Electrical machinery; a practical study course on installation, operation and maintenance

Front matter

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Preface

During the three years from January, 1917, to December, 1919, there was published in "Power" a series of articles on practical electricity under the title, "The Electrical Study Course." For the first eighteen months the author of this book wrote the Electrical Study Course Lessons, and during 1919 edited them. After many requests for these lessons in book form, he was induced to compile these articles into a volume which this work represents. An effort has been made to include such material as would make the work not only of value as a study on elementary theory of electricity and magnetism, but of practical worth.

The first eighteen months of the Electrical Study Course Lessons have been used almost as they appeared in "Power," and also parts of the lessons during the following eighteen months. In addition to this there has been incorporated a number of articles by the author on electrical-machinery operation and connections and kindred subjects, which have appeared in "Power" during the last seven or eight years.

In the author's fifteen years' experience in teaching industrial electricity, directing men on the job and in editorial work, it has been found that there are many features in the study of electricity that a large percentage of home students and those taking short courses find to be stumbling blocks. They get wrong conceptions of the theory or find it difficult to get a clear understanding of certain elementary laws. In the preparation of this material an effort has been made to present the subjects in such a way as to guide the student over these pitfalls and to have all the problems deal with things that the practical man comes in contact with in his vi PREFACE

daily work, so as to show how theory fits into practice. Although there are available a number of good books on elementary electricity and magnetism, the demand for the Study Course Lessons in book form has been interpreted as a justification for this volume, and it is presented with the hope that it has a real field of usefulness.

The author takes this opportunity to express his very deep personal appreciation of Mr. Fred R. Low, Editor in Chief of "Power," for his constructive suggestions and inspiration, which has made this book possible. He also desires to acknowledge with thanks the work of Mr. W. A. Miller, who wrote the Electrical Study Course Lessons during the last eighteen months they were published.

F. A. Annett.

New York City, March, 1921.

Contents

PAGE

Preface v to vi

CHAPTER I Introduction — Fundamental Principles 1 to 9

Electricity. — Sound Analogy. — Heat Analogy. — Light Analogy. — Electricity Similar to Heat, Sound and Light. — Electrical Generating System. — How Electric Pressure is Varied in a Wire. — What Happens when Current Flows in a Wire. — Magnetic Effects of an Electric Current. — Magnetic Field Varies with the Current. — Causes Changing Direction of Current.

CHAPTER II Elementary Magnetism 10 to 28

Discovery of the Property of Magnets. — Artificial Magnets. — Permanent Magnets. — Properties of Magnets. — The Earth a Magnet. — Poles of a Magnet. — Distribution of Magnetic Field. — Lines of Force Conventional. — Unit Magnetic Pole. — How Magnets Attract Pieces of Iron. — Why Iron can be Magnetized. — Theory of Magnetism. — How Magnets are Demagnetized. — Electricity and Magnetism. — Direction of Magnetic Field About a Conductor. — ■ Relation Between Direction of Current and Magnetic Field. — • Determining Direction of Magnetic Field About a Conductor. — Magnet Field Around a Number of Conductors. — Properties of Electromagnets.

CHAPTER III Fundamentals of the Electric Circuit 29 to 48

Production of Voltage. — Electric Pressure and Temperature. — Unit of Electric Current. — Electrical Resistance. — No Perfect Insulator of Electricity. — Pump Analogy of Electric Circuit. — Measurement of Pressure. — Measurement of Current Flow. — How to Increase Voltage. — Pump Analogy of Voltage Sources in Series. — How Current is Increased. — Pump Analogy of Current Sources in Parallel. — ■ Series-Parallel and Parallel-Series Connections.

