Electronics [1 ed.]

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$3’25

ELECTRONICS by Robert Irving illustrated by RUTH ADLER

As you sit in front of your TV set, or listen to your radio, do you wonder how these appliances work? We use many electrical appliances in our homes and in industry, but only some of these appliances are called electronic. Businessmen and engineers use electrons to guide and control the tools of industry. Simply, lucidly, this book describes the development, use and performance of such familiar pieces of electronic equipment as: the machinery used in television, radio broadcasting, and auto¬ matic welding; photo tubes, geiger counters, electron microscopes, radar, digital computors, radio telescopes, vari¬ ous servomechanisms and many other electronic devices. These all fall into the area and defini¬ tion of electronics—“an electrical appli¬ ance is electronic if it contains electronic tubes or substitutes for tubes such as transistors.” As in al! of Robert Irving s books, the explanations are clear and simple, and the illustrations by Ruth Adler are an integral part of the text. 12-lip

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OTHER BORZOI BOOKS FOR YOUNG PEOPLE

by Robert Irving

Electromagnetic Waves Sound and Ultrasonics Energy and Power Rocks and Minerals Hurricanes and Twisters

Published by Alfred A. Knopf

Electronics

ELEC TRONICS b

ROBERT IRVING

Illustrated by Ruth Adler

1961

:

ALFRED A. KNOPF

:

NEW YORK

1966

L.C. Catalog card number 61-6049

THIS

IS A BORZOI

BOOK,

PUBLISHED

BY ALFRED

A.

KNOPF,

INC.

Copyright © 1961 by Irving Adler. All rights reserved. No part of this book may be reproduced in any form without permission in writing from the pub¬ lisher, except by a reviewer who may quote brief passages and reproduce not more than three illustrations in a review to be printed in a magazine or news¬ paper. Manufactured in the United States of America. Published simultaneously in Canada by McClelland & Stewart, Ltd.

FIRST EDITION

J • /

JUL 31 ’61 0 03

CONTENTS

I II

What Is Electronics? Electricity and Magnetism

III

Members of the Electronics Team

IV

The Hot Cathode Tube and Radio

V VI VII VIII

The Phototube and Television

3 8 35 63 87

The Gas-filled Tube and Industrial Circuits

115

Atom Smashers and Electronic Brains

136

The Transistor, Old-timer and Newcomer

163

index

follows page 173

Electronics

2

gS

1 WHAT IS ELECTRONICS?

THE ELECTRONIC TUBE

We use many electrical appliances in our homes and in industry. In all these appliances there is a flow of electric current. The electric current is a mov¬ ing stream of tiny particles called electrons. It can be put to work for many purposes. In a broiler, the current is used to produce heat. In a lamp, it is used to produce light. In a radio, it is used to produce sound. In a motor, it is used to produce motion. Only some of these appliances are called elec-

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Electronics

tronic. In a broiler or in a motor, the electric current always flows through a wire. In a radio, for part of the time, the current is not in a wire, but flows through a vacuum or through a gas enclosed in a tube. A tube of this kind is called an electronic tube. An electrical appliance is called electronic only if, in addition to parts that may have wires in it, it has electronic tubes in it. A broiler or a motor is not elec¬ tronic, but a radio is. An incandescent lamp is not

An electric light bulb

Current flows through wires all the time

A simple electronic tube

Part of the time, current flows through a vacuum or gas

What Is Electronics?

« «

5

electronic, because the current in it flows through a wire. A fluorescent lamp is electronic, because the current in it flows through a gas in the lamp. Other examples of electronic devices are television trans¬ mitters and receivers, motion picture sound pro¬ jectors, atom smashers, and computing machines. There are also many electronic instruments for guid¬ ing and controlling the tools of industry.

EASIER CONTROL Electronic tubes are useful because they make it easier to control the flow of an electric current. In a wire, the electrons are part of a big crowd of par¬ ticles. The particles “tug” at the electrons and get in their way as they move. This makes it difficult to control the motion of the electrons. In an electronic tube, the electrons spread out for a while in a space

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Electronics

that is nearly empty. Here they can move more freely. During the short time when the electrons have more "elbow room,” it becomes easier to control their motion. When an electrical current flows through the tube, it is easy to speed it up or slow it down—or it can be stopped and then started again. The chapters that follow will describe how this control is carried out, and how the controlled currents are used.

THE TUBE HAS A RIVAL

The electronics industry grew up around the elec¬ tronic tube and its many uses. But now, in some parts of the industry, the tube is beginning to disappear. It is being replaced by a "rival,” the transistor. A tran¬ sistor does many things that an electronic tube can do, and it does them more cheaply. The transistor will be described in the last chapter of this book. The

What Is Electronics?

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y

appearance of the transistor makes it necessary to change slightly our definition of electronics. We should say that an electrical appliance is electronic if it contains electronic tubes or substitutes for tubes, such as transistors.

2 ELECTRICITY AND MAGNETISM

Before you can understand how electronic tubes work and how they are used, you must first know the main facts about electricity and magnetism. This chapter gives a quick review of these facts.

ELECTRICITY IN EVERYTHING

There are two kinds of electricity, called positive and negative electricity. They are part of the building

Electricity and Magnetism

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9

blocks out of which all material things are made. All material things, whether they are solids, liquids or gases, are made up of small particles called mole¬

cules. Each molecule is a cluster of smaller particles called atoms. There are about one hundred different kinds of atoms. Each atom is made up of a nucleus surrounded by some electrons. Inside each nucleus there are one or more protons. The proton is a piece of positive electricity. We say it has a positive charge. The electron is a piece of negative electricity. We say it has a negative charge. The more protons there are in a nucleus, the more positive charge it has. The more electrons there are in a crowd of electrons, the more negative charge there is in the crowd. Bodies that carry electrical charges push and pull on each other. If they have the same kind of charge, they repel each other and try to move apart. If they have opposite charges, they attract each other and try to move closer together. The electrons that sur-

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Electronics

round a nucleus are held in place by the force of attraction between the positive charge of the nucleus and the negative charge of the electrons. Large bodies always contain both protons and electrons. But they do not always behave as though they carry electrical charges. A body that has as many electrons as it has protons behaves like a body that has no charge. If a negative charge is brought near such a body, the protons pull on this negative charge —but the electrons push it the other way just as hard. The push and the pull cancel each other, so the body behaves as though it had no charge at all. We say the body is electrically neutral. If electrons are added to a body that is electrically neutral, the body will have more electrons than pro¬ tons. The extra electrons will give the body a negative charge. If some electrons are removed from a body that is electrically neutral, the body will have more protons than electrons. The extra protons will give

Electricity and Magnetism

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11

the body a positive charge. We can give a neutral body a negative or positive electrical charge by put¬ ting on or removing electrons. A body that has an electrical charge is surrounded by an electrical field. We picture the field as a col¬ lection of lines. The arrow on each line shows the direction in which the push or pull of the charge would make a proton move. The field around a charge

The field around a positive charge pushes a proton away

The field around a negative charge pulls a proton in

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Electronics

ELECTRICAL LEVELS, VOLTAGE, AND CURRENT

The pushes and pulls of positive and negative electrical charges behave in some ways like the pull of gravity. Electrons surrounded by electrical charges may be compared to water in a water tank. The water is pulled down toward the ground by the force of gravity. But the water does not move as long as it is held back by the walls of the tank. The elec¬ trons are pulled by the charges, toward the positive charges and away from the negative charges. But the electrons do not move as long as they are held back, usually by the wall of air that surrounds them. If the water tank is joined to an apartment beneath it by means of an open pipe, the water flows from the higher level of the tank to the lower level of the apartment. Just as there are different levels in the space above the ground, there are different electrical

Electricity and Magnetism

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13

levels in the space surrounding electrical charges. We do not call them higher and lower levels. In¬ stead we call them more positive and more negative levels. A copper wire joining two places that are at different electrical levels is like an open pipe for electrons. The electrons flow from the more negative level to the more positive level. The flow of electrons is an electric current. Water flows through the open pipe because it is pushed by water pressure. The amount of the pres¬ sure depends on the difference between the levels

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Electronics

that the pipe joins. Electrons flow through a copper wire because they are pushed by an electrical pres¬ sure. This electrical pressure is usually called voltage. The amount of voltage depends on the difference between the electrical levels that the wire joins. The unit used for measuring electrical pressure is called a volt. A single dry cell produces an electrical pressure of 1/2 volts. The electrical pressure supplied by the electrical outlets in your house is probably between 110 and 120 volts. We use the word current for the rate of flow of electrons. The unit used for measuring electric cur¬ rent is called an ampere. If the lamp on your desk has a 100-watt bulb, when you turn it on the current that flows through it is almost one ampere. A steam iron usually draws 1000 watts of power. The current flowing through it is about nine amperes.

Electricity and Magnetism

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15

RESISTANCE AND HEAT

When a voltage is connected to the ends of a wire, the voltage pushes the electrons in the wire, trying to make them move in a current. But the wire resists this push, and tries to hold the current back. The amount of this resistance depends on the length and width of the wire, and on the material that is used to make the wire. In some materials, the electrons are held tightly by nearby nuclei. In these materials, it is hard to make a current move. In other materials, many electrons are held loosely. In these materials, it is easy to make a current move. A wire is like an obstacle course for electrons. The longer the wire is, the more obstacles there are standing in the way of the electrons. So it is easier for a current to flow through a short wire than through a long wire of the same material and the same width. It is easier for a

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Electronics

current to flow through a thick wire than through a thin wire of the same material and the same length. The unit of resistance is called an ohm. When an electrical pressure of one volt pushes against a re¬ sistance of one ohm, the amount of current that flows through the resistance is one ampere. When a current flows through a wire, it produces heat. The amount of heat produced depends on the amount of current that is flowing, and on the resist¬ ance of the wire.

DIRECT AND ALTERNATING CURRENT

There are two kinds of current in use. In direct cur¬

rent (abbreviated DC) the electrons flow in only one direction through the wire. The dry cells in a flash¬ light, and the storage batteries in an automobile sup¬ ply direct current. In alternating current (abbrevi¬ ated AC) the electrons first flow in one direction,

Electricity and Magnetism

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17

then slow down to a stop and start flowing in the op¬ posite direction. The flow changes back and forth many times during a second. The current supplied by the electric power company is almost always alternat¬ ing current. In this current, the voltage changes from moment to moment. When we say that the power company supplies 110 volts, we mean that the aver¬ age of the changing voltage is 110 volts.

CLOSED AND OPEN CIRCUITS

The first diagram p. 18 shows a lamp connected by wires to a dry cell. The dry cell, the connecting wires, and the wire in the lamp form an unbroken loop. The electrons flow around this loop in the direc¬ tion shown by the arrow. Such an unbroken loop is called a closed circuit. If one of the connecting wires is cut, and the cut ends are separated, the loop is broken. The electrons cannot get across the gap be-

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Electronics

tween the cut ends. So the current stops flowing. The broken loop is called an open circuit. To control the flow of current in the loop, we usually put a switch into the circuit. A switch is simply a gap that can be opened or closed. When the gap is closed, current flows. When the gap is opened, the current stops. The second drawing is a schematic drawing of an open circuit. In a schematic drawing, each part of the circuit is shown by a special symbol.

Closed circuit with lamp and dry cell

Schematic drawing of open circuit with switch

Symbols: Switch Battery Switch

__®_ Lamp

Electricity and Magnetism

« «

19

ELECTRICAL STORAGE TANKS

There are containers in which electrical charges can be stored. They are called capacitors. (See Chap. Ill). A simple capacitor can be made out of two metal plates held close to each other without touching. If a battery is joined to the plates, it removes electrons from one plate, and piles them up on the other. So one plate gets a positive charge while the other one gets a negative charge. We say the capacitor was charged by the battery. The plates remain charged when the battery is disconnected. If the plates are then joined by a copper wire, the electrons rush from the negative plate to the positive plate in a short pulse of current. When this happens, we say that the ca¬ pacitor has been discharged. There are many different kinds of capacitors. Some are made of metal plates separated by air. Others are

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Electronics

made of sheets of metal foil separated by paper, and rolled up to fit into a small space. There are also ca¬ pacitors in which two sheets of metal foil are sepa¬ rated by a paste made of special chemicals. To show how well a capacitor serves as a storage tank for an electrical charge, we use the measure called capac¬

itance. The unit of capacitance is called a farad.

MAGNETIC FIELDS The needle of a magnetic compass is a small bar magnet. If it is free to turn, it swings around until one end points north and the other end points south. We call these ends of the needle the poles of the magnet. One is the north-seeking pole, and the other is the south-seeking pole. We usually call them north pole and south pole for short. Every magnet has a north pole and south pole. The poles of magnets push and pull each other in the same way that electrical

Electricity and Magnetism

« «

21

charges do. Poles of the same kind repel each other, while opposite poles attract each other. Every magnet is surrounded by a magnetic field. We picture the field as a collection of lines of force. The line of force through a point shows the direction in which a north pole held at that point would be pushed by the magnet. If iron filings are sprinkled over a sheet of paper resting on a magnet, the filings line up along the lines of force, as shown in the draw-

22

Electronics

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ing p. 21. Where the lines are crowded near the poles, the magnetic force is strong. Where the lines are spread out, the force is weak.

ELECTRIC CURRENTS MAKE MAGNETS

When an electric current flows through a wire, the wire is surrounded by a magnetic field. The lines of force of this field form circles around the wire. If a magnet is held near a wire through which a current

When the electrons flow in this direction..

The magnet pushes the wire in this direction

* this is the direction of the magnetic field

The magnetic field around a wire

How an electric motor works

Electricity and Magnetism

«

«

23

flows, the magnet pushes the wire, and can make it move. This is how an electric motor works. A wire that is wound around a cylinder is called a

coil or solenoid. When a current flows through the wire, the solenoid is surrounded by lines of force like those around a bar magnet, and the solenoid behaves like a bar magnet. We call it an electromagnet. It has a north pole, and a south pole. The strength of the magnet depends on the strength of the current flow¬ ing through the coil, on the size and shape of the coil, and the number of turns of wire in the coil. It also deSymbols used in a schematic diagram Resistance .—\f\J\f\f\—

An iron core electromagnet

Inductance —— with air core Inductance — with iron core Capacitance

An air core electromagnet

2^

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Electronics

pends on what kind of material is in the space inside the cylinder. This material is called the core of the electromagnet. If the core is made of air, the mag¬ netic field near the coil is weak. If the core is made of iron, the magnetic field is stronger. The ability of a coil to build up a magnetic field around itself when a current flows through it is called its inductance. The unit of inductance is called a henry.

