INTRODUCTION TO RADIO
WHAT IS ELECTRICITY?
MEET THE ATOM
A single atomic bomb demonstrated to a startled world
that the ATOM is a source of a lot of energy. Since then
the atom has been pictured as a new and untapped source
Actually, it is neither new nor untapped. For years,
man has known the atom to be composed of POSITIVE
and NEGATIVE charges of electricity-that these charges
have been used to turn the wheels of industry, power our
trains, and energize our radio transmitters.
The story of how your transmitter sends a message
begins with the atom itself. The ACTIVITY of the tiny
negative and positive charges within the atom is the
source of energy that sends your radio message to
Singapore or Saipan.
YOU KNOW SOMETHING ABOUT IT
You have experimented with atomic energy man. times.
Remember the fun you had rubbing your shoes on the
rug and then giving an electric shock to another person
by bringing your finger near the end of his nose? And
you probably have heard the snap and crack of electric
sparks, as you stroked a cat's back. These little demonstrations were experiments with the positive and negative
charges of the atom.
WHAT IS THE ATOM LIKE?
There are ninety-odd known kinds of atoms, ranging
from simple hydrogen with ONE POSITIVE and ONE NEGATIVE charge to the famous uranium atom with many
All atoms, whether simple or complex, have a similar
basic arrangement. They have a concentration of material in a central mass called the NUCLEUS and a number
of NEGATIVE charges revolving in ORBITS about the nucleus.
The structure of three single atoms-hydrogen, helium,
and lithium-is given in figure 1. Hydrogen has ONE
Figure 1.-Hydrogen, helium, and lithium atoms.
positive charge (PROTON) and ONE negative charge
(ELECTRON). The PROTON is in the NUCLEUS, and the
ELECTRON is floating about the nucleus in an ORBIT, like the
moon revolving about the earth.
The second atom, HELIUM, has four protons and four
electrons. ALL of the PROTONS and TWO of the ELECTRONS
are in the nucleus, and the other two electrons are in
The third element, LITHIUM, has seven electrons and
seven protons. ALL of the PROTONS and FOUR of the
ELECTRONS are in the nucleus, and the remaining three
electrons are in the orbits. Also notice that with
lithium, a SECOND ORBIT is added.
Figure 2.-Atoms of oxygen, neon, and sodium.
The atoms of oxygen, neon, and sodium, in figure 2,
continue to show a systematic arrangement of electrons
and protons. The atoms given so far show these facts-
ALL the PROTONS and approximately one-half of the
ELECTRONS are in the nucleus, and the REMAINDER of the
ELECTRONS are in the orbits. Each orbit has a maximum number of electrons that it can hold-for instance,
TWO on the first, and EIGHT on the second.
Of the six atoms so far described, NEUTRONS (N) are
present in each atom except hydrogen. Don't be alarmed.
The neutron is just one ELECTRON COMBINED with one
PROTON to form one NEUTRAL CHARGE (NEUTRON)-
1 electron + 1 proton = 1 neutron.
Neutrons are dead ducks so far as electricity is concerned, so don't let them trouble you.
Turn to figure 2 again. The helium nucleus contains
four protons and two electrons. The two electrons combine with two of the protons to form TWO NEUTRONS.
This leaves an excess of two PROTONS, which give the
nucleus TWO POSITIVE CHARGES.
Helium has TWO ELECTRONS in the first orbit. Therefore, an atom of helium is balanced, since it has TWO
POSITIVE CHARGES in the nucleus and TWO NEGATIVE
charges in the orbit.
How about lithium ? It has four neutrons, three positive charges, and three negative charges. You can see
that it is also a BALANCED atom. Similarly, oxygen, with
eight neutrons, eight positive charges, and eight negative
charges, is a balanced atom.
In each case an atom as an individual unit is a
BALANCED, UNCHARGED piece of matter.
THEORY OF BUILDING A CHARGE
From the information given so far, it appears that all
atoms have a balance of charges. That is true until you
do something to destroy the balance.
Most atoms are eccentric things. Some have a tendency to GIVE AWAY ELECTRONS; others have a tendency
to BORROW or STEAL ELECTRONS from other atoms.