Viii CONTENTS

Chapter IV Page

Ohm's Law, Series and Parallel Circuits 49 to 72

Three Electric Units.— Unit of Electric Current.— Relation Between Volts, Amperes and Ohms.— How to Remember Ohm's Law. — Electric Generator.— Factors that Influence Flow of Electric Current — Ohm's Law Problems. — What Constitutes Electrical Resistance. — Series Circuits. — Flow of Electric Current Compared to Water Flowing Through a Pipe. — Voltage Drop Along a Circuit is Similar to Pressure Drop in a Pipe Line.— The Value of the Volts Across Each Section of Resistance in Any Circuit May be Found by Ohm's Law. — Parallel Circuits. — The Total Resistance of Any Circuit is Equal to the Total Voltage Divided by the Total Current. — Joint Resistance of Two or More Unequal Resistances in Parallel. — Resistance and Conductance. — Formula for Finding Joint Resistance. — Hydraulic Analogy of a Parallel Circuit. — Two Methods of Working Out Joint Resistance Problems. — Volts Drop Across a Circuit. — Determining Value of Current by Volts Drop and Resistance of Section of a Circuit.

CHAPTER V Energy, Work and Power 73 to 81

Energy the Ability of Doing Work. — Steam Under Pressure Possesses Energy. — Foot-Pound Unit of Work. — Defining Unit of Power. — Definition of Energy, Work and Power. — Horse-power Defined by James Watt. — When Energy is Expended Heat is Produced. — Mechanical Equivalent of Heat. — British Thermal Unit. — Power of an Electric Circuit. — Heat Equivalent of an Electric Current.

CHAPTER VI Complex Circuits and Effects of Internal Resistance 82 to 92

Circuits in Which Elements are Grouped in Series and Parallel. — Internal Resistance. — How to Compute Value of Current when Effect of Internal Resistance is Considered. — Volts Drop in a Source of Electromotive Force. — Volts Drop Through Resistance of Armature. — Effects of Increasing the Load on Batteries.

CHAPTER VII Electrical Instruments 93 to 106

Principles on which Electrical Instruments are Constructed.

Movement for Modern-Type Instrument.— Instrument Connected as Voltmeter.— Turning Effort Proportional to the Current in the

CONTENTS ix

PAGE

Coil. — Not Voltage that Causes the Coil to Move, but Current. — Instrument Connected to Measure Current. — Instrument to Measure Both Volts and Amperes. — Direct-Reading Wattmeter. — Watt-Hour Meter.

CHAPTER VIII Methods op Measuring Resistance 107 to 116

Voltmeter-and-Ammeter Method. — Known-Resistance Method. — Measuring High Resistance. — Measuring Insulation Resistance. — The Wheatstone Bridge. — Hydraulic Analogy of a Wheatstone Bridge. — Condition to Give Balanced Pressure.

CHAPTER IX Calculating Size of Conductors 117 to 132

Wire Gage for Round Wire.— Circular Mil as a Unit of Measurement. — Some Features of the Wire Table. — Wire-Number Sizes. — Cross-Section and Diameter of Round Conductors. — Wire Tabic Worked Out from No. 10 Wire. — Allowable Carrying Capacities of Copper Wire. — Allowable Voltage Drop. — Economical Size of Conductors. — Voltage Regulation of Different Circuits. — Calculating Size of Conductors. — Derivation of Circular-Mil Formula. — Resistance per Foot of Conductor.

Chapter X

Fundamental Principles of Dynamo-Electric Machinery.. . 133 to 151 Electromagnetic Induction. — Rule for Determining Direction of Electromotive Force. — Lines of Magnetic Force. — Unit Electromotive Force. — Fundamental Principle of the Electric Motor. — Direction Conductor Will Move in a Magnetic Field Depends upon the Direction of the Current. — Counter-Electromotive Force. — How an Electric Generator is Loaded. — Simplest Form of Electric Machine. — Factors Governing Value of the Voltage. — Current Beverses in External Circuit. — Electromotive Force or Current Curve. — Machine with Ring-Type Armature.