IMPEDANCE

Three qualities found in an electrical circuit have been mentioned so far. They are resistance, capaci¬ tance, and inductance. Different parts of a circuit have these qualities in different amounts. A capacitor in a circuit may supply nearly all of its capacitance. A coil may supply nearly all of its inductance. Some¬ times a rod of carbon may supply most of the resist¬ ance. In schematic diagrams of electrical circuits,

Electricity and Magnetism

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25

special symbols are used for resistance, capacitance, and inductance. These symbols are shown p. 23. We have seen already that resistance interferes with the flow of direct current. All three qualities, re¬ sistance, capacitance, and inductance, interfere with the flow of alternating current. The combined effect they have in holding current back is called imped¬ ance.

INDUCED CURRENTS

Electric currents can be produced by means of magnetism. If a closed loop of wire moves through a magnetic field, or if the magnetic field surrounding the wire is changed, a current begins to flow through the loop of wire. It is called an induced current. The current supplied by your electric power company is an induced current. It is produced at the power sta¬ tion by turning coils of wire in a magnetic field.

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Electronics

TIME-FLOW DIAGRAMS

In the circuits we shall describe in later chapters, the voltage or current between two points in a circuit is rarely steady. Each of them keeps changing all the time. To understand these changes, we shall find it helpful to make a time-flow diagram. Suppose, for example, we want to keep track of the voltage be¬ tween the points A and B which are separated by a resistance in the circuit shown below. First we draw a horizontal line, and let each point on the line stand for an instant of time. Points further to the right stand for instants later in time. Then above or below each point, we put a dot to show the voltage between A and B. We put the dot above the line if A is more positive than B. We put it below the line if A is more negative than B. We show the amount of voltage by the distance of the dot from the line. We put in a

Electricity and Magnetism

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27

series of dots, one for each instant of time. The series of dots forms a curve. By moving a finger along the curve from left to right, we can see how the voltage changes. Several time-flow diagrams for voltage are shown on p. 28. In diag. I, the ‘‘curve” is a straight line. When you move your finger along it, it goes neither up nor down. This kind of diagram shows a voltage that is steady, with A more positive than B. Diagram II shows another steady voltage. But here the curve is below the time line, so it shows that A is more negative than B. In diagram III, the curve starts above the time line, then drops toward the line, and then rises again. This shows that the voltage dropped and then rose again. In all three cases, the voltage would produce a direct current. The flow of current can be shown by another set of diagrams, p. 29, diag. 1, we see the kind of current caused by the voltage shown in diagram I. The fact that the line is level

28

Electronics

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Diagram of a circuit with a resistance and a voltage source B

AMAAM Voltage source

To understand what happens between A and B w we use time flow diagrams Voltage

I

Steady voltage Amount by which A is more positive than B

II

Flow of time Time line Steady direct current. Electrons flow from B to A.

Voltage Flow of time Time line

Amount by which B is more positive than A

Steady voltage Steady direct current. Electrons flow from A to B.

Ill

Voltage

Amount by which A is more positive than B

Flow of time

Time line Pulsating direct current. Electrons flow from B to A.

Electricity and Magnetism

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29

shows that the current is steady. The fact that it is above the time line shows that the current flows in one direction only, from B to A. In diagram 2 we see the current caused by the voltage of diagram II. Here, too, the current is steady. But the line showing

1

Steady current Time line

the current is below the time line. This shows that the current flows the other way, from A to B. In diagram 3, we see the current caused by the voltage of diagram III. The flow of current at first is strong. Then it grows weaker and later grows stronger again. But it is always in the same direction, from B to A. Notice that the voltage diagram has the same shape

30

Electronics

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as the current diagram. In direct current circuits, the voltage and the current go up and down together. Diagrams IV and 4 show a different set of changes. The voltage curve starts above the time line, showing that at first, A is more positive than B. Then it drops to the time line, and crosses over. The point where it crosses the line represents a time when A and B are at the same electrical level. Then the difference of levels is zero, and the voltage is zero. Later, A

SV During this time electrons flow from A to B

During this time A is more negative than B

During this time A is more positive than B

During this time electrons flow from B to A Alternating current

Electricity and Magnetism

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3i

becomes more negative than B. The current curve shows that at first the current flows from B toward A. Then it gradually weakens and stops. Then, im¬ mediately, it starts flowing the other way, from A to B, and gradually grows strong again. This is the way the voltage and the current change in an alternating current. The changes shown in diagrams IV and 4 make up one cycle of change. In an alternating cur¬ rent, cycles like these are repeated over and over again, as shown in diagram V. The number of cycles that are repeated in one second is called the fre¬ quency of the current. The current supplied by your electric power company has a frequency of 60 cycles per second. Sometimes two different alternating voltages have to be compared. If their curves keep step with each other, see p. 32 diag. V, we say that the voltages are in phase. If their curves are out of step, as in diag. VI, p. 33 we say that the voltages are out of phase. When

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Electronics

the voltages are out of phase, one of them lags behind the other. Sometimes we have to compare the changes in a voltage with the changes in current caused by the voltage. In diagrams IV and 4, the voltage and cur¬ rent are in phase. They go up and down together. This is what happens when an alternating current flows through a circuit that has only resistance. If the circuit also has capacitance or inductance, then the voltage and current will be out of phase. One will

Electricity and Magnetism

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33

lag behind the other. The amount of lag, known as the phase shift, depends on the amount of inductance or capacitance that combines with the resistance to hold the current back. In the next chapter we shall see how a phase shift can be produced.

ELECTROMAGNETIC WAVES

When an alternating current moves back and forth in a wire, the electrical field and the magnetic field

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Electronics

around the wire keep changing back and forth. These changes travel away from the wire as a wave. The wave moves through space at the speed of light, 186,000 miles per second. The wave is often repre¬ sented by the same kind of diagram we used to pic¬ ture changes in an alternating voltage or current. The frequency of the wave is the frequency of the current that sends it out. Electromagnetic waves carry radio and television programs from the antenna of a broadcasting station to the antennas of our receivers.

3 MEMBERS OF THE ELECTRONICS TEAM

When electronic tubes are in a circuit, they work as part of a team. Each member of the team usually has a special job to carry out. This chapter describes some of the team members and the things that they can do. RESISTORS

There are fixed resistors, whose resistance is a defi¬ nite number of ohms. There are also variable resistors whose resistance can be made larger or smaller.

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Electronics

Some fixed resistors are made of wire. Others are made of carbon. They are connected to a circuit by wires joined to their ends. The number of ohms of resistance in a carbon resistor is shown by a code that uses colored lines painted near one end of the resistor. Each color stands for a number, as shown in the table below. The code is read from the end of the resistor towards the center. You get the num¬ ber of ohms in the resistor in this way: the first color tells you the first figure to write down; the second color tells you the second figure to write down; the third color tells you how many zeros to put down after these figures. For example, if the colors are red, black, and yellow, you write 2, then o, and then four more zeros. Then the resistor is 200,000 ohms. Color Code for Carbon Resistors o black

1 brown 2 red

3 orange 4 yellow

5 green

6 blue

8 gray

7 violet

9 white

Members of the Electronics Team A fixed resistor

/V\A/V—

Symbol —

« «

37

Two types of variable resistor

Symbol

A variable resistor is made of a bare wire wound around a support shaped like a doughnut. A metal arm, mounted on a shaft through the center of the doughnut, touches the wire. The arm can be turned so that it can touch the wire anywhere along the top of the doughnut. When the resistor is put into a circuit, the connections are made at one end of the wire and at the shaft. A current entering the wire at the connected end flows through the wire until it reaches the arm. Then it flows out of the wire through the arm and shaft. By turning the arm, you change

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Electronics

the length of wire through which the current flows. When the length is great, the resistance is high. When the length is small, the resistance is low. When a current flows through a resistance, the two ends of the resistance are at different electrical levels. The difference between these levels is called the voltage drop across the resistance. To calculate it, you multiply the number of amperes in the cur¬ rent by the number of ohms in the resistance. For example, if a current of 0.002 amperes flows through a resistance of 10,000 ohms, the voltage drop is 0.002 X 10,000 volts, or 20 volts. One of the main uses of resistors is to introduce a voltage drop be¬ tween two parts of a circuit. If the two parts were merely joined by a connecting wire that has almost no resistance, they would be at the same electrical level. But if they are joined by a resistor, then they are placed at different levels. A variable resistor can be used to divide a voltage

Members of the Electronics Team

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39

into parts, to allow only part of it to be used. A wire is connected to each end of the resistor, and another wire is connected to the shaft. Suppose 60 volts are connected to the ends of the resistor. Then the volt¬ age drop between the ends is 60 volts. The voltage drop across one-fourth of the length of the resistor is only 15 volts. If the arm is turned so that it touches the wire one fourth of the way from one end, then the voltage drop between the arm and that end will be 15 volts. By turning the arm to different positions, any voltage from o volts to 60 volts could be ob¬ tained. A variable resistor used in this way to tap part of a voltage is called a potentiometer. A variable resistor can also be used to change the current in a circuit. In the diagram p. 40, a variable resistor is in a circuit with a lamp, and a fixed volt¬ age is supplied. When the resistance is made high, the current decreases. When the resistance is made low, the current increases. When a current flows

40

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Electronics

-WWW

_f Voltage supply

Tapped voltage

Schematic of a potentiometer

Voltage supply

A variable resistor used to change the current in a lamp

through a resistor, it produces heat. Sometimes the heat is an unwanted waste product. But in some appliances, we purposely produce the heat in this way in order to use it. In an electric stove, we use the heat for cooking. In an electric welder, we use the heat to melt the edges of two pieces of metal that are to be joined. In an electric tube we pass a current through the filament in order to make it hot. The heat makes electrons leak off the filament into the space surrounding it in the tube. Chapter IV explains how these electrons are used.

Members of the Electronics Team

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41

CAPACITORS

There are fixed capacitors, whose capacitance is a definite number of farads. There are also variable capacitors whose capacitance can be made larger or smaller. The capacitance of a small capacitor is usually shown by three colored dots that are painted on it. The colors express a number by means of the same color code used for carbon resistors. The number is the number of micro-microfarads in the capacitance. A micro-microfarad is a millionth of a millionth of a farad. A variable capacitor is made of two sets of metal plates. In each set, the plates are separated by air gaps, but are joined at the ends. One set of plates is mounted so that it does not move. The other set is mounted so that its plates lie between the plates of

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Electronics

the first set. But it can be turned so that the plates can be moved out or in. Turning the plates changes the areas that face each other on the plates. When the area is large, the capacitance is large. When the area is small, the capacitance is small. Fixed capacitors are used for temporary storage of a voltage. When the capacitor is charged, the plates are placed on different electrical levels. The difference between these levels is the stored voltage. When the capacitor is discharged, this voltage is put to work to produce a current. When a capacitor forms part of a circuit loop, it is a gap in the loop. With such a gap in the circuit, direct current cannot flow through it. But alternating current can flow through it, if the capacitance has a properly chosen value. What happens then is that the current first charges the capacitor, and starts flowing the other way just when the capacitor begins to dis¬ charge. So the current swings back and forth in step

Members of the Electronics Team

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43

with the repeated charging and discharging of the capacitor. Because of this fact, a capacitor can be used to remove an unwanted frequency from a line. For example, in a radio circuit, the current that is fed to the loudspeaker is a mixture of frequencies. Some of them are frequencies of less than 20,000 cycles per second. These are the frequencies that will produce sounds in the loudspeaker. There are other frequen¬ cies in the mixture that are much higher. They can¬ not produce sound and they would interfere with the operation of the loudspeaker. To remove them, a capacitor is connected between the terminals of the speaker. The unwanted high frequencies then flow into the capacitor instead of flowing into the speaker. A capacitor used in this way is called a by-pass capacitor, p. 44.

44

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Electronics Loudspeaker

Symbol Voltage supply n^\

Variable capacitor

\

By-pass capacitor

INDUCTANCES

Inductances, like resistors and capacitors, may be fixed or variable. A typical fixed inductance is made of a solenoid wound around a hard plastic cylinder. The wire of the solenoid is insulated, that is, it is coated with some material that does not conduct electricity. As a result, although neighboring turns of the coil may touch each other, the current cannot leak across the contact points. There are several different ways of making a vari-

Members of the Electronics Team

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45

able inductance. One way is to scrape off the insula¬ tion at several places on the coil and attach wires there as well as at the ends of the coil. If any pair of these wires is connected into a circuit, only that part of the coil is used that lies between the wires. Smaller and larger sections of the coil have different lengths and different numbers of turns of wire, so thev have different inductances. j

The inductance of a coil depends on the nature of its core. So another way of changing the inductance of a coil is to change its core. When a coil is wound around a hollow cylinder, it has an air core. If a rod of iron is moved into the cylinder, the core becomes iron. If the rod is moved partly out of the cylinder, the core becomes partly iron and partly air. So the inductance of the coil can be changed gradually by moving the core in or out. A third way of making a variable inductance is

46

»

»

Electronics

shown in the diagram below. Coil A is wound around an iron ring. Coil B is wound around the same ring, and a direct current is passed through it. The direct

current produces magnetism in the ring. This gives coil A a magnetized core. The inductance of coil x4 is changed by changing the amount of magnetism in its core. This is done by using the variable resistor shown in the diagram to change the amount of cur¬ rent flowing through coil B. The chief uses of inductances are described in later paragraphs.

