MPA Logo, San Francisco Maritime National Park Association, USS Pampanito, Historic Ships at Hyde Street Pier, Education Programs Maritime Park Association Home Page Maritime Park Association Home Page Events Maritime Park Association Home Page Maritime Park Association Home Page Maritime Park Association Home Page Volunteer Membership Donate Maritime Park Association Home Page USS Pampanito Submarine Historic Ships at Hyde Street Pier Education Programs About Maritime Park Association Home Page Directions to Maritime Jobs at Maritime Facility Rental at Maritime Trustees of the Association Calendar Press Room Store Maritime Map

Drawing of a ship and compass.

CHAPTER 5
MAGNETISM
NATURAL MAGNETS

It may seem strange to you that, for centuries, magnets were of little practical value. Magnetism's first real use was in a compass to guide the ancient mariners. The first of these devices was little more than a magnetized needle on a block of wood floating in a dish of water. Crude as they were, these early compasses were the first step toward modern navigation.

Little by little other uses were found, but it was not until the 19th century that new discoveries and inventions showed magnetism to be one of the foundations of the science of electricity.

Not only are magnets important in many electrical devices that you will use directly, but also in hundreds of hidden and indirect ways, such as the generation of an emf to light the incandescent bulbs about the ship.

The discovery of magnetism dates back to the ancient shepherds of Asia Minor. They noticed that the iron tips of their staffs were attracted to certain types of stones. These stones were NATURAL MAGNETS known as LODESTONES, meaning "leading stone."

609198°-46-4

 
41

The shepherds also observed that the iron tips of the staffs, if left in contact with lodestones, soon acquired the ability to attract other pieces of iron. While these
Lodestones.
Figure 33 -Lodestones.
ancients did not understand WHY these things happened, they were observing two types of magnets, NATURAL and ARTIFICIAL.

A lodestone, the NATURAL magnet, is a piece of rich IRON ORE, magnetite, and its source of magnetism is the

Earth's magnetic and geographic poles.
Figure 34.-Earth's magnetic and geographic poles.
 
42

earth itself. As illustrated in figure 34, the core of the earth is assumed to consist of iron, or a high grade iron ore.

During the past ages, the core became magnetized. The EFFECT of this magnetism seems to be concentrated in two areas, which are located near the north and the south GEOGRAPHIC poles.

The area near the north geographic pole is called the NORTH MAGNETIC POLE and the other the SOUTH MAGNETIC POLE.

ARTIFICIAL MAGNETS

Most NATURAL magnets have many north and south poles. The nails, sticking to the lodestone in figure 33, indicate the presence of three poles. Actually it may have many more. In addition to having many poles, the magnetic strength of a lodestone is too weak to be useful.

A few metals-iron, cobalt, and nickel-have the ability to become magnetized. They are ARTIFICIAL magnets, having but two poles and a greatly increased magnetic strength.

THEORY OF MAGNETISM

Probably no one knows exactly what happens inside an iron bar when it becomes magnetized, but a good explanation has been given. A piece of iron is supposed

Unmagnetized iron bar.
Figure 35.-Unmagnetized iron bar.
 
43

to be made of millions of small magnets. When the bar is unmagnetized, these small magnets have a "helter-skelter" arrangement as illustrated in figure 35. The magnetic forces of one molecule cancel the field of its neighbor.
Magnetized iron bar.
Figure 36.-Magnetized iron bar.
When the bar is magnetized, the small magnets are arranged so that ALL the north poles point in one direction, and all the south poles in the opposite direction. This systematic "line-up" of the individual magnets causes the whole bar to act as a SINGLE MAGNET. All the magnetism seems to be concentrated at the two ends of the bar, with one end designated as NORTH and the other SOUTH.
Magnetic Poles.
Figure 37.-Magnetic Poles.
Breaking the bar in half or into many pieces does not separate one pole from the other. As illustrated in figure
 
44

37, a magnet may be cut into many pieces, and each will have a north and a south pole.

MAKING A MAGNET

A bar of iron may be magnetized by stroking it with a lodestone or with another magnet. You must be careful always to stroke in the same direction, as illustrated in figure 38. It doesn't make any difference which way you

Making a magnet by induction.
Figure 38.-Making a magnet by induction.
stroke the bar-just be sure you LIFT the stroking bar several inches away at the end of each stroke.

The stroking arranges the molecular magnets within the bar so that the N poles point in one direction and the S poles in the other.

If a bar of iron lies in contact with another magnet, the bar will in time become magnetized.

You may also produce a magnet by heating the bar to red heat and then placing it parallel to the magnetic field of the earth-that is, in a north and south direction. Heating the bar frees the molecular magnets so that they may arrange themselves in order with greater ease.

MAGNETIC FIELD

A magnet extends its influence a considerable distance away from the bar. This area of influence is known as the MAGNET'S FIELD.

If you place a pane of glass over a bar magnet, figure 39A, and sprinkle iron filings on the glass, you will get a

 
45

Magnetic field.
Figure 39.-Magnetic field.
pattern like figure 39B. The filings arrange themselves in DEFINITE LINES, with the GREATEST CONCENTRATION at the ENDS of the bar. The lines DO NOT cross, but run from one end to the other.

Notice only a few scattered filings are directly over the bar itself-indicating the presence there of only a few magnetic lines of force.