CHAPTER XI Direct-Current Machinery Construction 152 to 169

Types of Armature. — Objections to Ring-Type Armatures. — ■ Drum-Type Armatures. — Effects of Eddy Currents. — Eliminating Eddy Currents. — Objection to Smooth-Core Armatures. — Field Magnets. — Early Types of Field Magnets. — Consequent-Pole Type Machine. — Materials Used in Field Poles. — Function of the Commutator.— Commutator Construction.

s COXTEXTS

CHAPTER XH

PAGE

INDCCTANCE and Commutation 1"0 to 1*1

Magnetic Field About a Conductor. — Voltage Induced by Decreasing Current. — Mutual Inductance. — Self-Inductance. — Neutral Point on Commutator. — How Voltage in Armature is Maintained Constant. — Effects of Self-induction on Commutation. — Commutation Period Short.

CHAPTER XIH

Types of Direct-Current Generators 1S2 to 202

Exciting Field Coils of Direct-Current Generators. — How Voltage is Built up in the Armature. — Lines of Force in Field Poles Does Xot Increase in Proportion to the Current Flowing in the Field Coils. — Field-Coil Connections and Armature Rotation. — Reversing Rotation of Armature. — Series-Connected Generator. — Ampere- Turns on Field Poles. — Load on a Series Generator. — Voltage Control on Shunt Generator. — Diagrams of Direct-Current Generators. — Effects of Loading a Shunt Generator on Voltage Regulation. — How Voltage May be Maintained Constant. — Compound- Wound Generator. — Crossing Field Connections of Compound- Wound Generator.

CHAPTER XIV

DlRECT-CuRRENT GENERATOR CHARACTERISTICS 203 to 214

Voltage Curves of Shunt Generator. — Volts Drop in Armature with Fields Separately Excited. — Voltage Curve of Series Generator. — External Characteristic Curves. — Voltage Curve of Compound Generator. — Series Winding to Compensate for Volts Drop in Armature. — Voltage at Terminals of Compound Generator. — Proper Amount of Compounding. — Compounding Shunt. — Short- Shunt and Long-Shunt Connections.

CHAPTER XV

Losses in Direct-Current Machinery 215 to 221

Loss in Transformation of Energy. — Friction Losses. — Current to Excite the Field Coils. — Copper Losses in the Armature. — Eddy- Current Losses. — Hysteresis Losses. — Total Losses in an Electric Machine.

CHAPTER XVI

DlRECT-CuRRENT Motors 222 to 232

Fundamental Principle of Electric Motor. — Effects of Changing Brush Position. — Another Theory of Motor Operation. — Both

CONTENTS xi

PAGE

Motor and Generator Action Present in all Dynamo-Electric Machines. — Counter-Electromotive Force. — How Motor Operates When Loaded. — How Speed of Motor may be Adjusted.

CHAPTER XVII

Types of Direct-Current Motors 233 to 246

Five Types of Motors. — Torque of Series Motor Varies as the Square of the Current. — Shunt-Type Motors. — Torque of Shunt Motor Varies as the Armature Current. — Compound-Type Motors. — Effects of Armature Reaction. — Commutating-Pole Machines. — Number of Interpoles Used. — Motors with Compensating Windings.

CHAPTER XVIII

Stabting Rheostats and Connections to Direct-Current Motors 247 to 268

Starting Resistance. — Action Taking Place when Motor is Started. — Starting-Current Curve. — Starting Boxes. — Connecting Up a Series Motor. — Connecting Up Shunt Motors. — Mistakes in Making Connections. — Checking Connections. — Connecting Up Compound Motor. — Testing Polarity of Series and Shunt-Field Windings. — ■ Reversing Direction of Rotation. — Connecting Up Interpole Machines.

CHAPTER XIX

Three- Wire Systems and How They May Be Obtained 269 to 289

Voltages for Direct-Current Circuits. — Three-Wire System. — • Balancer Sets for Three-Wire Systems. — How Balancer Set Maintains Three-Wire Systems Voltage Balanced. — Compound-Wound Balancer Sets. — Three-Wire Generators. — Power Output of Three- Wire Generator.

