Audel Practical Electricity / Edition 5

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Everything begins with the basics

To succeed in any of today's electrical specialties, you must first understand the fundamentals. This concise guidebook, fully updated and revised to comply with the National Electrical Code,? provides that solid foundation in electrical theory, circuitry, and common applications. Whether you're pursuing an electrical career, need a refresher course, or simply want to understand the wiring in your home, you'll learn the basics from this book.
* Examine the fundamentals of magnetism and electric-ity, conductors, insulators, and circuits
* Study common applications including house wiring, lighting, cables, electric heating, and generating
* Become familiar with test procedures and electromagnetic induction
* Understand inductive and capacitive AC circuits and the principles of alternating current
* Explore alarm and intercom wiring, home circuiting, and multiple switching
* Find out how generating stations and substations function
* Learn from clear, specific text, functional illustrations, and review questions in every chapter

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Product Details

  • ISBN-13: 9780764541964
  • Publisher: Wiley
  • Publication date: 6/7/2004
  • Series: Audel Technical Trades Series, #19
  • Edition description: All New 5th Edition
  • Edition number: 5
  • Pages: 480
  • Sales rank: 316,700
  • Product dimensions: 5.28 (w) x 8.38 (h) x 1.04 (d)

Meet the Author

Paul Rosenberg is a leading voice in the electrical industry. A master electrician with extensive experience in all aspects of the industry, he teaches at Iowa State University and has written for all the major industry publications. He is the author of several Audel electrical books, including Audel Electrician’s Pocket Manual, 2nd Edition.

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Table of Contents


Chapter 1: Magnetism and Electricity.

Chapter 2: Conductors and Insulators.

Chapter 3: Electric Circuits.

Chapter 4: Series-Parallel Circuits.

Chapter 5: Electromagnetic Induction.

Chapter 6: Principles of Alternating Currents.

Chapter 7: Inductive and Capacitive AC Circuits.

Chapter 8: Electric Lighting.

Chapter 9: Lighting Calculations.

Chapter 10: Basic House Wiring.

Chapter 11: Wiring with Armored Cable and Conduit.

Chapter 12: Home Circuiting, Multiple Switching, and Wiring Requirements.

Chapter 13: Electric Heating.

Chapter 14: Alarms and Intercoms.

Chapter 15: Generating Stations and Substations.




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First Chapter

Audel Practical Electricity

By Paul Rosenberg Robert Gordon Middleton

John Wiley & Sons

ISBN: 0-7645-4196-X

Chapter One

Magnetism and Electricity

Early experimenters dating back to the dawn of history discovered that certain hard black stones attracted small pieces of iron. Later, it was discovered that a lodestone, or leading stone, pointed north and south when freely suspended on a string, as shown in Figure 1-1. Lodestone is a magnetic ore that becomes magnetized if lightning happens to strike nearby. Today we use magnetized steel needles instead of lodestones in magnetic compasses. Figure 1-2 illustrates a typical pocket compass.

Magnetic Poles

Any magnet has a north and a south pole. We know that the earth is a huge, although weak, magnet. In Figure 1-1, the end of the lodestone that points toward the North Star is called its north-seeking pole; the opposite end of the lodestone is called its south-seeking pole. It is a basic law of magnetism that like poles repel each other and unlike poles attract each other. For example, a pair of north poles repel each other and a pair of south poles repel each other, but a north pole attracts a south pole.

Magnetic forces are invisible, but it is helpful to represent magnetic forces as imaginary lines. For example, we represent the earth's magnetism as shown in Figure 1-3. There are several important facts to be observed in this diagram. Since the north pole of a compass needle points toward theearth's geographical North Pole, we recognize that the earth's geographical North Pole has a magnetic south polarity. In other words, the north pole of a compass needle is attracted by magnetic south polarity.

Another important fact shown in Figure 1-3 is the location of the earth's magnetic poles with respect to its geographic poles. The earth's magnetic poles are located some distance away from its geographic poles. Still another fact to be observed is that magnetic force lines have a direction, which can be indicated by arrows. Magnetic force lines are always directed out of the north pole of a magnet and directed into the south pole. Moreover, magnetic force lines are continuous; the lines always form closed paths. Thus, the earth's magnetic force lines in Figure 1-3 are continuous through the earth and around the outside of the earth.

The actual source of the earth's magnetism is still being debated by physicists. However, insofar as compass action is concerned, we may imagine that the earth contains a long lodestone along its axis. In turn, this imaginary lodestone will have its south pole near the earth's north geographic pole; the imaginary lodestone will have its north pole near the earth's south geographic pole.

Experiments with Magnets

If we bring the south pole of a magnet near the south pole of a suspended magnet, as shown in Figure 1-4, we know that the poles will repel each other. It can also be shown that magnetic attractive or repulsive forces vary inversely as the square of the distance between the poles. For example, if we double the distance between a pair of magnetic poles, the force between them will be decreased to one-fourth. It can also be shown that if the strength of the magnet in Figure 1-4 is doubled (as by holding a pair of similar magnets together with their south poles in the same direction), the force of repulsion is thereby doubled.