NORTH AND SOUTH MAGNETISM

One pole of a magnet is designated as being NORTH, and the other SOUTH. Just how it was decided which should be S and which N is not definitely known, but all the laws of magnetism have been built around this convention.

Flux pattern about a bar magnet
Figure 40.-Flux pattern about a bar magnet
 
46

The lines of magnetic force are called FLUX. Like current, the flux is said to flow. The direction of flow is FROM the NORTH pole TO the SOUTH pole.

The arrows on the lines of force in figure 40 indicate the flux to be LEAVING the NORTH and ENTERING the SOUTH pole.

The STRENGTH of a magnet is expressed by the NUMBER OF LINES OF FORCE in the CROSS SECTION AREA of the field. A FLUX DENSITY of 10,000 lines means there are 10,000 lines of magnetic force in a square inch cross section area of the field.

LIKES REPEL-UNLIKES ATTRACT

If you bring two north poles or two south poles together, a force of repulsion will exist between them. In figure 41, when the north pole of the suspended magnet

Likes repel-unlikes attract.
Figure 41.-Likes repel-unlikes attract.
is brought near the north pole of the bar magnet, the needle will swing AWAY. The same thing is true when you bring two south poles together. But if the north pole of the suspended magnet is brought near the south pole of the bar magnet, the needle will move TOWARD the bar, indicating a force of ATTRACTION.

When iron filings are sprinkled over the ends of two like poles, figure 42A, the filings arrange themselves in a

 
47

Likes repel, unlikes attract.
Figure 42.-Likes repel, unlikes attract.
manner that indicates a REPULSION. But with unlike poles, figure 42B, an ATTRACTION is evident.
North magnetic pole has south pole magnetism.
Figure 43.-North magnetic pole has south pole magnetism.
 
48

THE EARTH'S MAGNETISM

You were told in the first page of this chapter that the earth is a huge magnet and that the NORTH MAGNETIC POLE is near the north geographic pole. That is very true, but the north magnetic pole has SOUTH POLE MAGNETISM.

To help clear this confusion, look at figure 43. Think of the earth as having a huge bar magnet extending from pole to pole with the SOUTH POLE OF THE MAGNET pointing toward the NORTH geographic pole.

Now put together two things you know-

1. Unlikes attract.
2. Lines of magnetic force leave at the North pole and enter at the South pole.

Which end of a compass points north? The NORTH! Applying the first of the above points, the north magnetic pole must have SOUTH POLE MAGNETISM or it could not attract the north magnetism of the compass needle.

Second, experiments show the earth's lines of force moving from the SOUTH GEOGRAPHIC to the NORTH GEOGRAPHIC pole. Since magnetic flow is from north magnetism to south magnetism, the north geographic pole must have SOUTH POLE MAGNETISM.

MAGNETIC MATERIALS

Only a few substances are capable of being magnetized. The most common is iron. Nickel and cobalt also have magnetic properties.

While both iron in various forms, and its derivative, STEEL, are magnetic, they have different characteristics. Soft iron is easily magnetized, but it also loses its magnetism very quickly. Certain forms of steel require a great deal of energy to magnetize them, but when magnetism is once established it remains a long time.

Since steel keeps its magnetism a long time, it is said to have a high degree of RETENTIVITY, while soft iron is said to have little RETENTIVITY. The magnetism that remains AFTER the magnetizing force has been removed is called RESIDUAL magnetism.

 
49

Several ALLOYS have been developed in order to produce substances that have HIGH RETENTIVITY and others that have LOW RETENTIVITY. A mixture of aluminum, nickel, and cobalt, properly heat treated, produces a metal (ALNICO ) with EXTREMELY HIGH RETENTIVITY and field strength. An alloy of iron and nickel, properly heat treated, produces a substance known as PERMALLOY. It is magnetized very easily, but loses its strength the instant the magnetizing force is removed.

ALNICO is used in the construction of loudspeakers, while PERMALLOY is used for transformer cores. More will be said about this later.

RELUCTANCE AND PERMEABILITY

Many metals, like copper, lead, silver, and aluminum, are without magnetic properties. Actually they STOP THE FLUX from passing through them.

Some substances like glass have a more or less neutral effect on the flux. They do not seem to stop the lines of force, neither do they aid their movement.

The DEGREE to which a substance STOPS the MOVEMENT of flux is described as the RELUCTANCE of the material. RELUCTANCE in magnetism can be compared with resistance in electricity. Both express the degree of opposition to flow. Metals with extremely high reluctance are used as MAGNETIC INSULATORS, just as substances with high resistance are used as electrical insulators.

SOFT IRON is an extremely GOOD CONDUCTOR of FLUX, or you may say that it is very PERMEABLE. Soft iron is so

Permeability of a piece of iron.
Figure 44.-Permeability of a piece of iron.
 
50

permeable that when it is placed in a magnetic field, the flux is actually concentrated into a small space. You can observe this in figure 44.

PERMANENT MAGNETS

Magnets of the type discussed in this chapter are called PERMANENT MAGNETS because they retain their magnetism AFTER the magnetizing force has been removed.

The magnets that lose their magnetism as soon as the magnetizing force has been removed are temporary magnets, a common example of which is the ELECTROMAGNETS.

 
51

Previous Chapter
Previous Chapter
Radio Home Page
Radio Home Page
Next Chapter
Next Chapter

 

Copyright © 2013, Maritime Park Association.
All Rights Reserved.
Legal Notices and Privacy Policy
Version 3.00