CHAPTER XX

Direct-Current Generators in Parallel 290 to 306

Reasons for Operating Generators in Parallel. — Effects of Generator Characteristics on Parallel Operation. — Compound Generators in Parallel. — Equalizer Connection. — Putting Generators into Service. — Parallel Operation of Three-Wire Generators.

CHAPTER XXI

Fundamental Principles of Alternating Current 307 to 321

Dynamo-Electric Machines Fundamentally Generate Alternating Current. — Distance Traveled and Rate of Motion. — Principles of the Dynamo-Electric Machines. — Alternating Electromotive Force.

xii CONTENTS

PAGE

— Electrical Degrees. — A Number of Conductors Connected in Series Generating Voltage.— Relation Between Cycles, Poles and Speed.

CHAPTER XXII

Measurement and Addition of Alternating Voltages and Current 322 to 332

Instruments for Alternating-Current Circuits. — Voltmeter and Ammeter Readings. — Addition of Voltages and Currents.

CHAPTER XXIII

Two-Phase and Three-Phase Circuits 333 to 349

Two-Phase Systems. — Three-Wire Two-Phase Systems. — Three- Phase Systems. — Three-Phase Machine with Ring Armature. — ■ Delta and Star Connections.

CHAPTER XXIV

Operating Alternators in Parallel 350 to 362

Condition for Parallel Operations. — Adjusting Load on Two or More Alternators. — Putting an Alternator into Service. — Other Methods of Synchronizing. — Electro-Dynamometer Type of Synchronoscope. — Synchronoscope Connections.

CHAPTER XXV

Kilowatts, Kilovolt-Amperes and Power Factor 363 to 375

How Current Builds Up in a Direct-Current Circuit. — Lenz's Law. — • Current Lags Behind Voltage. — Pump Analogy of Alternating Current. — Apparent Watts and Useful Watts. — A Wattmeter Reads True Watts. — Power in an Alternating-Current Circuit. — Effects of Capacity. — Power of Polyphase Circuits.

CHAPTER XXVI

Potential and Current Transformers 376 to 385

Types of Potential Transformers. — Principles of Transformer Operation. — Core-Type and Shell-Type Transformers. — Relation of Primary and Secondary Volts. — Grouping of Transformers.— Current Transformers.

CHAPTER XXVII

Alternating-Current Motors 3§q to 401

Parts of an Induction Motor. — Revolving Magnetic Field. — Relation Between Poles, Frequency and Speed. — How Rotation is Pro-

CONTENTS xiii duced. — Rotor Must Run Slower than the Magnetic Field. — The Induction Generator.

CHAPTER XXVIII

Starting Polyphase Motoks 402 to 420

Wound-Rotor Induction Motors. — Types of Rotors. — Wound- Rotor Motors that Start Automatically. — How to Reverse the Direction of Rotation. — Comparison of Squirrel-Cage and Wound- Rotor Motors. — How Auto-Transformers Operate. — Starting Compensators for Polyphase Motors. — Rheostats for Starting Polyphase Motors. — Starting Two or More Motors from the Same Compensator.

Index 421

Chapter I Introduction— Fundamental Principles

Electricity. — The object of this introduction is to give a clear conception of the electric current and its fundamental actions as used in commercial electrical machinery. It will be confined to a pictorial description rather than a mathematical analysis or study of the laws of electricity, because it will be easier for the student to understand the operation of the machinery that will be described later if a practical view is obtained of what is happening on the inside rather than to try to understand the results of laws or equations. Later, these fundamental principles will be referred to a great many times to get an insight of the actual workings of the different electrical machines, therefore it will pay the reader to get them firmly fixed in his mind.