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47

SERIES AND PARALLEL CIRCUITS

Complicated circuits are made by joining together simple parts. There are two main ways of joining the parts. One way is to place the parts one after the other, so that an electric current must pass through one part before reaching the other. When parts are joined in this way, we say they are in series. Parts are joined in series when we want the parts to be at

k

different electrical levels. Another way of joining electrical parts is to connect them side by side, like two roads branching off from a single highway. Then an electric current that approaches the junction di¬ vides into two currents. One current flows through one part, while the other current flows through the other part, until the second junction is reached. There the two currents combine into a single cur-

48

Electronics

» »

A___®__®-_®__ Lamps in series

@

_

1

*

i

®

_

)

*

Lamps in parallel

Arrows show direction of flow of current

rent again. When parts are joined in this way, we say they are in parallel. Parts are joined in parallel when we want them to be at the same electrical level. In the diagram on p. 46, coil B and the variable re¬ sistor are connected in series. In the diagram on p. 44, the by-pass capacitor and the loudspeaker are connected in parallel. The ordinary lamps in your house are connected in parallel. If one lamp burns out, the current stops in the branch of the circuit that contains that lamp. But the other lamps can still draw current because they are in other branches of the circuit. Lamps for

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49

Christmas tree decorations are sometimes connected in series. If one of these lamps burns out, the current stops in all of them.

TRANSFORMERS

Two coils held close to each other can be used to change low voltage to high voltage, or vice versa. A combination of coils used in this way is called a

transformer. If the coils are wound side by side on a hollow cylinder, it is called an air core transformer. If they are wound around an iron ring, it is called an iron core transformer. Here is how a transformer works. One coil, called the primary coil, is connected to an alternating cur¬ rent. The other coil, called the secondary coil, is con¬ nected to some appliance (the load) for which a voltage supply is wanted. The changing current in the primary coil produces a changing magnetic field

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Electronics

Plastic cylinder

1

< —

Air-core transformer and symbol

Iron-core transformer and symbol

in the core. The changing magnetic field in the core then induces a changing current in the secondary coil. If the secondary coil has more turns of wire than the primary coil, the voltage coming out of the secondary is higher than the voltage put into the primary. In this case the transformer is a step-up transformer. If the secondary coil has fewer turns of wire than the primary coil, the voltage coming out of the secondary is lower than the voltage put into the primary. In this case the transformer is a step-

down transformer.

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51

RELAYS

A relay is a switch that is controlled by an electric current. A simple relay is shown in the diagram be¬ low. Its main parts are a coil wound around an iron core, and an armature that is the movable part of a switch. A spring is attached to the armature. When no current flows through the coil, the spring keeps the armature away from the contact point and the

When current flows through the coil....

When no current flows through the coil....

Voltage

Voltage

... the lamp circuit is open

... the lamp circuit is closed

How a relay works

52

» »

Electronics

circuit is open. When a current flows through the coil, the core is magnetized. The magnetized core then pulls the armature until it touches the contact point. In this way the circuit is closed.

FILTERS

A coil has the property that it resists any changes in a current flowing through it. If the current begins to increase, the inductance of the coil tries to decrease it. If the current begins to decrease, the inductance tries to increase it. As a result, the induct¬ ance has a levelling effect on the current. It can be used to change an unsteady current into a steady current. A device used for this purpose is called a

filter. A capacitance also has a filtering effect. In the diagram p. 53, a capacitance is in parallel with a resistor. Suppose an unsteady current approaches

Members of the Electronics Team

« «

53

Two types of filter

AWV\A

if

w

Symbols —VVVV\A— Resistor ===== Inductance with iron core ”"j ( Capacitor

the junction. When the current approaching is high, part of it flows through the resistor and part onto the capacitor. So the current flowing through the resistor is less than the current that approaches the junction. Meanwhile the capacitor is charged. Then, as the current approaching the junction falls, the capacitor discharges through the resistor. The dis¬ charge boosts the current, so that it doesn’t fall too much. In this way the unsteady current is levelled

off. To make a more effective filter, capacitors are

54

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Electronics

rrrrrrrrr\ Current before passing through filter

Current after passing through filter

often joined to inductors or resistors, as shown in the drawing below. The time-line diagrams on this page show the effect of a filter.

TANK CIRCUITS

A capacitor and a coil connected in parallel is called a tank circuit. If the capacitor is charged and then allowed to discharge through the coil, a current begins to surge back and forth in the tank circuit with a regular rhythm. Every tank circuit has its own

Members of the Electronics Team

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55

natural rhythm or frequency. The frequency de¬ pends on the capacitance of the capacitor and the inductance of the coil. Most of the current in the circuit will have this frequency. In addition, small amounts of current are generated with a frequency that is a whole number times the natural frequency. It may be double the natural frequency, or triple the natural frequency, for example. These higher fre¬ quencies are called harmonics of the natural fre¬ quency. Suppose an alternating current is fed into a tank circuit. If the frequency of the current matches the natural frequency of the tank circuit, it flows easily in the circuit. If the frequencies do not match, some¬ thing else happens. While the current tries to move with one rhythm, the tank circuit tries to push it with another rhythm. The rhythms clash, like two people trying to push the same thing in opposite directions at the same time. The result is that hardly

56

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Electronics

any current flows in the tank circuit at all. A tank circuit is very selective about what kind of current it allows to enter. It admits only currents that vibrate with its natural frequency. This selective behavior of a tank circuit is called resonance. A tank circuit can be made with a variable capac¬ itor. As the capacitance is changed, the natural fre¬ quency of the circuit is changed, too. Changing the frequency is called tuning. Suppose a current that is a mixture of many different frequencies is fed to the tank circuit. If the circuit is tuned to one of these frequencies, this one frequency will be admitted to the circuit. All the others will be kept out. So a tuned tank circuit can be used to select one of many fre¬ quencies in a mixture. Tuned circuits do an impor¬ tant job in radio and television receivers, as we shall see.

Members of the Electronics Team

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57

TIMING CIRCUITS

When a capacitor is charged, a voltage is stored on it. If the capacitor is discharged through a resistor, the voltage between the plates of the capacitor drops gradually. How fast it drops depends on the capac¬ itance of the capacitor and the resistance of the re¬ sistor. If the number of farads in the capacitance is multiplied by the number of ohms in the resistance, the result is the number of seconds it takes for the capacitor to lose two-thirds of its stored voltage. This number is known as the time constant. For ex¬ ample, if the capacitance is one millionth of a farad, and the resistance is one million ohms, then the capacitor will lose two-thirds of its voltage in 1 second. When a capacitor is discharged through a resistor, it loses over 99% of its voltage in a length of time equal to five times the time constant. The delay

58

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Electronics

between the beginning and end of the discharge gives us a way of controlling the time when things shall happen in an electrical circuit, and the length of time during which they happen. A simple use of a time delay circuit is pictured in the diagram p. 59, where a capacitor is shown in parallel with the coil of a relay. When switch S is closed, the coil of the relay is magnetized, the arma¬ ture is pulled to the left, and switch T is closed. At the same time, the capacitor C is charged. When switch S is opened, the current through the coil does not stop at once. The capacitor C discharges through the resistance of the coil. This keeps the magnetism in the core for a little while longer. The armature is released, and switch T is opened only after the core becomes too weak a magnet to oppose the pull of the spring. In this way, a time delay is introduced be¬ tween the opening of switch S and the opening of switch T,

Members of the Electronics Team

«

«

59

|-Voltage T supply r— 1

'

-■

Me

Voltage supply

^

-1

Tank circuit

Relay circuit with time delay

A time delay circuit also has another use. It introduces a phase shift between voltage and current.

MAGNETIC AMPLIFIERS

By putting together a few of the simple parts already described, it is possible to make a magnetic

amplifier. In a magnetic amplifier, small changes in a direct current can be used to control large changes in an alternating current. The parts used to make a magnetic amplifier are a variable resistor, and a variable inductance made of two coils wound on an

6o

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Electronics

iron core. The coils are wound on the core as shown in the drawing. When the voltage in the direct cur¬ rent coil is increased, the inductance of the alter¬ nating current coil is decreased. This combination of coils and core is called a saturable core reactor. The symbol for it is shown alongside the drawing. The diagram for the magnetic amplifier is shown p. 61. An alternating current supply is in series with the load and the alternating current winding

Saturable core reactor

Members of the Electronics Team

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61

of the saturable core reactor. The direct current supply is in series with a variable resistor and the direct current winding of the saturable core reactor. When the resistance of the variable resistor is made high, the current through the direct current coil is low. Then the core is magnetized only slightly, and

AC

Magnetic amplifier

the inductance of the alternating current coil is high. As a result, the coil uses up a large share of the alter¬ nating current voltage, and leaves little of it for the load. If the resistance in the variable resistor is made

62

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Electronics

lower, more current flows through the direct current coil. The core becomes more strongly magnetized. The inductance of the alternating current coil be¬ comes lower. As a result, it uses up less of the alter¬ nating current voltage, and leaves more of it for the load. A small change in the voltage fed to the direct current coil can lead to a very large change in the voltage supplied to the load. The saturable core reactor also has another use. When the inductance of the alternating current coil is changed, a phase shift is produced between the voltage and the current flowing through it. We shall see in Chapter VI how such a phase shift can be used to control the timing of a welding machine.

4 THE HOT CATHODE TUBE AND RADIO

There are three main types of electronic tube. Each of them uses a different way of starting a cur¬ rent through the tube. One type uses heat as a starter. The second type uses light as a starter. The third type starts the current by first cracking molecules of gas in the tube. Each type of tube and some of its uses will now be described in a separate chapter.

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Electronics

THE HOT CATHODE TUBE

In a hot piece of metal, the molecules are all jump¬ ing around violently. They have many collisions, and often knock each other’s electrons out of place. If the metal is surrounded by a vacuum, some of these electrons leak off the metal into the space around it. We take advantage of this fact in the hot cathode tube. A simple example of such a tube is shown in the diagram p. 65. A filament or wire is sealed into a tube from which the air has been re¬ moved. The tube also contains a metal plate, sep¬ arated from the filament by empty space. A battery (called an A battery) pushes a current through the filament. The current makes the filament hot. Then electrons leak off the filament into the space around it. Another battery (called a B battery) is connected to the filament and the plate to make the plate posi-

The Hot Cathode Tube and Radio

« «

tive and the filament negative. Then the electrons flow from the negative filament to the positive plate. The plate, the B battery, the filament, and the space between the filament and the plate form a closed circuit, called the plate circuit. As more electrons leak off the filament, they are pulled toward the

A diode

plate. So a steady current flows in the plate circuit. The result is different if the connections of the B battery are reversed. Then the plate is made nega¬ tive, and the filament is made positive. The negative

66

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Electronics

plate does not pull the electrons towards it, and no current flows through the tube. A current flows in the plate circuit only if the plate is made positive. A tube like this is called a diode. The plate is called the anode, a name used for a receiver of electrons. The filament is called the cathode, a name used for a releaser of electrons. Now suppose an alternating voltage, instead of a direct current B battery, is connected to the filament and the plate. As the voltage changes back and forth, the plate is sometimes positive and sometimes neg¬ ative. A current will flow through the tube only when the plate is positive. The current passes through in a series of pulses. In each pulse, the current flows from the filament toward the plate. So they are pulses of direct current. The diode behaves like a one-way street. It allows current to flow through it in only one direction. As a result, it has used an alternating voltage to produce a direct current.

The Hot Cathode Tube and Radio

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6y

In some diodes, the cathode is separate from the filament, as shown in the diagram below. The cath¬ ode is another piece of metal mounted near the fila¬ ment. The filament is used as a heater to make the cathode hot. Then electrons leak off the cathode. The filament here does not become part of the plate circuit.

Plate Cathode

•- Heater

Indirect-heater type diode

r\ r\ The part that gets through

RECTIFIERS

When electric power is sent over long distances, it is easier and cheaper to send it in the form of alter-

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Electronics

nating current. That is why the current you get from the power company is alternating current. But some appliances we use need direct current. So we have to change the alternating current to direct current. This is done by a rectifier. A diode can serve as a rectifier. What the diode does to the alternating current fed to it is shown in the diagrams p. 67. The first dia¬ gram shows the kind of current usually produced by an alternating voltage. It is a series of complete cycles. The upper half of each cycle represents the time when the anode is positive. The second diagram shows the part of it that the diode allows to pass through. It is a series of bumps. Each bump is the positive half of a cycle. It represents a pulse of direct current, first growing stronger and then weaker. Although the current is direct, it is not steady. To make it steady it is passed through a filter circuit, which is part of the rectifier circuit.

The Hot Cathode Tube and Radio

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6g

AN AMPLIFIER

Now let us change a diode by placing a wire mesh screen between the cathode and the plate. This screen is called a grid. Because the tube now contains a third element, it is called a triode. The grid gives us a means of controlling the flow of current through the tube. Suppose we connect a B battery to the cathode and plate to make the plate positive. Then a current flows from the cathode to the plate, passing

A triode

70

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Electronics

through the holes of the grid. Now we connect an¬ other battery between the cathode and the grid. If we connect the positive terminal of the battery to the grid, the grid becomes more positive than the cath¬ ode. Then the grid helps the cathode pull electrons away from the cathode. Because of the extra pull from the grid, the current to the plate becomes stronger. If we connect the negative terminal of the battery to the grid, the grid becomes more negative than the cathode. While the positive plate pulls the electrons that have leaked off the cathode, the nega¬ tive grid pushes them back. This weakens the current flowing to the plate. Now suppose that an alternating voltage, rather than a battery, is connected between the grid and the cathode. As the voltage changes back and forth, the grid is sometimes positive and sometimes nega¬ tive. It sometimes helps the plate, and sometimes opposes the plate. As a result, the plate current rises

The Hot Cathode Tube and Radio

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71

and falls in step with changes on the grid. The changes in the plate current will have the same fre¬ quency as the changes in the grid current. If we think of the changes in the grid current as a signal, the changes in the plate current are a copy of this signal. Because the grid is close to the cathode, small changes in the grid current produce large changes in the plate current. So the signal in the plate current is stronger than it was in the grid current. The triode has served as an amplifier to strengthen a weak signal.

THE TRIODE AS RECTIFIER

Sometimes a battery (known as a C battery) is connected to the grid and cathode, with the negative terminal of the battery joined to the grid. Then, if an alternating current is fed to the grid, when the cur¬ rent flows in one direction through the grid, the grid

J2

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Electronics

is positive and a current flows in the plate circuit. When the current flows the other way through the grid, the grid is negative enough to cut off this cur¬ rent altogether. Then the grid turns alternating cur¬ rent into direct current in the same way that a diode does. So a triode, too, can serve as a rectifier.