Figure 3.-How an atom becomes positively charged.
Lithium in figure 3 is an element that tends to GIVE
AWAY one of its electrons. When it does this, the remaining charge will be-
|with ||-leaving a net charge of 1 proton
So by giving AWAY AN ELECTRON, lithium becomes positively charged.
Figure 4 shows how an atom of chlorine becomes negatively charged. It borrows an electron from some other
atom to form-
|with ||-leaving a net charge of 1 electron
In each case it is the ELECTRON that DOES THE MOVING.
BY GIVING ELECTRONS AWAY a POSITIVE CHARGE is produced;
Figure 4.-How an atom of chlorine becomes negatively charged.
BORROWING ELECTRONS produces a NEGATIVE
CHARGE. Remember those statements. They are THE
BASIS OF ELECTRICITY.
But why doesn't the proton move? In some cases it
does, but since the proton is about 2,000 times heavier
than the electron, the proton will move only when great
force is applied. It is like moving an aircraft carrier as
compared with moving a whale boat.
With the exception of lithium, the atoms so far discussed are normally gases. Don't let that trouble you-at high enough temperatures lithium too becomes a gas.
The same holds for all other atoms.
You now know the theory of producing a charge, and
you're ready for some practical examples.
HOW YOU CHARGE AN OBJECT
Go back to the old trick of rubbing your shoes on the
rug. The FRICTION between the sole of your shoe and
the rug removed some electrons from the leather, leaving a
POSITIVE charge. Then, when you touched your finger to
another fellow's nose, ELECTRONS jumped between your
finger and his nose.
That little track of giving somebody an electric shock
brings up some important questions-
How did the charge get from the shoe to your finger?
Why did the spark jump?
What was the spark?
The answer to the first question-the electric charge
did not remain concentrated in one spot but distributed
itself evenly over your whole body.
Why did the spark jump? Nature does not like
INEQUALITIES. Since the other person's body had a different number of electrons than yours, some electrons
moved from one body to the other to EQUALIZE the number
of electrons on both.
HERE IS A LAW which applies to those first two
"Nature always attempts to distribute EQUALLY the
number of ELECTRONS on all objects, with the EXCESS electrons MOVING to areas where they are FEWER in number."
Now, what was the spark? A stream of electrons.
CREATING A CHARGE-ADDING OR SUBTRACTING
When you create a charge-by stroking a cat's back,
by combing your hair, or by running a leather belt over a
pulley-the charge is produced by FRICTION REMOVING
ELECTRONS from one object and ADDING them to the other.
The object that LOSES electrons becomes positive ; the one
that gains electrons becomes negative.
LIKES REPEL, UNLIKES ATTRACT
The law of LIKES and UNLIKES needs little introduction.
You've seen it in operation when a charged comb picks
up bits of paper.
In figure 5, two negatively charged and two positively
charged balls are placed near each other, and with each
LIKE charge a force of repulsion exists.
Figure 5.-Likes repel, unlikes attract.
Now if one ball is given a positive charge and the others
a negative, as illustrated in figure 6, a force of attraction
Figure 6.-Unlikes attract.
Since the positive charge is 2,000 times heavier than
the negative, the positive will REMAIN almost FIXED while
the negative charge is FREE TO MOVE. It is said, "the
ELECTRON is attracted to the POSITIVE charge."
The electrical charges that appear on your shoes, a
comb, or a cat's back, are called STATIC, because they are
Surrounding the charges is an area that is influenced
by the charges. This area is called the ELECTROSTATIC
field. The stronger the charge, the stronger the field.
If the charge is increasing in strength, the field is expanding. A decreasing charge will produce a contracting field.
Electrostatic fields are important in radio circuits. In
some places you want them, in others you do not. Look
inside any receiver or transmitter, and you will see metal
walls or cans isolating certain coils, vacuum tubes and
condensers from other elements in the circuit. These
SHIELDS keep the electrostatic fields confined to the places
where they are wanted, and away from areas where they
can cause trouble.