Electricity is only one of several peculiar happenings in nature which are very common to us, but which are more or less mysterious. Sound, heat and light are in many respects similar in action to electricity, and on account of their being more easily understood it will be advantageous to compare a few of their actions. In each case there is a generating, a transmitting and a receiving apparatus. In each the product is invisible as it travels along the transmission system ; consequently, what has happened must be judged from the effects produced.

Sound Analogy. — If a pencil is tapped on the desk, the contact generates a sound. This sound is transmitted in

l

CHAPTER I INTRODUCTION— FUNDAMENTAL PRINCIPLES

Electricity. — The object of this introduction is to give a clear conception of the electric current and its fundamental actions as used in commercial electrical machinery. It will be confined to a pictorial description rather than a mathematical analysis or study of the laws of electricity, because it will be easier for the student to understand the operation of the machinery that will be described later if a practical view is obtained of what is happening on the inside rather than to try to understand the results of laws or equations. Later, these fundamental principles will be referred to a great many times to get an insight of the actual workings of the different electrical machines, therefore it will pay the reader to get them firmly fixed in his mind.

Electricity is only one of several peculiar happenings in nature which are very common to us, but which are more or less mysterious. Sound, heat and light are in many respects similar in action to electricity, and on account of their being more easily understood it will be advantageous to compare a few of their actions. In each case there is a generating, a transmitting and a receiving apparatus. In each the product is invisible as it travels along the transmission system ; consequently, what has happened must be judged from the effects produced.

Sound Analogy. — If a pencil is tapped on the desk, the contact generates a sound. This sound is transmitted in

l every direction along many paths from the point of eontact. One path is through the air to the ear. The air is the transmission and the ear is the receiving apparatus. If an easier path having less resistance to the sound as it travels along is substituted, the listener will get more sound at the other end. For instance, if the ear is held to a railroad track and someone hits the rail a blow a half-mile away, the sound can be heard through the rail. It could not be heard through the air. The rail is the better conductor. It will also be noted that the sound cannot be seen as it travels along the rail or through the air. If the pencil is tapped on the desk with a greater speed, more sound will be transmitted to the ear than if tapped with a slow motion. There are two ways, therefore, of getting more sound — improving the conductor by using one of less resistance, and increasing the speed of the generator. This is similar to the conditions in an electrical generating system.

Heat Analogy. — If one end of an iron rod is held against an emery wheel and the other in the hand, the emery cutting across the iron will generate heat, and the heat will flow along the rod to the hand. Thus we have a heat-generating system — a generating, a transmitting and a receiving apparatus. The emery wheel cutting across the iron is the generator, the rod is the transmission and the hand is the receiver. Here, too, the heat transmitted will depend upon the speed of cutting and the nature of the conductor. If a wooden rod is substituted, very much less heat will flow. If a vacuum is inserted in the line of transmission, practically no heat would flow across it ; that is, it is an insulator to the flow of heat. It will be noticed in heat transmission also that nothing can be. seen moving along the line of flow. Its effects must be dealt with, and it will be found that electricity is a close relative of heat and similar to it in some respects.

Light Analogy. — Light also has many characteristics that are similar to electricity. "When the filament of an incandescent lamp is heated to a certain temperature, it be°ins to

INTRODUCTION— FUNDAMENTAL PRINCIPLES 3

send out light. The light is transmitted along a line to the eye. It travels at a very rapid rate, much faster than heat or sound. Electricity travels at even a faster speed than light. As light travels along lines in space, it is also invisible to the eye. It is only when the end of one of these lines reaches the eye, either direct or reflected, that the light is detected by the eye. This is clearly shown by the moon. At night the space around the moon is full of light rays, or lines of light from the sun, but it is only those lines that strike the moon and are reflected to the eye that give the sensation of light. The other lines of transmission of light are invisible and the passage of light along them cannot be seen.