THE TRIODE AS OSCILLATOR

In the diagram p. 73, the current in the plate circuit of a triode is fed into a tank circuit. In the tank circuit the current surges back and forth as an alter¬ nating current with a definite frequency. The tank circuit has some resistance. Working against this resistance, the current produces heat. The more heat it produces, the weaker the current becomes. If nothing were done to boost it, the current in the tank circuit would gradually die out. But something is being done to boost it. Part of the current from the

The Hot Cathode Tube and Radio

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73

tank circuit is drawn off and connected to the grid of the tube. The weak signal in the grid produces a strong signal in the plate current, and this strong signal is fed back into the tank circuit. As a result, the current in the tank circuit is boosted enough to keep it from dying out. In this way, an alternating current with a strong steady rhythm is kept going. A combination of tube and tank circuit so used is called an oscillator. The part of the current that is carried back from the plate circuit to the grid is called

feedback.

Grid

Tank circuit

An oscillator

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Electronics

A MIXER TUBE

It is possible for a tube to have more than one grid. Suppose the tube has two grids. Feed to one of the grids an alternating current with a high fre¬ quency. Feed to the other grid an alternating current with a lower frequency, produced by an oscillator circuit. Both grids pass their frequencies on to the plate current. The combined effect of the two fre¬ quencies is that the lower one is subtracted from the higher one. For example, if the higher frequency is 1,500,000 cycles per second, and the lower frequency is 1,200,000 cycles per second, then the frequency of the plate current will be 300,000 cycles per second. A tube used in this way is called a mixer. Sometimes the jobs of the oscillator and the mixer are combined in one tube called a converter. The mixing of two

The Hot Cathode Tube and Radio

« «

75

frequencies to produce a lower frequency equal to their difference is called heterodyning.

RADIO BROADCASTING We now know enough about electronic tubes to understand how they are put to work in radio broadcasting. There are two different systems of broadcasting that are used. One is called AM, foramplitude modulation. The other is called FM, for frequency modulation. We shall describe AM broad¬ casting first. The announcer on a radio program speaks into a microphone. The microphone is like the transmitter of a telephone. It picks up the sound vibrations pro¬ duced by the speaker’s voice and converts them into electrical vibrations of a current flowing through a wire. The frequency of these vibrations may be any-

76

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Electronics

where between 20 cycles per second and 20,000 cycles per second. The current from the microphone is fed into one or more amplifying circuits to strengthen it. In the station’s radio transmitter there is an oscil¬ lator circuit producing alternating current with a high frequency. The frequency is controlled by the tank circuit. The station uses a frequency assigned to it by the Federal Communications Commission. The frequency lies somewhere between 535,000 cycles per second and 1,605,000 cycles per second. The current from the oscillator is fed into one or more amplifying circuits in order to strengthen it. A tank circuit in each of these circuits helps to keep the frequency fixed. In the last amplifier circuit, the strengthened signal from the microphone is fed into the grid of the tube. The effect is shown in the dia¬ grams p. 77. Diagram 1 shows the amplified current that came from the microphone. This is the sound or

The Hot Cathode Tube and Radio

«

«

77

1. Audio signal '■'N

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0

r\

n

L

Kj

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7

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r\

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Pi

p

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2. Radio signal

n

r\

limn

AAl

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3. Amplitude modulated radio signal (Audio signal carried on back of radio signal)

audio signal. Diagram 2 shows the amplified current that came from the oscillator. This is the radio signal. Diagram 3 shows the plate current that results when the two are mixed. It is the modulated radio signal. The radio signal is carrying the audio signal on its back. The modulated signal is fed to the antenna, which sends a radio wave out into space. Diagram 3 also represents the vibrations in the radio wave.

78

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Electronics

A RADIO RECEIVER

A radio receiver picks up the signal sent out by the broadcasting station and converts it into sound. It does this in a series of steps. Step one: When the radio waves pass the antenna of the radio receiver, they

Block diagram of an AM receiver

make an electric current flow in it, in step with the vibrations in the wave. The diagram of this current is the same as diagram 3, p. 77. Step two: The current from the antenna is fed into a converter. This reduces

The Hot Cathode Tube and Radio

« «

79

the frequency of the radio signal. The new signal still carries the audio signal on its back, as shown in dia¬ gram 4. Step three: The signal is passed on to an amplifier, to strengthen it. Step four: The strength¬ ened signal is fed to the grid of a tube called a de¬

tector. This tube changes the alternating current into pulsating direct current. The result is shown in dia¬ gram 5. Step five: The pulsating direct current is fed into a filter which fills in the gaps in the current. The result is shown in diagram 6. The output of this stage

nnnnnnnnnflfln

5. Pulsating direct current

6. Exact copy of original audio signal

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Electronics

is an exact copy of the audio signal that originally came from the microphone at the radio broadcasting station. This signal is fed into the loudspeaker which converts it into sound the way a telephone receiver does. The tank circuits in a radio receiver have variable capacitors in them. When we turn the dial of a re¬ ceiver we are turning the capacitors and changing the capacitance in the tank circuits. This changes the frequency of the current that may vibrate in them. Different stations broadcast at different frequencies. Each setting of the dial selects one frequency that is admitted into the circuit. In this way the radio picks up only one station at a time.

POWER SUPPLY

The power to operate a transmitter or receiver comes to us from the electric power company as

The Hot Cathode Tube and Radio

« «

81

alternating current at no to 120 volts. A transformer in each set changes the voltage by stepping it up or down to the levels needed in different parts of the circuits. A rectifier changes some of it into direct current to take the place of the B battery and C battery needed for the tubes.

FM BROADCASTING In AM broadcasting, the audio signal is used to change the strength or amplitude of the radio signal. In FM broadcasting the audio signal is used to change the frequency of the radio signal. The FM system has the advantage that its broadcasts are not spoiled by static or noise that comes from electrical disturbances in the air. It has the disadvantage that FM broadcasts cannot travel as far as AM broadcasts. FM stations use much higher frequencies than AM stations. Each FM station is assigned a frequency

82

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Electronics

between 88 million cycles per second and 108 million cycles per second. As in AM broadcasting, an FM station produces an audio signal and a radio signal, and then mixes them to produce a modulated signal. But it uses different methods for producing the radio signal and then modulating it. An FM transmitter produces its assigned radio frequency in several steps. First it produces a fraction of that frequency. Then it builds it up to the higher frequency that is wanted by doubling it a number of times. For example, suppose the assigned frequency is 96 million cycles. One fourth of this is 24 million cycles. The tank circuit of an oscillator is tuned to produce this frequency. When it oscillates it pro¬ duces a 24 million cycle frequency signal, but it also produces a 48 million cycle frequency, a harmonic, as a weaker signal. The two signals are fed into an amplifier whose tank circuit is tuned to 48 million cycles. The 48 million cycle signal is admitted to the

The Hot Cathode Tube and Radio

« «

83

circuit and strengthened. At the same time a 96 mil¬ lion cycle signal is produced as a harmonic. Now these two signals are fed into an amplifier whose tank circuit is turned to 96 million cycles. Now the 96 mil¬ lion cycle signal is admitted and strengthened. To modulate the radio signal, we take advantage of a special fact about alternating current circuits. We have already seen that putting a capacitor or an inductance into a circuit introduces a phase shift, so that the voltage and current are no longer in step with each other. It also works the other way round. If a phase shift between voltage and current is injected into a circuit it is like putting in more capaci¬ tance or inductance. If this is done to a tank circuit, it changes the frequency with which current oscil¬ lates in the tank circuit. In an FM transmitter the audio signal that comes from the microphone is used to control this change, with the help of a reactance

tube circuit. A simplified diagram of such a circuit is

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Electronics

shown below. The audio signal is fed into the grid of the reactance tube. It is also connected between the capacitor Ci and the resistor R. The capacitorresistor combination introduces a phase shift in the grid circuit. The grid circuit passes the phase shift on to the plate circuit, and the plate circuit passes it on, through the capacitor C2, to the tank circuit of the oscillator. The amount of change in the frequency of the tank circuit depends on the strength of the signal on the grid. This in turn depends on the loudness of the sound picked up by the microphone. A loud sound causes a large change in frequency. A soft

To tank circuit of oscillator

Audio signal

The Hot Cathode Tube and Radio

« «

£5

sound causes a small change in frequency. The changes are repeated many times, in step with the vibrations of the audio signal. The effect of this system on the radio signal can be shown by a series of diagrams. Diagram 1 represents the radio signal before it is modulated. The even spacing of the wave shows that the frequency is steady. Diagram 2 represents the modulated signal. Where the waves are more crowded, the frequency has been made higher. Where the waves are spread out more, the frequency has been made lower. This

n n

J u u u u u u u U U U U U U U U U v u u u c u

1. Radio signal

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Frequency modulated radio signal

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86

» »

Electronics

is the kind of signal that is amplified and sent out from the antenna of the FM transmitter. In the FM receiver the modulated signal is picked up by the antenna. This signal is a mixture of the radio signal and the audio signal. Special circuits which we shall not describe separate the audio signal from the radio signal, so that it may be fed to the loudspeaker.

5 THE PHOTOTUBE AND TELEVISION

THE PHOTOTUBE

Some metals are very sensitive to light. When light falls on them, the light knocks electrons out of the metal. This fact makes it possible to use these metals for changing light signals into electrical signals. The simplest device for doing this is a phototube. A phototube is a special kind of diode whose cathode is not heated. Instead, it is coated with a metal that is sensitive to light. When light falls on the cathode, electrons are released into the space near the cath-

88

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Electronics

ode. If a voltage is connected between the cathode and the anode so that the anode is made positive, then the anode pulls these electrons, and a current flows through the tube. When no light falls on the cathode, no electrons are released, and no current flows. If the light falling on the cathode is weak, not many electrons are released, and the current through the tube is small. If the light falling on the cathode is strong, many electrons are released, and the current is somewhat larger.

THE PHOTOMULTIPLIER TUBE

A single stone rolling down a mountainside can start an avalanche. The stone may strike two other stones and loosen them. Then three stones will be rolling down. Each of them may then loosen two others. Then nine stones will be rolling down. More and more collisions will set more and more stones

The Phototube and Television

« «

rolling until thousands of them come roaring down together. A photomultiplier tube is a device for releasing an avalanche of electrons. Like a simple phototube, it has a light-sensitive cathode and an anode. But several other electrodes called dynodes are mounted

Cathode

Cathode First dynode Second dynode

Third dynode

Anode

How a photo-multiplier works

between them. When light falls on the cathode, electrons are released. They are pulled toward the first dynode. When an electron strikes the dynode, it knocks three or more electrons out of it. These

go

» »

Electronics

electrons are then pulled toward the second dynode, where again each electron releases three or more electrons. As the electrons move from dynode to dynode, more and more electrons are released. In this way, a single electron released from the cathode can send thousands of electrons to the anode. Be¬ cause of this fact, a photomultiplier tube can convert a very weak light signal into a strong electrical signal. It is a super-sensitive phototube.

THE MAGIC EYE You may have seen in a large store or in a railroad station a door that is controlled by a “magic eye.” When somebody approaches the door, the door opens by itself. Here is how it works. The 'magic eye” is a phototube mounted in front of the door, at one side of the approach to the door. A lamp on the other side of the approach shines a beam of light onto

The Phototube and Television

«

«

91

Photo tube

the phototube. As long as the light falls on the tube a current flows through it. When someone approaches the door, he passes between the lamp and the phototube. He blocks the beam from reaching the phototube. When no light reaches the phototube, the current through it stops. The interruption of this current operates a relay which closes the switch of the mechanism that swings the door open.

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Electronics

In industry, a phototube is often used for count¬ ing objects moving by on a conveyor belt. A lamp sends a beam of light across the conveyor belt to the

phototube. Every time an object on the belt passes by it interrupts the beam, so it interrupts the current through the tube. The tube is connected to a counting device that counts the interruptions.

The Phototube and Television

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93

SOUND PICTURES

A phototube plays a part in producing the sound you hear when you watch a motion picture. The sound is recorded in the sound track, made up of a series of light and dark bands that lie along one edge of the motion picture film. As the film is run through the projector that puts the picture on the screen, the sound track passes a phototube. A thin beam of light shines through the track into the tube. When the beam passes through a light band in the sound track, most of the light gets through to the phototube. When the beam passes through a dark band, less of the light gets through. So, as the film moves through the projector, the amount of light reaching the phototube keeps chang¬ ing. As a result, the phototube produces a changing electric current. This changing electric current is an

94

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Electronics

audio signal that is passed on to amplifiers and then to a loudspeaker.

THE PHOTOTUBE THERMOMETER

When a metal is heated it begins to glow. As the temperature of the metal rises the color of the glow changes. First it is red, then orange and yellow, and finally, white. At the same time that the color of the glow changes, the amount of light also changes. The higher the temperature of the metal becomes, the more light it sends out. So the temperature of a glow¬ ing metal can be measured by means of the fight that comes from it. The fight is allowed to fall on the cathode of a phototube. The current that flows through the tube is then fed into an electric meter where it makes a pointer turn. The pointer shows the temperature directly on the dial of the meter.

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95

A CHEMICAL ANALYSER

White light is a mixture of colors. When a beam of white light is sent through a chemical solution, the chemical in the solution removes some of the light of each color, and allows the rest to pass through. How much light is removed depends on the nature of the chemical.

Different

chemicals

remove

different

amounts of the various colors found in white light. This fact makes it possible to use a phototube as a chemical analyser. To identify an unknown chemical with the help of a phototube, the chemical is first dissolved in water. A beam of light is sent through the solution. The light that comes through the solu¬ tion is then analysed to see how much light of each color is left in it. This is done in several steps. First the light is passed through a color filter that allows only one color to go through. Then the light is caught

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Electronics

by a phototube, so the amount of light of that color can be measured. By using different filters different colors can be allowed through, one at a time, to be measured. The chemical in the solution can be recog¬ nized by the amount of light of each color that it allowed to pass through.

