INTRODUCTION TO RADIO
EQUIPMENT

CHAPTER 1
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 of power.

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

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 charges.

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.

HYDROGEN ATOM

The structure of three single atoms-hydrogen, helium, and lithium-is given in figure 1. Hydrogen has ONE

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 orbit.

The third element, LITHIUM, has seven electrons and seven protons. ALL of the PROTONS and FOUR of the

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.

THE NEUTRON

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

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.

So by giving AWAY AN ELECTRON, lithium becomes positively charged.

In each case it is the ELECTRON that DOES THE MOVING. BY GIVING ELECTRONS AWAY a POSITIVE CHARGE is produced;

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

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 questions-

"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 ELECTRONS

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.

The electrical charges that appear on your shoes, a comb, or a cat's back, are called STATIC, because they are standing still.

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 electrons.

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.

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.

THE VOLT

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 charge.

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.

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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 be large.

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

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 ANOTHER POINT.

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.

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.

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 shows the schematic symbol for a resistance as used in radio circuits. More will be given about it in chapter 3.

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

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 resistance.

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.