Electricity Similar to Heat, Sound and Light. — Electricity is in several ways similar to these phenomena of heat, sound and light. It is invisible and for that reason we must use our imagination and be governed by its effects. It is not known exactly what it is, but that need not confuse the student, because there are other very common things in the same class. "When the rod was held on the emery wheel, heat passed along the rod to the hand. We do not know exactly what it is that moves along the rod, but we know its effects. Light passing from the lamp must send something along the line to the eye. It is said to be wave motion and that no material substance actually passes. To make this clearer suppose a wire on a pole line is struck with a pair of pliers. This will cause a wave to travel along the wire to the next pole. It was the wave that went to the next pole and gave it a sensation, no material substance actually passed along the wire. That is the way light travels through space, but at a speed a million times faster. There is some sort of a vibration at the generator of light — a wave is sent along a line in space and finally reaches the eye and gives it a sensation similar to that which the pole received. Electricity is a similar action of the internal substance of the wire. Nothing actually passes, but as with light, heat and sound, we say it flows along the wire, which is the transmitting medium.

Electricity must, therefore, be studied from its effects, or rather, the effects of electricity must be studied, because they are, after all, what we deal with in all electrical machinery. As in the case of heat generating, take a simple example of an electrical generating system and study the effect in each unit apart. By understanding the fundamental actions that occur in each part, the more complicated electrical machinery, which is simply an enlargement of these actions, will be simplified.

Electrical Generating System. — If a magnet is used in place of an emery wheel and a copper wire in place of the rod, we have an electrical generating system, as shown in Fig. 1. A magnet (for our purpose) is a piece of steel that

Fig. 1. — Illustrates fundamental principle of electric generator.

gives out magnetic lines of force. If this magnet is rotated so that the end passes across the copper wire, not quite touching it, and the magnetic lines coming out of the end of the magnet are cut across by the wire, electricity will be generated in the loop. The emery, by actual contact with the rod when cutting across it, generates heat in the rod. The magnetic lines, by actual contact with the copper wire when cutting across the latter, generate electricity in the conductor. In the first case it is said to be the friction of the particles of emery on the rod that generates the heat. In the second case it may be said that the friction of the

INTRODUCTION— FUNDAMENTAL PRINCIPLES 5

magnetic lines cutting across the wire produces electricity. We do not use the word friction, however, because that is used in connection with heat generation. We say in electricity generation it is the induction of the magnetic lines cutting across the wire, but it means practically the same thing.

This, therefore, is the all-important principle to remember — magnetic lines cutting across a closed wire loop generate electricity. This action is made use of in generators, transformers, motors and in fact, all commercial apparatus except heating or chemical appliances. The magnetic lines are made to cut across wires by several methods, but fundamentally it should be remembered that the cutting action of the magnetic lines across the wire generates the electricity.

How Electric Pressure Is Varied in a Wire. — The reader has probably noticed one thing in the figure for electricity generation that is different from that of heat generation — a return path is shown from the receiving apparatus. This is necessary in electricity or the current will not flow. The cutting of the magnetic lines across the wire generates an electrical pressure tending to send the ctirrent along the wire, but no current will flow until the complete path is made. This electrical pressure is called voltage. If the magnetic lines cut across the wire at a very slow rate, there is not as much friction or induction, and the pressure, or voltage, is low. If the speed of cutting the magnetic lines is increased, the pressure or voltage is higher. The pressure is directly proportional to the speed of cutting.

If the number of the magnetic lines is decreased there will not be as much friction, or induction as it is called, hence, the pressure or voltage will be less; also, if the number of lines cut is increased, it will proportionally increase the voltage.

It may therefore be seen how a pressure, or voltage, is generated in the wire, which tends to send an electric current along the wire. The amount of current that will flow due to this pressure depends upon the resistance in its path. The amount of sound that came from the blow on the rail depended upon the resistance to the flow of the sound. Very little came through the air ; most of it came through the rail. The rail had less resistance to the sound. The electric current will follow the path of least resistance in its endeavor to return to the generating point, and the amount of current, called amperes, that flows around this path will be governed by the resistance of the path for a given voltage. If the voltage is raised by speeding up the cutting or by increasing the number of lines, there will be more amperes flowing for a given resistance in the path. Therefore, the current depends upon two things — the resistance of the path and the pressure, or voltage, behind it. The voltage depends upon two things also — the speed of cutting the magnetic lines and the number of magnetic lines cut.

