, etc. The electrons do not, it is true, create this current of
themselves. It is necessary for us to provide some external influence in order to
initiate and maintain the flow or "drift" of electrons. This is done, for instance,
when we connect a battery to the two ends of a copper wire.
The fact that the positive terminal of the battery attracts electrons and the
negative terminal repels them, inaugurates a condition of electrical pressure which
causes the electrons to drift, or flow around through the entire electrical circuit
under its influence. The greater this "electrical pressure" the more electrons will
be torn away from their atomic nuclei and thus the amount of electric current flowing
in the wire depends on the amount of pressure. This electrical pressure we know
under several names such as "electromotive force," or "voltage," etc.
It is the ordinary conception that the current flow in a wire thus connected
to a battery, is from the "positive" terminal of the battery, through the wire and
back to the "negative" terminal of the battery. On the other hand, it has just been
explained that the electrons actually flow in just the opposite direction. Why this
For a complete and detailed answer we would have to trace the history of man's
study and developing knowledge of electricity. It is only in relatively recent years
that the electron theory was formulated and brought a true understanding of the
subject. Earlier, electricity itself was a mystery, although its uses and applications
were becoming widely known. In these earlier days it was recognized that there were
opposite poles, or opposite influences evidenced within an electrical circuit. These
were logically enough, called "positive" and "negative," but the error (in the light
of our more modern understanding) was made in arbitrarily assuming that the current
flow was from the "positive" terminal of the battery or other source of voltage,
through the circuit and back to the "negative" battery terminal. Nevertheless, a
system of terminology was established on this basis and, while it is not in agreement
with modern electrical theory, it is for practical reasons still continued in use
among electricians and many other workers outside of the electronic field.
To avoid confusion it has become common practice among engineers and others to
follow the old, conventional standards when speaking of the direction of flow electric
current, but to specifically state "electron flow" when referring to the actual
direction of movement of the electrons. These distinctions are made clear in Fig.
3. This is the practice that will be followed in the future articles of this series
and the student is urged to think in these same distinguishing terms.
Thus, in considering the vacuum tube, it is generally stated that the plate current
flows from the positive "B" voltage source, to the tube plate, across the intervening
space inside the tube to the cathode, then back to the negative side of the supply
source. We will find, however, that the actual electron flow within the tube is
from the cathode to the plate and the internal operation of the vacuum tube can
never be fully understood until one has full comprehension of this important factor.
With the foregoing basic discussion of modern electrical theory out of the way,
we can proceed directly to more specific consideration of electrical circuits. In
a way it is unfortunate that it has been necessary to start this instruction course
with a discussion of one of the most complex phases of the entire study of radio
or electricity. However, once the student grasps the fundamentals of the electron
theory, and visualizes what takes place during the flow of an electric current,
his path of learning will be made much easier. If this theory is not fully comprehended
at this time it is not a cause for worry or discouragement. Its practical applications
will become apparent as we delve more deeply into the later subjects. For that reason
the student is urged to keep this first article handy for future reference. Reviewing
it from time to time as the study progresses will help to clear up points which
may not be entirely clear at this first reading.
(To be continued. - Ed.)
Posted February 3, 2022