The strength of a magnetic field is measurede in gauss (G). For example, the strength of the earth's magnetic field is a approximately 0.5 G. The gauss unit is a measurement of flux density-that is, it is a measure of the number of magnetic force lines that pass through a unit area. One gauss is defined as one line of force per square centimeter. In turn, one gauss is equal to 6.452 lines of force per square inch. For example, the strength of the earth's magnetic field is approximately 3.2 lines of force per square inch.

Note that there are 2.54 centimeters in 1 inch, or 0.3937 inches in 1 centimeter. Therefore, there are 6.452 square centimeters in 1 square inch, or 0.155 square inch in 1 square centimeter. Since one gauss is defined as one line of force per square centimeter, it follows that one gauss is also equal to 6.452 lines of force per square inch.

A unit of magnetic pole strength is measured in terms of force. That is, a unit of magnetic force is defined as one that exerts a force of one dyne on a similar magnetic pole at a distance of 1 centimeter. If we use a pair of like poles, this will be a repulsive force; if we use a pair of unlike poles, it will be an attractive force. There are 444,800 dynes in one pound; in other words, a dyne is equal to 1/444,800 of a pound. It is not necessary to remember these basic definitions and conversion factors. If you should need them at some future time, it is much more practical to look them up than to try to remember them.

Another important magnet experiment is shown in Figure 1-5. If we break a magnetized needle into two parts, each of the parts will become a complete magnet with north and south poles. No matter how many times we break a magnetized needle, we will not obtain a north pole by itself or a south pole by itself. This experiment leads us to another basic law of magnetism, which states that magnetic poles must always occur in opposite pairs. Many attempts have been made by scientists to find an isolated magnetic pole (called a magnetic monopole). All attempts to date have failed, though scientists are still trying.

It has been found that iron and steel are the only substances that can be magnetized to any practical extent. However, certain alloys, such as Alnico, can be strongly magnetized. Substances such as hard steel and Alnico retain their magnetism after they have been magnetized and are called permanent magnets. Since a sewing needle is made from steel, it can be magnetized to form a permanent magnet. On the other hand, soft iron remains magnetized only as long as it is close to or in contact with a permanent magnet. The soft iron loses its magnetism as soon as it is removed from the vicinity of a permanent magnet. Therefore, soft iron is said to form a temporary magnet.

Permanent magnets for experimental work are commonly manufactured from hard steel or magnetic alloys in the form of horseshoe magnets and bar magnets, as shown in Figure 1-6. The space around the poles of a magnet is described as a magnetic field and is represented by magnetic lines of force. The space around a lodestone (Figure 1-1), around a compass needle (Figure 1-2), around the earth (Figure 1-3), and around a permanent magnet (Figure 1-4) are examples of magnetic fields. Since a magnetic field is invisible, we can demonstrate its presence only by its force of attraction for iron.

Consider the patterns formed by magnetic lines of force in various magnetic fields. One example has been shown in Figure 1-3. It can also be easily shown experimentally that when a bar magnet is held under a piece of cardboard and then iron filings are sprinkled on the cardboard, the filings will arrange themselves in curved-line patterns as shown in Figure 1-7. The pattern of iron filings formed provides a practical basis for our assumption of imaginary lines of force to describe a magnetic field. The total number of magnetic force lines surrounding a magnet, as shown in Figure 1-8, is called its magnetic flux.

A similar experiment with a horseshoe magnet is shown in Figure 1-9. The iron filings arrange themselves in curved lines that suggest the imaginary lines of force that we use to describe a magnetic field. Note that the magnetic field is strongest at the poles of the magnet in Figure 1-7. Since the field strength falls off as the square of the distance from a pole, a magnet exerts practically no force on a piece of iron at an appreciable distance. A magnet exerts its greatest force on a piece of iron when in direct contact.

Formation of Permanent Magnets

Another important and practical experiment is the magnetization of steel to form a permanent magnet. For example, if we wish to magnetize a steel needle, we may use any of the following methods:

The needle can be stroked with one pole of a permanent magnet. The needle can be stroked several times to increase its magnetic strength, but each stroke must be made in the same direction. If the needle is held in a magnetic field (such as between the poles of a horseshoe magnet) and the needle is tapped sharply, it will become magnetized. We can heat a needle to dull red heat and then quickly cool the needle with cold water while holding it in a magnetic field, and the needle will become magnetized.