CURRENT ELECTRICITY-MOVING ELECTRONS
When electrons of a static charge MOVE, it is no longer
STATIC electricity, but CURRENT electricity. Think back
to the electric spark again. The spark that jumped between your finger and some other object was a STREAM of
Certainly you have noticed that some sparks are large
and others are small. More electrons are flowing in a
large spark than in a small one. It's like comparing
rivers of different size. The flow of a river is measured
in units of gallons or cubic feet that pass a point each
minute. The flow of electricity is measured by the NUMBER of ELECTRONS that pass a point each SECOND.
UNIT OF ELECTRICITY-COULOMB
No one has ever seen an electron or probably ever will ;
so to simplify the job of counting them, individual electrons are grouped together into a large unit. It's like
grouping grains of sugar into a large unit, the pound.
You probably never troubled to count the grains in a
pound of sugar, but some one did CALCULATE the NUMBER
of electrons in the UNIT of electricity, the COULOMB. He
found that it contained 6.3 billion billion ELECTRONS.
That number is 63 with 17 zeroes after it. And that is
a lot of electrons.
RATE OF FLOW-AMPERE
The name given to the unit of electrons is the coulomb.
Now when ONE COULOMB of electricity passes a point in a
SINGLE SECOND, ONE AMPERE of electricity is flowing.
Thus an AMPERE is to ELECTRICAL FLOW as the GALLON-PER-MINUTE is to WATER FLOW. It is the RATE of FLOW.
One-half coulomb per second is ½ ampere; 1/1000
coulomb is 1/1000 ampere or one milliampere, abbreviated (ma.).
In radio work, the most-used unit of current is the
MILLIAMPERE. With receiving circuits, the range is
from one or two to about 50 milliamperes, while with
transmitters, the current flow will range upwards of
SEVERAL HUNDRED milliamperes.
Volume of CURRENT is not always the same. It varies
directly with the size of the charge. Since work is required to move electrons and create a charge, the size of
charge may be expressed in units of WORK DONE to move
The VOLT is the unit used to express the amount of work
done to create a charge. One VOLT of charge is created
when one JOULE of work is done in moving a COULOMB.
A volt actually expresses more than degree of charge.
When you pile up a surplus of electrons, you are creating
a RESERVE OF ENERGY. ENERGY IN RESERVE IS POTENTIAL
ENERGY. Thus a volt may also be used as an expression
of the potential energy of an object.
VOLTAGES ARE DIFFERENCES IN POTENTIALS
Since no object is of zero potential, and it is possible
to create a charge by either adding or removing electrons,
the energy of two points is not expressed in ACTUAL potentials but in DIFFERENCES of potential.
So when you say an object has a potential of 200 volts,
all you are actually stating is the DIFFERENCE in the potentials of two points.
Since all objects have some potential, it is a common
practice to designate some point as ZERO potential. In a
radio, zero potential is usually the frame or chassis of the
set. Hence, when you say the plate of a vacuum tube is
positive 200 volts, you are only stating that the plate is
200 volts more positive than the chassis.
The rate of current flow is influenced by the magnitude
of the difference between the two charges. If the difference between the charges is small, the rate of flow will
be low, but if the difference is large, the rate of flow will
THERE ARE NEGATIVE POTENTIALS ALSO
Although the chassis of a radio is given as "zero" potential, it is possible for CERTAIN PARTS of a receiver or
transmitter to be at a lower potential than the chassis.
All these parts are said to have NEGATIVE potentials.
You will find the GRIDS of vacuum tubes stated as being
-5, -10, or -50 volts. It means that the grids of those
tubes are at a LOWER positive potential than the chassis,
by 5, 10, or 50 volts.
Don't let a NEGATIVE potential fool you. There is just
as much "wallop" between -200 volts and the chassis as
between +200 volts and the chassis.
ALL VOLTAGES ARE ONLY RELATIVE
Now you are beginning to get the whole picture.
Voltage is only a RELATIVE THING. Look at figure 7.
Point A is given as being -200 volts in comparison to
the chassis. And B is 200 volts more positive than the
chassis. Hence you may say, point B is 400 volts more
POSITIVE than A. Turn things around-point A is 400
volts NEGATIVE in respect to B.