What Happens When Current Flows Through a Wire. — The current is now flowing along the wire. Let us consider the things that are happening in and around the wire while the current is flowing through it. It has already been said that electricity is closely related to heat. It is a peculiar fact that electricity flowing along a wire produces heat. The amount of heat produced will depend upon the amount of current flowing and the amount of resistance offered. If practically all the resistance is in one place, that part will get the hottest. Copper wire is used to conduct the current because it offers very little resistance to the flow. When it is desired to produce heat, as in a flat iron, a section of conductor is placed in the path which offers a high resistance.

A close relative of heat is light. The wire in the electric bulb, if heated by forcing an electric current through it, quickly becomes hot enough to send out light. Light waves go out from the wire, due to the heat, and the heat is due to the electric current passing through a high resistance.

Besides the heating effect, there is the chemical effect of an electric current, but on account of its very small use in commercial machinery, it will not be taken up at this time.

INTRODUCTION— FUNDAMENTAL PRINCIPLES 7

Magnetic Effect of an Electric Current.— There is one other effect of the electric current — the magnetic effect. This is the effect from which all mechanical power from electricity is secured. As the current passes along the wire from the generator it sends out circular lines of magnetism around it. Electricity and magnetism are as closely related as heat and light. There may be heat without light and magnetism without electricity, but not light without heat, and similarly electric current cannot be had without magnetic lines. Whenever current is flowing in a wire, there are magnetic lines surrounding the conductor, and it is often for the sole purpose of getting these magnetic lines that an electric current is used. Except for heating, the magnetic lines surrounding the wire are what we are after in all electrical machinery.

When the path, or circuit, is completed in the simple generating system, the current flows and the magnetic lines spread out around the wire. When the circuit is opened, they close in and vanish. This will, perhaps, be made clearer by watching the hairspring of a watch. Let the shaft of the balance wheel represent the wire and the hairspring the magnetic lines that surround it. Let the tick of the escapement represent the opening and closing of the circuit. On one tick the circuit is closed, the current flows and the magnetic lines spread out around the wire. Another tick, the circuit is broken, the current stops and the magnetic lines close in. If the current is left on and is continuous in the same direction, the magnetic lines will spread out to a certain value and remain stationary as long as the current remains the same. Magnetic lines cannot be produced with pressure or voltage alone on the wire; the circuit must be closed and current flowing.

Magnetic Field Varies with the Current.— The number of lines surrounding the wire will depend upon the amount of current, or amperes, flowing. If the current in the wire is reduced, the surrounding magnetic lines close in, and if the current is reduced to zero, they do not immediately vanish, but close to zero gradually. If the current is increased, the magnetic lines spread out and more take the place of those spreading out. This spreading out and closing in of the magnetic lines is extremely important, because the motion of these lines is made use of in cutting across near-by wires. This action will be referred to a good many times in later chapters.

•Nothing has been said as yet regarding the direction of current in the wire. This is important, because it governs the direction of the magnetic lines about the wire. Returning to the simple generating system, Fig. 1, it will be seen that the ends of the permanent magnet are marked N (north) and S (south) pole. The magnetic lines are supposed to come out from the north pole of the magnet and return to the steel through the south pole. There will be a different effect produced in the wire when it cuts through the lines of force from the north pole than when through those from the south pole. One will induce a pressure tending to send the current around the loop in one direction, and the other will tend to send the current around in the opposite direction. That is, the direction of the current in the wire will alternate every time the magnetic lines from a different kind of a pole cut the wire. In other words, an alternating current is produced. Also, if the direction of the cutting is changed from upward to downward with the same pole, the direction of the induced current will be changed.