PHOSPHORS

A phototube changes light energy into electrical energy. There is a way of reversing this change with the help of special chemicals known as phosphors. If a beam of fast-moving electrons strikes against a phosphor, the phosphor begins to glow. In this way electrical energy is changed into light energy. Photo¬ tubes and phosphors together give us the foundation for television broadcasting. At a television studio a light signal from a scene or picture is changed into an electrical video signal. The video signal is sent out

The Phototube and Television

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97

into space on the back of a radio wave. In a television receiver the video signal is separated from the radio wave, and is used to fire a stream of electrons against a phosphor screen. The glowing screen makes a copy of the scene or picture that was in the studio. We shall now describe some of the steps by which this is done.

THE ELECTRON GUN

First we have to get acquainted with the electron

gun. An electron gun is a device for shooting out a stream of fast-moving electrons. It is really a special kind of hot cathode vacuum tube. As in a simple diode, a heater makes a cathode hot, so that it re¬ leases electrons. Two collars, mounted in front of the cathode, serve as anodes. A high positive voltage on the second anode pulls the electrons, making them move in a rapid stream. As the electrons move along,

98

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Electronics

they pass through the first anode. The electrical force from this anode, called the focussing anode, squeezes the electrons into a narrow beam. The moving stream of electrons fired by an elec¬ tron gun is like an electric current. It is surrounded by a magnetic field. Because of this magnetic field, the stream can be pushed by a magnet. This pushing is usually done by two electromagnets in a deflection

yoke mounted around the neck of the tube. One of the magnets pushes the stream sideways. The other one pushes the stream up or down. By the combined action of these two magnets, the stream can be aimed

Focussing anode

Accelerating anode

The electron gun

The Phototube and Television

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gg

at any point of the target at the other end of the tube. Alternating currents through the coils of the mag¬ nets can make the stream of electrons sweep across the face of the target in a series of lines, zigzagging back and forth and moving down from top to bottom at the same time. This process is called scanning, because it resembles the way in which we scan the printed lines of a book. Electron guns are found in television cameras, as well as in the picture tubes of television receivers. They have many other uses, too.

Electron Cathode stream

Target

*

X-ray tube

IOO

» »

Electronics

One of the oldest is in the X-ray tube, where the elec¬ tron gun shoots electrons at a metal target. In this tube, when the electrons strike the metal, the metal sends out X rays, p. 99.

THE TELEVISION CAMERA

A television camera has the job of converting the light signal from a scene into an electrical signal. It does this in three steps. First, as an ordinary camera does, it forms a picture of the scene on a screen. Then, secondly, it makes an electrical image of the picture. Finally, it uses the electrical image to create an electrical signal. There are several different kinds of television cameras, each working in a different way. We shall describe only one of them, known as an image orthicon. In an ordinary camera, light from a scene being photographed first passes through a lens. The lens

The Phototube and Television

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101

focusses the light onto a sheet of film, where an image of the scene is formed. The film is covered with an emulsion. When the light strikes the emul¬ sion, the light image is changed into a chemical image. In the image orthicon, light from a scene also passes through a lens first. The lens focusses the light onto a screen that is inside the envelope of the tube. The screen in this case is a photocathode. It releases electrons when it is struck by light. The im¬ age formed on the screen has dark spots and light spots, matching the dark parts and light parts of the scene. A dark spot is a place where not much light has struck the photocathode. So, at a dark spot, not many electrons are released. A light spot is a place where much light has struck the photocathode. So, at a light spot, a larger number of electrons are released. Different parts of the photocathode release different amount of electrons. An acceler¬

ator grid, carrying a positive voltage, pulls the

102

Electronics

» »

electrons and makes them move. A focus coil, through which a direct current flows, creates a mag¬ netic field in the tube. This field forces the electrons to move in parallel lines to a glass target. There is a metal screen in front of the target. The electrons pass through the holes of the screen to reach the target. Now let us trace the action that starts at a small spot on the photocathode. Light that struck this spot knocked out some electrons. The electrons, pulled by the accelerator grid and guided by the focus coil, moved to the target in a narrow beam. When the Photocathode Lens

;:;•; • ••’£5 Accelerator grid '

Dynodes Collector plate

1 >1 1

►Video signal

1 V

1

c'CQ'tt

Metal screen

Image orthicon

The Phototube and Television

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103

beam strikes the target, it knocks electrons out of it. These electrons are caught by the screen that is near the target, and are removed. At the spot where the beam struck, since electrons were lost, a positive charge remains. A dark spot on the photocathode sends out a weak beam of electrons. A weak beam knocks out only a few electrons from the target, leaving a weak positive charge. A light spot on the photocathode sends out a strong beam of electrons. A strong beam knocks out many electrons from the target, leaving a stronger positive charge. In this way each spot on the photocathode produces a matching charged spot on the surface of the target. The light image on the photocathode has become an electrical image on one side of the target. The charged spots lie side by side on the surface of the target. It is difficult for an electric current to flow along the surface, so each spot keeps its own charge without sharing it with its neighbors. How-

104

>y >y

Electronics

ever, an electric current can flow through the target. Each charged spot is positive, so it pulls electrons away from the opposite face of the target. The electrons flow through the target from one face to the other. So each charged spot on the first side of the target causes another charged spot just like it to appear on the other side of the target. In this way, the electrical image of the scene, originally formed on one side of the target, is transferred to the other side. The next job of the image orthicon is to “read” this image, spot by spot and line by line, the way we read a book, word after word, and line by line. This is done with the help of an electron gun. The electron gun is mounted at the other end of the tube. It fires a beam of electrons at the target. The beam bounces back from the target and is caught on a collector plate. The beam starts out from the gun moving fast. It is slowed down just before it reaches the target, so

The Phototube and Television

«

«

*°5

it does not hit it hard enough to knock out electrons. The deflection yokes that control the movement of the beam make it scan the target, so that the beam strikes spot after spot on line after line in a definite order. Now let us see what happens when the beam of electrons from the electron gun strikes a spot on the target. This spot carries a positive electrical charge. The charge pulls some electrons out of the beam. So the beam, after it bounces away from the target, has fewer electrons in it than it had when it struck the target. A spot that has a large charge pulls out more electrons than a spot that has a small charge. So, as the beam scans the target, moving from spot to spot, the reflected beam changes in strength. The changes match the charges at the spots that are scanned. The reflected beam does not go directly to the collector plate. It first bounces off several dynodes, which amplify the beam the way they do in a

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Electronics

photomultiplier tube. The amplified beam comes out of the collector plate as a changing current. This is the video signal that will be broadcast.

SCANNING

The scanning of the image on the target is done in a special way, somewhat different from the way in which we read a book. When we read a book we scan the lines one after the other without skipping any. When the beam from the electron gun scans the target it skips lines. The target is divided into 525 lines, one under the other. Imagine these lines num¬ bered from 1 to 525, starting at the top. The electron beam at first scans only the odd numbered lines. Then it starts at the top again to scan the even numbered lines. It completes the job of scanning all 525 lines in one-thirtieth of a second. Then it starts all over again with the odd numbered lines.

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107

When the camera is directed at a moving scene, the picture focussed onto the photocathode keeps changing. As a result, the electrical image that matches it on the target keeps changing, too. The beam of the electron gun scans a complete image in one-thirtieth of a second. At the end of that time, it starts scanning the image again. But by that time the image has changed, so it is a slightly different image that is scanned next. The beam scans a line by moving from left to right and downwards at a very small angle. When it com¬ pletes the line, it is at the right side of the target. It must move back from right to left before it can scan another line. It is necessary to keep the beam from striking the target when it sweeps back to the left. This is done by means of a blanking pulse to the anode, which stops the flow of electrons during the short time in which the beam moves from right to left. These blanking pulses, controlled by timing cir-

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Electronics

cuits, are repeated so that one occurs after each line is scanned. When the beam reaches the bottom of the target, it has to swing back to the top. A special blank¬ ing pulse makes sure that the beam is cut off while it moves up. The movements of the beam, left and right, and down and up, are controlled by other pulses known as sync pulses. The blanking pulses and the sync pulses are added to the video signal, and the combined signal is sent to the transmitter.

THE TV BROADCAST

A television program produces both a picture and sound. The sound is picked up by a microphone and the audio signal that comes from it is broadcast by the FM system. The picture is picked up by a televi¬ sion camera, and the video signal that comes from it, combined with sync and blanking pulses, is broadcast by the AM system.

The Phototube and Television

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log

THE TELEVISION RECEIVER

A television receiver is made up of two parts. One part is an FM radio receiver. This part picks up the radio wave that carries the audio signal of the tele¬ vision broadcast. It removes the audio signal from the radio wave and feeds it to a loudspeaker which pro¬ duces sound. The other part is like an AM radio re¬ ceiver. It picks up the radio wave that carries the video signal of the television broadcast. It removes the video signal from the radio wave and feeds it to a picture tube which produces a picture. The picture tube is a large vacuum tube with a phosphor screen painted on the inside of its face. An electron gun is mounted at the other end of the tube. A beam of electrons is fired from the electron gun at the phosphor screen. The beam strikes the screen in a small spot and makes that spot glow for a short

no

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Electronics

time. A deflection yoke mounted on the neck of the tube pushes the beam back and forth and up and down, so that the beam scans the screen. The sync pulses and blanking pulses received from the broad¬ casting station are used to control the movements and the blanking of the beam. As a result, the beam scans the screen of the picture tube in the same way that the beam in the image orthicon scanned the tar¬ get. The video signal is fed to a grid in the tube. The changing signal puts a changing voltage on the grid. The changing voltage makes the strength of the elec¬ tron beam change in step with the changing video signal. Because of its changing strength, the electron beam causes spots of different brightness in the glow¬ ing phosphor. When the beam is strong, the spot it strikes glows brightly. When the beam is weak, the spot it strikes glows faintly. Because the beam in the picture tube moves in the same way that the beam in the image orthicon does, the bright spots on the

The Phototube and Television

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m

screen of the picture tube have the same arrangement as the charged spots on the target of the image orthicon. In this way a copy of the image on the target is reproduced on the screen of the picture tube. As the image on the target changes, the picture on the screen changes. The new pictures follow each other on the screen at the rate of thirty pictures per second. When we look at the screen, our brains combine the pictures that follow each other into a single moving image, just as it does when we look at the screen of a motion picture theater.

THE OSCILLOSCOPE

There is a very useful instrument that enables us to “see” the changes in an electrical current. It is known as a cathode ray oscilloscope. It contains a tube that is almost like a television picture tube. An electron gun fires a beam of electrons at the phosphor

112

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Electronics

screen on the face of the tube. A deflection yoke makes the beam move back and forth at a fixed rate. If only this yoke were working, the beam would pro¬ duce a horizontal line in the center of the screen. But there is another yoke working to make the beam move up and down at the same time. So the line has bumps or waves in it. The current whose changes we want to “see” is used to control the up and down mo¬ tion of the beam. When the voltage of this current rises, the beam swings away from its center position. When the voltage drops, the beam swings back to¬ ward the center. So the bumps that appear on the screen match the rise and fall of the voltage. If an audio signal is fed into an oscilloscope, the line produced on the screen is a picture of the sound vibrations that produced the audio signal. The draw¬ ings p. 113 show the vibrations produced by some musical instruments.

The Phototube and Television

Vibrations produced by a French horn

« «

113

Vibrations produced by a trumpet

An oscilloscope can be used to measure the fre¬ quency of an alternating current. A current of known frequency is used to move the beam in the tube back and forth. The current whose frequency is unknown is used to move the beam up and down. The com¬ bined movements back and forth and up and down produce a complicated pattern on the screen. The shape of the pattern, known as a Lissajous figure, then tells you the ratio of the unknown frequency to

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Electronics

the known frequency. The drawings below show what the Lissajous figures look like when the ratio is i to 1,2 to i? 3 to l, and 4 to 3.

ltol

2tol

3tol

4to3

Lissajous figures

Radio and TV servicemen use an oscilloscope for analyzing the voltages that appear in different parts of a radio or TV circuit. By comparing the voltages that appear with the voltages that should appear, they can track down the cause of any trouble.

6 THE GAS-FILLED TUBE AND INDUSTRIAL CIRCUITS IF HIGH CURRENTS ARE NEEDED

The tubes described in Chapters IV and V are vacuum tubes. They can produce currents of only one half an ampere or less. There are some industrial cir¬ cuits which need currents that are much higher than that. Sometimes a current as high as 10,000 amperes may be called for. Where high currents are wanted, a gas-filled tube is used instead of a vacuum tube. The gas in a gas-filled tube may be mercury vapor or one

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Electronics

of the inert gases (not chemically active) like argon, neon, or helium.

BREAKING MOLECULES

The gas in a gas-filled tube is like a wall between the cathode and the anode. This wall prevents any flow of current through the tube. In order to start a current through the tube, it is necessary first to cut a pathway through the wall. This is done by breaking the molecules in the gas. In each molecule there are negative electrons surrounding positive nuclei. The whole molecule is electrically neutral. The molecule can be broken into pieces by ripping electrons out of it. The pieces that result are charged. The electrons pulled away have a negative charge. The rest of the molecule, from which the electrons were separated, is left with a positive charge. These charged pieces of a molecule are called ions. The process of breaking the

Gas-filled Tube ir Industrial Circuits

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117

molecules of a gas into ions is called ionization. When the gas between a positive anode and a negative cath¬ ode is ionized, current can flow through it. The nega¬ tive ions move toward the anode and the cathode supplies electrons to take their place.

THE GEIGER COUNTER

A well-known example of a gas-filled tube is the

Geiger tube used for catching and counting the high

Counter The Geiger counter

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Electronics

energy particles that are fired out by the exploding nuclei of radioactive elements. The Geiger tube is a metal cylinder with a wire through it. The cylinder is filled with gas. A voltage difference is applied be¬ tween the wire and the wall of the cylinder. If a high energy particle crashes into the tube, it ionizes some of the molecules of gas. This opens a pathway for current to flow through the tube. In this way, each particle that enters the tube releases a surge of cur¬ rent. The particles are counted by counting the surges of current. One of the ways in which surges of current can be counted is described in Chapter VII.