The formation of permanent magnets is explained in terms of molecular magnets. Each molecule in a steel bar is regarded as a tiny permanent magnet. As shown in Figure 1-10, the poles of these molecular magnets are distributed at random in an unmagnetized steel bar. Therefore, the fields of the molecular magnets cancel out on the average, and the steel bar does not act as a magnet. On the other hand, when we stroke an unmagnetized steel bar with the pole of a permanent magnet, some of the molecular magnets respond by lining up end-to-end. In turn, the lined-up molecular magnets have a combined field that makes the steel bar a magnet. If the steel bar is stroked a number of times, more of the molecular magnets are lined up end-to-end, and a stronger permanent magnet is formed, as shown in Figure 1-11A.

Steel is much harder than iron; therefore, it is more difficult to line up the molecular magnets in a steel bar than in a soft-iron bar. To make a strong permanent magnet from a steel bar, we must stroke the bar many times with a strong permanent magnet. A soft-iron bar becomes fully magnetized as soon as it is touched by a permanent magnet but will return to its unmagnetized state as soon as it is removed from the field of a permanent magnet. Once the molecular magnets have been lined up in a hard steel bar, however, they will retain their positions and provide a permanent magnet.

Although there are very large numbers of molecules in an iron or steel bar, the number of molecules to be lined up is not infinite. Therefore, there is a limit to which the bar can be magnetized, no matter how strong a field we use. When all the molecular magnets are aligned in the same direction, the bar cannot be magnetized further, and the iron or steel is said to be magnetically saturated. The ability of a magnetic substance to retain its magnetism after the magnetizing force has been removed is called its retentivity. Thus, retentivity is very large in hard steel and almost absent in soft iron. Magnetic alloys such as Alnico V have a very high retentivity and are widely used in modern electrical and electronic equipment. The Alnico alloys contain iron, aluminum, nickel, copper, and cobalt in various proportions depending on the requirements.

A permanent magnet that weighs 1 1/2 lbs may have a strength of 900 G and will lift approximately 50 lbs of iron. This type of magnet is constructed in a horseshoe form and is less than 3 in. long. A 5-lb magnet may have strength of 2000 G and will lift approximately 100 lbs of iron. A 16-lb magnet 5 1/2 in. long may have a strength of 4800 G and will lift about 250 lbs of iron.

Bar magnets can be magnetized with sufficient strength so that one of the magnets will float in the field of the other magnet, as shown in Figure 1-11B. A similar demonstration of magnetic forces is provided by circular ceramic magnets. Each circular magnet is about 2 1/2 in. in diameter and has a hole in the center that is 1 in. in diameter. One surface of the disc is a north pole, and the opposite surface is a south pole. When placed on a nonmagnetic restraining pole, with like poles adjacent, the circular magnets float in the air, being held in suspension by repelling magnetic forces.

Aiding and Opposing Magnetic Fields

An experiment that demonstrates the repulsion of like magnetic poles was illustrated in Figure 1-4. The question is often asked how magnetic force lines act in aiding or opposing magnetic fields. Figure 1-12 shows the answer to this question. Note that when unlike poles are brought near each other, the lines of force in the air gap are in the same direction. Therefore, these are aiding fields, and the lines concentrate between the unlike poles. It is a basic law of magnetism that lines of force tend to shorten as much as possible; lines of force have been compared to rubber bands in this respect. In turn, a force of attraction is exerted between the unlike poles in Figure 1-12A.

On the other hand, a pair of like poles have been brought near each other in Figure 1-12B. The lines of magnetic force are directed in opposition, and the lines from one pole oppose the lines from the other pole. In turn, none of the lines from one magnet enters the other magnet, and a force of repulsion is exerted between the magnets. We observe that the magnetic fields in Figure 1-12 are changed in shape, or are distorted with respect to the field shown in Figure 1-8. If the magnets in Figure 1-12 are brought more closely together, the fields become more distorted. Hence, we recognize that forces of attraction or repulsion between magnets are produced by distortion of their magnetic fields.


Electromagnetism is the production of magnetism by an electric current. An electric current is a flow of electrons; we can compare the flow of electrons in a wire to the flow of water in a pipe. Today we can read about electronics and electrons in newspapers, magazines, and many schoolbooks. However, the practical electrician needs to know more about electrons than a mechanic or machinist does. Therefore, let us see how electric current flows in a wire.

An atom is the smallest particle of any substance; thus, the smallest particle of copper is a copper atom. We often hear about splitting the atom. If a copper atom is split or broken down into smaller particles, we can almost say that it is built from extremely small particles of electricity. In other words, all substances, such as copper, iron, and wood, have the same building blocks, and these building blocks are particles of electricity. (This is technically not an entirely true statement, but it is close enough for our use here.) Copper and wood are different substances simply because these particles of electricity are arranged differently in their atoms. An atom can be compared to our solar system in which the planets revolve in orbits around the sun. For example, a copper atom has a nucleus, which consists of positive particles of electricity; electrons (negative particles of electricity) revolve in orbits around the nucleus.

Figure 1-13 shows three atoms in a metal wire.


Excerpted from Audel Practical Electricity by Paul Rosenberg Robert Gordon Middleton Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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