Figure 7.-Relative potentials.
How about point C? It is 100 volts positive in respect
to the chassis, but 100 volts more NEGATIVE than point B.
So in respect to A, C is 300 volts positive.
Now point D. It is 50 volts NEGATIVE in RESPECT to the
CHASSIS but 150 volts positive in respect to A. Thus
point D is also 150 volts more negative than C, and 250
volts more negative than B.
So you see all potentials (voltages) are ONLY RELATIVE
THINGS. When you state the voltage of an element, remember that what you state is true ONLY in RESPECT TO
ELECTRONS FLOW TOWARD THE MORE POSITIVE
Here is a little statement to remember. "Electrons
flow toward the MORE positive potential." Even if all
potentials are given as negative, the electrons move from
the MOST negative toward the LEAST negative potential.
Figure 8.-Direction of electron flow.
It may look in figure 8 as if electrons are flowing up
hill. Well, maybe so, but that shouldn't trouble you.
You have seen other things pulled upwards, for instance,
a magnet picking up a pin or nail. To place your mind
at ease, just think of elections being PULLED TOWARD the
MORE POSITIVE POTENTIAL.
The HIGHER the VOLTAGE-the greater the potential difference-the greater the flow of electrons.
RESISTANCE-OPPOSITION TO FLOW OF ELECTRONS
Thus far, only the voltage has been given as a factor influencing the rate of flow of electrons. But OBSTACLES in the path of the electrons have a great effect on electron movement.
Electrical obstacles are called RESISTANCES. All materials have resistance. In the case of most metals, the resistance is low. But with some substances, such as glass, rubber, and cotton, the resistance is great enough to stop the flow completely.
CONDUCTORS AND INSULATORS
The amount of resistance offered by a material depends upon the NUMBER OF FREE ELECTRONS in the substance. As an example, COPPER and SILVER have many free electrons, and offer a low resistance. These metals are called good CONDUCTORS.
Substances like GLASS and RUBBER with few FREE ELECTRONS have high resistance and are called INSULATORS.
Not all metals conduct current with equal ease. Some offer considerably more resistance than others. The table below shows six conductors arranged in order, with silver the best and iron the poorest. The insulators in the right hand column are not arranged in order.
In addition to the KIND of conductors, three other factors-SIZE, LENGTH, and TEMPERATURE-also affect the resistance of a wire.
The larger the wire is in cross section, the lower its resistance. A long wire naturally has more resistance than a short one, and with most metals the resistance rises as the temperature of the wire goes up.
UNIT OF RESISTANCE
The unit of resistance is the OHM, which is usually stated in a roundabout manner. An ohm is defined as
Figure 9.-Schematic symbol for a fixed resistor.
the amount of opposition that will permit one ampere of current to flow in a circuit with an applied potential of one volt.
Figure 9 shows the schematic symbol for a resistance as used in radio circuits. More will be given about it in chapter 3.
Figure 10.-Carbon resistors.
RESISTORS USED IN RADIO CIRCUITS
Radio circuits use a great variety of resistors. Some are simple and small, like the CARBON types given in figure
10 ; others are more complicated, like the tapped, wirewound varieties of figure 11.
Figure 11.-Wire-wound resistors.
The carbon resistors are made by fusing and burning
a mixture of carbon and clay. The amount of resistance
is determined by the relative mixtures used.
Wire-wound resistors are formed by winding high resistance wire on a ceramic tube. The specific resistance
of the wire and the length of the winding determine the
With the exception of a narrow strip down one side,
the whole resistor is covered with a coat of enamel. The
exposed strip of bare wire is made to permit you to TAP
the resistor and obtain the desired resistance.
Figure 12.-Variable resistor.
The resistor in figure 12 is of the VARIABLE type. It is
made by wrapping high resistance wire about a short
section of a paper tube. The arm is movable, and by
turning the knob, this arm is made to tap-off any value
between zero and the maximum resistance.
Figure 13.-Variable resistors.
Other forms of variable resistors are given in figure
13. When you turn up the volume on your radio receiver, it is one of these resistors you are adjusting.