THE PHANOTRON

The simplest of the gas-filled tubes used in in¬ dustry is the phanotron. It is a diode with a hot cath¬ ode. When the anode is made more positive than the cathode, both electrodes begin tugging at the

Gas-filled Tube & Industrial Circuits

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ng

charged particles in the molecules of gas in the tube. The anode tugs at the negative electrons. The cath¬ ode tugs at the positive nuclei. If the voltage differ¬ ence is high enough, this tugging in two directions breaks the molecules, and the gas is ionized. Once the gas is ionized, current will flow through the tube as long as the anode remains more positive than the cathode. The phanotron, like an ordinary vacuumtube diode, is often used as a rectifier. If an alternat¬ ing voltage source is connected across the tube, the anode is made sometimes negative and sometimes positive. Current will flow through the tube only dur¬ ing the half-cycles when the anode is positive. The electrons will flow in only one direction then, from the cathode to the anode.

120

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Electronics

THE THYRATRON

The thyratron is like a phanotron into which a grid has been inserted. The presence of the grid makes it possible to control the time when the current begins to flow through the tube. Two different methods of control are described in a later paragraph. A thyra¬ tron can supply currents up to about 100 amperes. Like an ordinary vacuum-tube triode it can work as an amplifier as well as a rectifier.

THE IGNITRON

The ignitron is a triode in which the cathode is a small pool of mercury. Evaporated mercury in the space above the cathode makes up the gas that fills the tube. A rod of boron or silicon carbide has its

Gas-filled Tube i? Industrial Circuits

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121

point resting in the pool of mercury. This rod is the

ignitor of the tube. When a current is passed from the ignitor to the mercury pool an arc is created between the two. The arc ionizes some of the mercury vapor

The dot • in the symbol means this is a gas-filled tube Symbol

and permits current to flow through the tube. An ignitron can produce currents up to 6,000 amperes. The tube becomes very hot, so it has to be cooled by water. The water flows through a jacket that sur¬ rounds the envelope of the tube.

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Electronics

FIRING A THYRATRON

Starting a current through a gas-filled tube is called firing it. A thyratron will fire only when the anode is more positive than the cathode. But even then it may not fire at all. Whether the tube fires and when it fires is controlled by the grid. The tube fires when the gas in it is ionized. The gas is ionized when the pull toward the anode is strong enough to break the molecules. If the anode were pulling alone it would have to have a certain minimum positive volt¬ age before it could ionize the gas. But the anode does not pull alone. There is a grid in the tube that can help or hinder it. If the grid is positive, the pull of the grid is added to the pull of the anode. Because it has help, the anode need not pull as hard as it would have to if it were pulling alone. Then the tube can fire with a lower than usual voltage on the anode. If the grid is

Gas-filled Tube & Industrial Circuits

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123

negative it opposes the pull of the anode. Then the anode has to pull harder than it would have to if it were pulling alone. In this case a higher than usual voltage is needed on the anode to make the tube fire. If the anode never gets this higher voltage, the tube will not fire at all. If an alternating voltage is supplied to the anode, the tube will fire only when the voltage is positive. But even then it will fire only when the voltage reaches the proper level. What this level is depends on what the voltage is on the grid. So, by changing the voltage on the grid, we can change the time when the tube fires. The way in which the time of firing is controlled by the grid voltage is shown in the chart p. 124. The upper curve shows the voltage on the anode at different times during a positive half-cycle. Later time is indicated by points that are further to the right. The bottom curve, called the grid control

locus, shows the lowest voltage on the grid that would

124

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Electronics

How the grid voltage controls the firing of a thyratron

permit the tube to fire at each of these times. Now suppose the grid is made 2 volts more negative than the cathode. This is shown on the chart as —2. Draw a horizontal line from —2 until it crosses the grid con¬ trol locus at A. Then draw a vertical line until it crosses the upper curve at B. When the grid voltage is —2, the tube will fire when the voltage on the anode reaches the level shown by B. Now suppose

Gas-filled Tube & Industrial Circuits

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125

the grid voltage is 3 volts more negative than the cathode. Draw a horizontal line from —3 until it crosses the grid control locus at C. Then draw a verti¬ cal line until it crosses the upper curve at D. When the grid voltage is —3, the tube will fire when the voltage on the anode reaches the level shown by D. But D is further to the right than B. So, when the grid voltage is —3 the tube fires later than it does when the grid voltage is —2. After the thyratron fires, current flows through the tube only until the end of the positive half-cycle is reached. It will fire again during the next positive half-cycle when the proper level is reached once more. The later the tube fires during the half-cycle, the less is the time during which the current flows through the tube. When the current flows for a shorter time, the average voltage it supplies is lower. So, when the voltage on the grid is changed, it

126

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Electronics

changes the average voltage of the output current, as well as the time when the current starts in each cycle. The diagram below shows a simple circuit for con¬ trolling the output of a thyratron amplifier by con¬ trolling the voltage on the grid. If the arm of the potentiometer is moved to the left, the grid becomes

6

AC supply

—AM Wv

Grid control of a thyratron

more positive, the tube fires earlier in each positive half-cycle, and the current supplied to the load is higher.

Gas-filled Tube {? Industrial Circuits

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12J

PHASE-SHIFT CONTROL

It is also possible to control the firing of a thyratron by putting an alternating voltage on the grid. Then the time when the tube fires depends on how much the voltage on the grid is out of phase with the voltage on the anode. The phase shift can be intro¬ duced by using a combination of resistance and ca¬ pacitance as shown in the diagram p. 128. By varying the resistance, the amount of the phase shift can be changed. The effect of the phase shift is shown in the graph p. 128. Suppose the grid voltage follows curve I. This curve crosses the grid control locus at A. The tube fires when the anode voltage reaches the level marked by B. Suppose the grid voltage follows curve II. This curve crosses the grid control locus at C. The tube then fires when the anode voltage reaches the level marked by D. Since D is further to the right

128

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Electronics

than B, this level is reached at a later time. The more the grid voltage lags behind the anode voltage, the later the tube will fire.

AUTOMATIC WELDING

In the manufacture of metal products, it is often necessary to join one piece of metal to another. This can be done by welding them together. There are automatic welding machines in which gas-filled tubes play an important part.

Gas-filled Tube i? Industrial Circuits

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i2g

To weld two pieces of metal together, they are squeezed together between two electrodes. The elec¬ trodes are usually made of copper or a copper alloy. An electric current is passed from one electrode to the other, flowing through the two pieces of metal on the way. The current, as it works against the resist¬ ance in its path, produces heat. The resistance is highest where the two pieces of metal touch each other. So this is where the most heat is produced. By using a high current, enough heat is produced to melt the edges of the metal that touch each other. Away from the edges, where the resistance is lower, the heat is not high enough to make the metal melt. When the current is turned off, the melted metal cools and hardens into one joint connecting the two pieces of metal. The diagram p. 130 shows in a very simplified form what happens in a welding machine. When the switch is closed, an AC current flows through the pri-

130

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Electronics

mary winding of the transformer. An AC current is induced in the secondary winding, and it flows through the pieces of metal that are between the elec-

Electrode Metal to be welded Electrode

Switch

Simplified welding diagram

trodes. A step-down transformer is used in order to produce a low voltage and a high current in the sec¬ ondary winding. However, there is more to welding than just pass¬ ing a current through the metal. The whole process has to be timed accurately. There are four intervals of time that must be controlled carefully. First the two pieces of metal that are to be welded are

Gas-filled Tube i? Industrial Circuits

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13*

squeezed together until the right pressure is built up. It takes time to build up this pressure. This is called the squeeze time. Then the current is turned on, and is kept on for a fixed amount of time while the weld¬ ing takes place. This is called the weld time. The cur¬ rent is turned off, but the two pieces of metal are still squeezed together for a long enough time to allow the welded joint to harden. This is called the hold time. Then the electrodes are separated, the welded metal is removed, and the pieces that will be welded next are moved into place. The time during which all this happens is called the off time. Then the whole process is ready to start all over again. To carry out all these steps automatically, a con¬ tactor circuit is used instead of a simple switch. The contactor circuit contains a thyratron or an ignitron. The action of the contactor circuit and the action of the press that squeezes the metal between the electrodes are controlled by special timing circuits.

132

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Electronics

The timing circuits form a chain, so that each one passes a signal on to the next one in the chain. The squeeze-time circuit signals the press to start squeez¬ ing, and begins measuring out the squeeze time. At the end of the squeeze time it signals the weld-time circuit to turn on the current through the electrodes. After turning on the current, the weld-time circuit measures out the weld time. At the end of the weld time, it turns off the current through the electrodes, and signals the hold-time circuit. The hold-time cir¬ cuit then starts measuring out the hold time. At the end of the hold time, it signals the press to release the pressure on the electrodes and pull them apart. It also signals the off-time circuit to start measuring out the off time, during which the old job is removed and the next job is put in place. At the end of the off time, the off-time circuit signals the squeeze-time circuit, and the chain of steps begins again.

Gas-filled Tube & Industrial Circuits

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133

DC MOTORS In many factories, DC motors are used. Gas-filled tubes are often used to feed electric current to these motors and control them. The part of a motor which turns is called the armature. It lies inside the mag¬ netic field created by an electromagnet. A direct cur¬ rent is fed into the armature winding. The current makes the armature a magnet. This magnet is pushed by the magnetic field surrounding it, so the armature is forced to turn. Thyratrons may be used to supply the direct current that the armature needs, and to control the speed at which the armature turns. The current supplied by the electric power com¬ pany is alternating current. To get the direct cur¬ rent needed for the armature, the alternating current must be rectified. A thyratron can serve as the recti-

134

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Electronics

fier. We saw on p. 126 that the voltage output of a thyratron depends on the time when the thyratron is fired. By changing the time of firing of the thyratron, we can change the amount of DC voltage that it sup¬ plies. This makes it possible to control the speed with which a DC motor turns. The speed of a DC motor depends on the voltage supplied to the armature winding. A high voltage produces a high speed. A low voltage produces a low speed. To vary the speed, a saturable core reactor is used to introduce a phase shift between the grid volt¬ age and the anode voltage of the thyratron. Here is how it is done. There is a variable resistor in series with the DC winding of the saturable core reactor p. 61. When this resistance is changed, a phase shift occurs in the AC winding of the saturable core reactor. This phase shift is fed to the grid of the thyratron. It causes a change in the time when the

Gas-filled Tube i? Industrial Circuits

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135

thyratron fires, see p. 128. The change in firing time causes a change in the output voltage which is fed to the armature. The change in voltage changes the speed with which the armature turns.

7 ATOM SMASHERS AND ELECTRONIC BRAINS

Electronics has invaded nearly every field of activ¬ ity. Wherever men have learned how to do a difficult job and do it well, electronic circuits have given them a way of doing it better. A microscope helps us see things that are very small. With an electron micro¬ scope we can see things that are smaller. Strong chemicals help us break up molecules. With elec¬ tronic particle accelerators we can crack the smaller nuclei of atoms. Telescopes help us see things that are

Atom Smashers b Electronic Brains

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137

far away. With radar and with radio telescopes we can see things that are further away. Electronic "brains” can solve in a few seconds complicated prob¬ lems that, in the past, would have taken a team of mathematicians many years. Servomechanisms, run electronically, can steer ships, aim guns at a moving target, or run a factory. Each of these uses of elec¬ tronics is described briefly in this chapter.

THE ELECTRON MICROSCOPE In an ordinary miscroscope we form a large image of a small object. This is done by passing light through the object, and then using lenses to bend the rays of light. In an electron miscroscope we pass cathode rays, or streams of electrons, through the ob¬ ject. Then we use electrical or magnetic "lenses” to bend the rays and bring them to a focus. Charged plates can serve as an electrical lens. They can bend

138

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Electronics

a cathode ray by pushing against the electrical charge of the electrons in the ray. Electromagnets can serve as a magnetic lens. They can bend a cathode ray by pushing against the magnetic field that surrounds it. The way in which an electron microscope works is shown in the diagram p. 139. A hot cathode and a positively charged anode work together as an elec¬ tron gun. The electrons that leak off the cathode are pulled towards the anode. They fan out as they move ahead, and pass through a hole in the anode. Beyond the anode, they pass a condenser lens which bends the streams of electrons to make them run parallel. The parallel streams of electrons then pass right through the object that is being viewed in the micro¬ scope. After that the objective lens bends the elec¬ tron rays and focusses them to form an enlarged but invisible image. Another lens, the projection lens, re¬ ceives rays from the center of the image, and repeats the process, enlarging the image of the center again.

Atom, Smashers ir Electronic Brains

«

«

139

140

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Electronics

The second enlarged image is also invisible. To make it visible it is caught on a screen coated with a phos¬ phor. If a permanent photograph is desired, it is caught on a photographic plate instead. Then the image can be enlarged again by enlarging the photo¬ graph. By this triple enlargement, an electron micro¬ scope can magnify things 100,000 times.

ATOM SMASHERS

To smash an atom, you have to hit it hard with a fast-moving “bullet.” The bullets used are ions, which are charged particles. The instrument that makes them move is something like an electron gun. It is a giant vacuum tube in which the particles move. Elec¬ tric or magnetic fields are used to push the particles along and build up their speed. The particle used most often is the proton. It makes a better bullet than

Atom Smashers & Electronic Brains

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141

an electron because it is about two thousand times as heavy. There are many different types of atom smash¬ ers. They are rated by the amount of energy they can give the bullets that they fire at a target. The energy is measured in units called MEV (million electron volts). Where only a few MEV are needed, a linear

accelerator is used. A linear accelerator is a series of pipes arranged one after the other in a straight line. There are gaps between the pipes. An alternating electrical field is created in each gap. When an elec¬ tron or proton moves through the pipes, each time it passes through a gap it gets a push from the electrical field. The repeated pushing makes the particle go faster and faster. Higher energies can be built up in a cyclotron. In a cyclotron, the particle moves in a space that is made of two D-shaped parts separated by a gap. An elec-

142

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Electronics

tromagnet built around this space guides the particle so that it moves around in a circle. When the particle crosses the gap, it meets an alternating magnetic field which gives it a push. The push forces the parti¬ cle to move in a larger circle, and speeds it up at the same time. The particle crosses the gap many times. Moving along larger and larger circles, it spirals out as it gains speed, and then it is hurled at its target. A cyclotron can reach energies of 30 MEV or more. One of the accelerators that can produce energy measured in billions of electron volts is the synchro¬

tron. Here the particles move inside a gigantic tube that is shaped like a doughnut. They are pushed by an alternating magnetic field. As the particles pick up speed, the magnetic field vibrates faster in order to push them in the right direction at the right time.

Atom Smashers & Electronic Brains

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143

RADAR

Radar uses pulses of radio waves in an echo system for locating distant objects and measuring how far away they are. If you shout toward a cliff, the sound of your voice moves to the cliff, strikes it, and then bounces back as an echo. A radar transmitter sends out a pulse of radio waves instead of a sound. If the pulse strikes an object in its path, it bounces back as an “echo” which can be picked up by a radio receiver. A radar transmitter sends out pulses in a narrow beam. The beam reaches out like a probing finger in one direction at a time, so that if an echo comes back we can tell which direction it came from. To change the direction in which the beam travels, the radar antenna keeps turning. In this way it scans the space surrounding it. It takes time for a radar pulse to travel. It travels

144

>y >y

Electronics

with the speed of light, which is 186,000 miles per second. When a pulse strikes an object and comes back as an echo, there is an interval between the time when the pulse left the antenna and the time when it came back. The length of this interval is the amount of time it takes for the pulse to make a round trip from the antenna to the object and back again. By measuring this interval, we can calculate how far away the object is. In the simplest radar receiver, a moving electron beam produces a line of light on the face of an oscil¬ loscope tube. When a pulse is sent out by the trans¬ mitter, an electrical signal is sent to the oscilloscope to produce a“pip” or bump in the line. When an echo is received, it produces another pip in the line. The time interval between the two pips can be calculated from the distance between them on the face of the tube. Then the distance that the pulse traveled is cal¬ culated from the time interval. Half of this distance

« «

Atom Smashers & Electronic Brains

145

Pulse Echo

“Pips” on an oscilloscope screen

is the distance to the object that caused the echo. For example, if the echo comes back in one ten-thou¬ sandth of a second, it means that the pulse traveled 18.6 miles to reach the object and come back. So the object must be 9.3 miles away. A more advanced type of radar receiver uses a PPI, or Plan Position Indicator. The radar antenna scans an entire region, and all the echoes sent back com¬ bine to form a picture of the region on the face of a tube like a television picture tube.

146

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Electronics

Not all radio waves can be sent out in a narrow beam. To produce a narrow beam, it is necessary to use waves of a very high frequency, much higher than the frequencies used for radio or television broadcasts. Radar broadcasts use ultrahigh frequen¬ cies, in the range between one hundred and one mil¬ lion megacycles (a megacycle is one million cycles). To work with these frequencies, special equipment is needed. Radar sets use special tubes. They use pipes instead of wires. They use bowl-shaped anten¬ nas. For tank circuits in which the ultrahigh fre¬ quencies are produced they use hollow cyclinders called cavity resonators. Radar is used for such varied purposes as seeing through fog at sea or at an airport, guarding a coun¬ try’s frontiers, tracking meteors, and measuring the distance to the moon.

Atom Smashers ir Electronic Brains

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147

RADIO TELESCOPES

A radio telescope is a radar receiver designed to catch radio waves that come from outer space rather than reflected pulses that come from the radar set in the first place. There are radio waves that come from the stars and from atoms in the space between the stars. These waves can pass through dust clouds that block visible starlight. So radio telescopes can "see” right through these dust clouds to stars that had never been seen before. The United States Navy is building a radio telescope whose bowl-shaped an¬ tenna will be 600 feet in diameter. When it is com¬ pleted, it will be the largest radio telescope in the world.

148

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Electronics

ELECTRONIC BRAINS

“Electronic brain” is the popular name for a digital computer. It is an electronic instrument that can do “thinking” at high speed. In some business offices, a digital computer is used to keep track of supplies, or to calculate bills. In a science laboratory, it may solve equations. At a station for launching rockets, it may calculate the right path for a rocket, and guide it along this path. Because a computer is called a “brain,” and can do such complicated things, you may think that a computer is very clever. Actually it isn’t clever at all. In fact it is rather simple-minded. It can carry out only a few simple steps. Whenever a computer has a complicated job to do, someone first breaks this job down into a chain of simple steps. Then the com-

Atom Smashers

6- Electronic Brains

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149

puter is given instructions to carry out these steps in the proper order. A digital computer contains electrical elements such as switches, relays, and tubes. All these elements have one feature in common. They can be in one of two conditions. A switch may be open or closed. A relay may make a contact or break the contact. A tube may have current flowing through it, or it may have no current flowing through it. One of the simple things a computer can do is change from one con¬ dition to another in one of these elements. The engineers who design computers have figured out ways of using long chains of such changes to carry out complicated calculations. As an example, we shall show how chains of such changes can be used to do arithmetic. All arithmetic can be based on counting. Addition can be done by repeated counting. For example, to

15° » »

Electronics

add 7 and 9, first count from 1 to 7, and then, starting with the number that follows 7, count out 9 more numbers. Multiplication can be done by repeated addition. For example, to multiply 3x8, add 8 and 8 and 8. But the addition can be done by repeated counting. So multiplication, too, can be done by counting. Once a computer is designed so that it can count, then it can add and multiply, too. When we do arithmetic with pencil and paper, we count things in groups of ten. This is shown by the way in which we write numbers. To show any number of things from zero to nine, we use the special symbols: o, 1, 2, 3, 4, 5, 6, 7, 8, and 9. To show ten things, we write 10. The 1 in this symbol stands for "one group of ten.” The o stands for "no additional units.” When we write 12, it means "one group of ten and two additional units.” We keep track of units in the first column from the right. We keep track of tens in the second column from the right. We keep

Atom Smashers 6- Electronic Brains

« «

track of tens of tens, or hundreds, in the third column from the right, and so on. We might, if we wish, count things in groups of two, instead of ten. But then it would be convenient to use a different way of writing numbers. To show zero or one, we could use the symbols o and 1. To show two things, we could write 10. The 1 in this symbol stands for "one group of two,” and the o stands for "no additional units.” The symbol 11 would mean "one group of two and one additional unit.” In other words, in this system, 11 would stand for three. Numerals like this, based on counting things in groups of two are called binary numerals. In binary numerals, four is written as 100, five is written as 101, and six is written as 110. A computer counts things by counting electrical pulses, one for each thing. The elements of a digital computer can be in one of two conditions. This makes it natural for a computer to count by means

» »

Electronics

of binary numerals. To show how this may be done, we first describe two components that are found in digital computers. One is called a flip-flop circuit. The other is called an and circuit.

THE FLIP-FLOP AND THE AND

A flip-flop circuit consists of two triodes connected as shown in the diagram p. 153. The plate current of each is fed to the grid of the other. When no current flows through tube o, a high voltage is placed on the grid of tube 1. When current flows through tube 1, this puts a low voltage on the grid of tube o, which prevents any current from flowing in it. A current always flows through one of the tubes, but never through both. This makes it possible to use the flipflop to stand for the digits used to write binary nu¬ merals. The digit o can be represented by current flowing through tube o. The digit 1 can be repre-

Atom Smashers & Electronic Brains

« «

153

A flip-flop circuit

sented by current flowing through tube 1. Numerals written with several digits can be represented by several flip-flops side by side. Three flip-flops ar¬ ranged from left to right could be used to represent the number six by having current in the 1 tube, the 1 tube, and the o tube of the three flip-flops, in that order. The and circuit is a switch designed to receive several signals and send out one signal. The signal in each case is a voltage, and it may have two values, high or low. The circuit is constructed so that the

154

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Electronics

signal sent out is high only if all the signals received are high. The diagram p. 155 shows an and circuit made with two diodes. Each diode permits current to flow in only one direction, from the cathode to the plate. Current flows through the diode only when the plate voltage level is higher than the cathode voltage level. The input at the cathode of each diode may be —15 or +5 volts. If a diode conducts cur¬ rent, since the resistance in the tube would be very low, the output voltage would be about the same as the input voltage. If both input voltages are --15, the output voltage will be —15. If both input volt¬ ages are +5, the output voltage will be +5. Sup¬ pose one input is —15, and the other is +5. Current flows through the tube with a low input voltage. The low voltage is transferred to its output. But the out¬ put is joined to the plates of the other tube. In this tube, the plate has a lower voltage than the cathode, so the current in this tube is cut off. The output

Atom Smashers & Electronic Brains 4- lOOv

If one input voltage Is ^ and the other is +5v

« «

155

4-100v

then the output voltage is —►—15v

If one input voltage is +5v and the

then the output voltage is —► +5v

°'SS The AND circuit

voltage has the high value of +5 only if both input voltages are high. The special symbol used for an and circuit is shown in the diagram. It is called an and circuit because the output signal is high only if the first and second input signals are high. In our ordinary way of writing numbers, we show higher and higher numbers by first writing higher and higher digits. After we reach the number 9, we don’t write new digits. To show the next higher number after 9, we replace the 9 by o, and carry 1 to the next column. Something similar happens when

i$6

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Electronics

we write binary numerals. After we reach the digit 1, we don’t write new digits. To show the next higher number after 1, we replace the 1 by o, and carry 1 to the next column. The diagram p. 157 shows how this procedure is carried out by a flip-flop and two and circuits. The flip-flop can have two settings, o or 1. A pulse from one of the and circuits will change it from one setting to the other. The output of each tube in the flip-flop is connected to the input of one of the and circuits. But only one tube is active at a time. So only one of the and circuits receives an input signal from the flip-flop. Suppose the flip-flop starts out set at o, that is, with current flowing in its o tube. Now we send a pulse along the wire labeled ‘rnput.” The and circuit on the right receives two signals, one from the input, and one from the o tube of the flip-flop. So this and circuit has an output. The output resets the flip-flop

Atom Smashers ir Electronic Brains

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157

Carrying in binary addition

from o to 1. As a result, current then flows only through the 1 tube of the flip-flop. Meanwhile, the and circuit on the left has received only one input signal, so it has no output. When another pulse is sent in to be counted, the right-hand and receives only one input signal, so it has no output. The lefthand and receives two input signals, so it sends out an output pulse. This pulse divides into two parts. One part resets the flip-flop. The other part “carries one” to the next flip-flop.

158

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Electronics

SERVOMECHANISMS

A great change is taking place in industry today. We call this change automation. It is introducing a more advanced way of using machinery. In the past, when a machine was used in a factory, there was always a human being there to guide and supervise the work of the machine. Now automation is displac¬ ing this human being. In an automated factory, a machine is guided and supervised by another ma¬ chine. Often, the guiding machinery is built into the working machine, so that the machine guides and supervises itself. To see how a machine can guide itself, let us see first how a human being guides his own movements. Suppose you are walking across a room, and you have to pass a table that stands in the way. If you

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159

are walking in the dark, you may stub your toes against the table. But if the light is on, you can pass close to the table without touching it at all. You can do so, because you have help from your eyes. Your eyes can see where your foot is as you move it. If your foot comes too close to the table, your brain signals your foot to move aside. Your feet, eyes and brain work as a team. Their teamwork has these features: 1) Your feet are given a definite job to do. 2) Your eyes can see the results of what your feet do. 3) Your brain can compare these results with the results that you wanted. It can notice the error, if there is one. 4) If there is an error, your brain signals your feet to correct the error. Machines can be designed so that their work also has these four features: 1) The machine has a defi¬ nite job to do. 2) The machine has a way of measur¬ ing the result of what it does. 3) It also has a way

160

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Electronics

of comparing the result with a standard for what was wanted. If the result differs from the standard, an electrical error signal is generated. 4) This elec¬ trical signal causes the machine to correct the error. A machine that can control its own work in this way is called a servomechanism. The error signal used for correcting the work of a machine is called feed-back. On page 73 we saw an example of feed-back in the design of an oscillator. Here is a simple example of automatic control. Suppose a DC motor is being used to turn a large spool on which some metal wire is being wound. Suppose, too, that the wire is fed to the spool at a fixed speed. As the wire is wound around the spool, each new turn of wire is wound around the turns already there. It is wound in a larger circle than the earlier turns, and therefore has more wire in it. So later turns wind up the wire at a higher speed. The

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161

spool begins to tug at the wire with more and more force. If this situation is not corrected, either the machinery jams or the wire breaks. To prevent this from happening, it is necessary to slow the motor down. One way of doing this is to use a thyratron to control the speed of the motor, and use the motion of the spool to control the thyratron. The spool is mounted so that it can move when it is pulled. A metal core is mounted on the spool, so that when the spool moves forward, the core moves into a solenoid. If the spool begins to wind up the wire too fast, the spool is pulled forward. The core enters the solenoid and changes its inductance. The change in induc¬ tance produces a phase shift, and this phase shift is used to reduce the speed of the motor, by the method described on p. 134. In this way the spool slows itself down so that it does not wind the wire faster than the wire is fed to it.

162

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Electronics

Servomechanisms are taking over more and more jobs that used to be done by men and women. They open up the possibility of reducing the hours that people must work, while increasing at the same time the amount of products that they can enjoy.

THE TRANSISTOR, OLD-TIMER AND NEWCOMER

The electronics industry grew up, at first, around the many uses of the hot cathode vacuum tube. Now, in some of these uses, the tube is being replaced by the transistor. The transistor is a new improved version of an old piece of radio equipment, the crystal, that used to be found in old-fashioned radio sets. In the early days of radio, it was known that cer¬ tain crystals, such as galena, behave like a one-way

164

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Electronics

street to an electric current. They permit current to flow through them in only one direction. For this reason, they could do the same jobs that a diode could do. In the old crystal radio sets, they served as detectors. Later they were replaced by tubes, which could amplify a current as well as rectify it. The amplified currents made it possible to replace headphones by a loudspeaker. But, now, radio en¬ gineers have learned how to make crystals that can serve as amplifiers, too. So the old crystal, in the form of the new transistor, is back again, with more jobs to do.

THE JUNCTION DIODE We shall describe here only the simplest type of transistor, and how it works as a diode. It may be made of two crystals of germanium that touch each other at a junction. The crystals are carefully made so

Transistor, Old-timer ir Newcomer

i65

« «

that they are almost pure. The electrical behavior of each crystal is the result of the impurities that are in it. We see below that every atom consists of a nu¬ cleus surrounded by electrons. The electrons are arranged around the nucleus in layers called “shells.” In an atom of germanium, there are four electrons in the outer shell. This shell has room for eight elec¬ trons. When germanium atoms come together to form a crystal, each atom is surrounded by four other atoms. An electron from each atom joins with

Atoms of

Germanium crystal

I

I

I

Q — Germanium

—

I \

II /

o I

X

II

Arsenic

1

N

I

II

II

I

I

Aluminum

II

i66

» »

Electronics

an electron from a neighboring atom to form a pair, and then the pair belongs to the outer shell of both atoms. In this way, each atom fills out its outer shell with eight electrons. The arrangement of the atoms in the crystal can be pictured roughly by the diagram p. 165. When neighboring atoms share their elec¬ trons in this manner, they hold the electrons very tightly. An ordinary DC voltage cannot pull the elec¬ trons loose and make them move. So a pure germa¬ nium crystal does not conduct current. Now suppose a small number of atoms of arsenic are scattered among the germanium atoms. An ar¬ senic atom has five electrons in its outer shell. Each arsenic atom takes the place of a germanium atom in the crystal. It shares four of its outer electrons with neighboring atoms of germanium. Then there is no room in any of the shells for the fifth electron. It is squeezed out of place and lies in the space between the atoms. So a crystal of germanium that has a small

Transistor, Old-timer & Newcomer

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167

amount of arsenic scattered in it has extra electrons in it. Because these extra electrons are negative, it is called an N type crystal. These electrons are free to move. When a DC voltage is applied across an N type crystal, the extra electrons in it flow toward the positive terminal and leave the crystal. At the same time, other electrons flow into the crystal from the negative terminal. Current flows through the crystal, and the number of free electrons in it remains the same. A different type of crystal is formed if a small number of aluminum atoms are scattered among the germanium atoms. Each aluminum atom has only three electrons in its outer shell. It shares these electrons with neighboring germanium atoms. But then there is room for one more electron, which the aluminum atom did not supply. It is as though there were a hole in the crystal that can be filled by an electron. This hole behaves like a positive charge. So

168

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Electronics

N-type crystal

P-type crystal

a crystal containing such holes is called a P type crystal. A hole always tries to capture an electron from a nearby atom. If it gets some help from an out¬ side voltage, it may succeed. But when the hole pulls an electron away from another atom, a new hole appears in the space that the electron came from. It is as if the hole had moved from one place to another. If a DC voltage is applied across a P type crystal, the holes move toward the negative terminal, where they are filled. Meanwhile, the positive ter-

,

Transistor Old-timer & Newcomer

« «

i6g

minal pulls some electrons out of the crystal, making new holes to take their place. So a current flows through the crystal, and the number of holes in it remains the same. A junction diode is made by joining an N type crystal to a P type crystal. Let us see what happens if a DC voltage is connected across this double crystal, with one terminal at the N end, and the other terminal at the P end. Suppose the negative terminal is at the N end of the crystal. There are free electrons in the N section. The negative terminal

—Free electron + Hole

N

P

The junction diode

lyo

» »

Electronics

drives these free electrons toward the junction. The positive terminal of the voltage source is at the P end. There are free positive holes in the P section. The positive terminal drives these free holes toward the junction. Here they meet the free electrons lined up on the other side, and the free electrons cross over to fill the holes. In this way, the P section loses holes, and the N section loses electrons. But as fast as they lose them, new ones appear at the terminals. So a steady flow of current passes through the crystal. The outcome is different if the terminals of the voltage source are interchanged. The only things free to move in the crystal are the free electrons in the N section, and the free holes in the P section. When the positive terminal of the DC voltage source is at the N end, and the negative terminal is at the P end, the free electrons and the free holes are pulled away from the junction. Then the free electrons can¬ not cross the junction to fill the holes. So no current

Transistor, Old-timer & Newcomer

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171

flows through the crystal at all. The double crystal behaves just like a diode, allowing current to flow through it in only one direction. The N section be¬ haves like a cathode, and the P section like an anode. Transistor triodes are made by joining a crystal section of one type to two sections of the opposite type. If an N section lies between two P sections, it is called a P-N-P triode. Transistor triodes, like hot cathode triodes, can amplify a current. Transistors have many advantages over vacuum tubes. They are small and light in weight. They do not get hot. They use up little power. They work at lower voltages than tubes do, and they last longer. Because of these advantages, transistors are already being used instead of tubes in many electronic de¬ vices, such as hearing aids, electronic computers, and telephone amplifiers. A hearing aid is like a small telephone system, with a microphone, an amplifier, a receiver, and a battery

1J2

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Electronics

power supply. Since a person using a hearing aid carries it around with him all day long, it should be light in weight and small enough to be kept out of sight. Hearing aids made with transistors can be made so small that they fit into the frame of a person’s eyeglasses! The first electronic computers that were built con¬ tained thousands of electronic tubes. The filaments of these tubes produced a lot of heat. Special cooling systems had to be built into the computers to carry the heat away. Because they contained so many tubes, the computers were big enough to fill a large room. Repairs had to be made often, whenever a tube burned out. Now, in the new computers being built, transistors are used instead of tubes. As a result, the new computers take up less space than the old ones did. They do not need a cooling system, and they don’t have to be repaired so often, since there are no tubes burning out.

Transistor, Old-timer & Newcomer

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173

The telephone company sends telephone messages across great distances through special cables. A telephone cable joining Europe to the United States lies under the Atlantic Ocean. At intervals along this cable there are amplifiers attached to boost the cur¬ rent. If these amplifiers were built with tubes, they would have to be hauled to the surface for repairs every time a tube burned out. The telephone com¬ pany avoided this trouble by using transistors instead of tubes in these amplifiers. The number of transistors in use increases from day to day. At the same time we still use many tubes because they are cheaper than transistors. As more and more transistors are used, it will become cheaper to make them. Soon transistors will be cheaper than tubes, and the transistor will reign over the elec¬ tronics industry that the electron tube created.

INDEX A battery, 64 Accelerator grid, 101-2 Air core transformer, definition,

Automatic welding, 128-32 Automation, 158-62

49 Alternating current, definition, 16-17 Aluminum, 167 Ampere, 14 Amplifier, 83,120,171 Amplifying circuit, 76 Amplitude modulation broad¬ casting, 75-81, 108, 109 And circuit, 153-7 Anode, 66-135, 1.37-4° Argon,116 Arithmetic, 149-51 Armature, 133, 135 Arsenic, 166 Atom, 9,165 Atom smashers, 137, 140-42 Audio signal, 76-7, 82, 84, 93, 111-12

B battery, 64, 81 Blanking pulse, 107, 108, 110 Boron, 120 Binary numerals, 151 By-pass capacitor, definition, 43

C battery, 81 Camera, 100-101 Capacitor, capacitance, 19-20, 24, 41-3, 52-4, 54-7, 57-9, 80,

83-5 Carbon resistor, 36 Cathode, 66-135,137-40 Cathode ray oscilloscope, 111-14 Cavity resonator, 146

ii

» »

Index

Chemical image, 101

Electric appliances, 4

Closed circuit, 17-18, 42

Electrical charge, definition, 9-

Coil, 23, 24, 44-6, 49, 51-2, 54-7, 59-60, 83-5, 161

II, 12 Electrical image, 103

Color codes, 36, 41, 95-6

Electrical levels, 12-13, 42

Contactor circuit, 131

Electricity, 8-34

Converter tube, 74

Electrodes, 129

Copper, 129

Electromagnet, 23,133,138

Core, 24, 44-6, 59-60, 83-5, 161

Electromagnetic waves, 33-4

Crystals, 164-73

Electron, defined, 3, 9

Current, definition, 3, 14

Electron gun, 97-100, 104-5, 109,

Cyclotron, 141

III, 138 Electron microscope, 136, 13740 Electron shells, 165

DC motors, 133-5,160-61 DC voltage, 167 Deflection yoke, 98, 110, 112 Detector, 79,164

"Electronic brains,” 137, 148-57, 173., 172 Electronic devices, 4-5, 7 Electronic tube, 3-7, 64-135

Digital computor, 148-57 Diode, 65-135 Direct current, definition, 16 Dynode, 89-90, 105-6

Farad, 20, 41-3, 57 Federal Communications Com¬ mission, 76 Feedback, 73, 160 Filament, 64

Electric field, 11,140

Filter, 52-4

Electric wire, 4-5

Fixed capacitor, definition, 41-3

Index Fixed inductance, definition, 44-

6

« «

Hi

Hole in crystal, 167-9 Hot cathode, 138

Fixed resistors, definition, 35-40

Hot cathode tube, 64-86, 97

Flip-flop circuit, 152-3,155-7 Focus coil, 102 Focussing anode, 98 Frequency, definition, 31 Frequency

modulation

Ignitor, 121 broad¬

casting, 81-6,108, 109

Ignitron, 120-21, 131 Image orthicon, 100-8, 110-11 Induced current, 25 Inductance, inductors, 24, 44-6, 52-4, 59-60, 83-5, 161

Galena, 163

Impedance, 24-5

Gas-filled tube, 115-35

Inert gas, 115-16

Geiger counter, 117-18

Ion, 116, 140

Germanium, 164,165-9

Ionization, 116-17

Gravity, 12

Iron core transformer, definition,

Grid, 69-135

49

Grid control locus, 123,127

Junction, 164 Harmonics, 55, 82-3

Junction divide, 164, 169-73

Hearing aid, 171-2 Heat, 16, 39-40, 72 Helium, 116 Henry, 24

Light, 87-114

Heterodyning, 75

Light image, 101-4

Hold time, 131-2

Linear accelerator, 141

iv

» »

Index

Lines of force, 21-2

Objective lens, 138

Lissajous figure, 113-4

Off time, 131-2

Load, 49, 61-2

Ohm, 16, 30-35, 57 Open circuit, 17-18, 42 Oscillator, 72-4, 82 Oscillator circuit, 76

MEV, 141

Oscilloscope tube, 144

“Magic eye,” 90-92 Magnet, 20-24, 133 Magnetic amplifier, 59-62 Magnetic field, 20-22, 49, 102, 133>140 Mercury, 120 Mercury vapor, 115 Metal, 64 Micro-microfarad, 41 Microphone, 75-6 Microscope, 137 Mixer tube, 74 Modulated radio signal, 77, 82,

85 Molecules, 9, 64,116-17

P type crystal, 167-71 P-N-P triode, 171 Parallel circuit, 47-9 Phanotron, 118-19 Phase, 31 Phase-shift control, 127-8 Phosphors, 96-7, 110,140 Phosphor screen, 109 Photocathode, 101-8 Phototube, 87-104 Phototube thermometer, 94 Photomultiplier tube, 88-90 Picture tube, 109, 111 Pip, 144 Plan Position Indicator, 145 Plate circuit, 64-5, 72

N type crystal, 167-71

Potentiometer, 39

Neon, 116

Power supply, 80-81

North pole, 20

Projection lens, 138

Nucleus, 9, 10, 116

Protons, 9, 10,140

Index Radar, 136,143-6

Sync pulse, 108,110

Radar sets, 146

Synchrotron, 142

« «

v

Radio crystal, 163-4 Radio receiver, 78-80 Radio signal, 77, 82, 85 Radio telescopes, 137, 147 Radioactive elements, 118

Tank circuit, 54-7, 72-3, 76, 80, 82-5, 146

Reactance tube circuit, 83-5

Telephone amplifiers, 171, 173

Rectifier, 68, 71-2, 81, 120, 133-4

Television, 96-7, 108

Relay, 51-2, 149

Television camera, 100-108

Resistance, 15, 24, 30-35

Television receiver, 109-11

Resistor, 35-40, 57-9, 59-62, 107-

Thyratron, 120, 122-8, 131, 133-

8, 131-2 Resonance, 56

5>161 Time delay circuit, 58 Time-flow diagrams, 26-33 Timing circuits, 57-9, 107-8, 1312 Transistors, 6-7,163-73

Saturable core reactor, 59-60, 134

Triode, 69-75, *52

Scanning, 99, 104-5, 106-8, 110

Tuning, 56

Series circuit, 47-9, 134 Servomechanisms, 158-62 Silicon carbide, 120 Solenoid, 23, 44, 161

Vacuum tube, 109,140, 171-3

Sound track, 93-4

Variable

Speed of light, 34, 144

4i-3 Variable inductance, definition,

Squeeze time, 131-2 Step-down transformer, 50 Step-up transformer, 50 Switch, 18, 149

capacitor,

definition,

44-6 Variable resistors, definition, 3540

vi

»

»

Index

Vibrations, 75, 77, 85

Weld time, 131

Video signal, 96, 108, 109-11

White light, 95

Volt, definition, 14 Voltage, definition, 14 Voltage drop, 38-9 Watt, 14

X-ray tube, 99

A NOTE ON THE

TYPE IN WHICH THIS BOOK IS SET

the text

of this book is set in Caledonia, a Linotype face

designed by

W.

A.

Dwiggins.

It belongs to the family of

printing types called “modem face” by printers—a term used to mark the change in style of type-letters that oc¬ curred about 1800. Caledonia borders on the general de¬ sign of Scotch Modern, but is more freely drawn than that letter.

the book

was composed, printed, and bound by H. Wolff,

New York, Grove, Pa.

paper

made by P. H. Glatfelter Co., Spring

typography

by Tere LoPrete.

electronicsOOadle electronicsOOadle

TK73ZO e

7)

3

Boston Public Library

WEST ROXBURY BRANCH LIBRARY 1961 Centre Street West Roxbury 32 The Date Due Card in the pocket indi¬ cates the date on or before which this book should be returned to the Library. Please do not remove cards from this pocket.

Other Book ROBERT IRVING ELECTROMAGNETIC WAVES An explanation of the seven kinds of electromagnetic waves, their discovery and uses. Illustrated by Leonard Everett Fisher.

SOUND AND ULTRASONICS An exploration of the world of sounds, both heard and unheard, with simple ex¬ periments, and definitive illustrations.

Illustrated by Leonard Everett Fisher.

Energy an

power

The evolution of the uses of power and the many, ways man has benefited from it.

Illustrated by Leonard Everett Fisher.

HURRICANES AND TWISTERS A treasury of facts about some of nature’s most damaging weather freaks.

Illustrated by Ruth Adler, and also by photographs.

ROCKS AND MINERALS Fascinating and informative introduc¬ tion to geology. Illustrated by Ida Scheib,

and also by photographs.

Pri rted in l .S.A.

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