This is a very long page due to the fact that next to the study of
        the electron flow, the study of magnetism is the most important part 
        of understanding electricity.


Magnets and Electromagnets
        Magnets and electromagnets have many uses, every electric motor,
generator or transformer requires a magnetic field for it's operation.
With the exception of a few special types, all use electromagnets. The 
magnets mounted on large cranes are used to lift heavy loads. Magnetism 
makes the generator supplying the electricity to your home work and the 
radio, telephone and most other electrical gadgets work.

        The properties of Magnetism were known to the Greeks as early
as 700 B.C. It was found that a certain type of ore had the power 
to attract pieces of iron which were in it's vicinity. The discovery was
made in a province called Magnesia, and the ore was given the name Magnetite
after its place of discovery. The type of magnetite which exhibits magnetic
properties is commonly known as Lodestone. Any material which exhibits these
magnetic properties is called a Magnet. 

        The first uses of magnetism were recorded by the Chinese, who are 
believed to have used suspended pieces of magnetite as compasses nearly
2000 years ago. Compasses were also used by the European navigators, but
not until about 1200 A.D. Christopher Columbus was interested in the properties
of magnetic compasses, and he made some important observations on the 
accuracy of compasses during his voyage to America in 1492. However, the 
first true study of magnetic properties was not attempted until 1600, when
William Gilbert, an English physician, published a report on his experiments
with magnets.

        A careful and through study of magnets and their actions shows that
all materials are affected to some extent when brought close to a strong
magnet. By testing all the known substances, it has been found that iron and
steel are affected very strongly, cobalt and nickel are affected to some 
extent, while all other materials are only slightly affected. Iron and steel
are called magnetic, or ferromagnetic, substances, the prefix "ferro" being
taken from the Latin word "ferrum", which means iron. The magnetic effect
on iron is much greater than on other materials, but certain combinations
or alloys of iron, nickel and cobalt are in common use today.

        A magnet found in a natural state is known as a Natural Magnet.
deposits of magnetic ore (magnetite) have been discovered at various places
one of these being Labrador. Pieces of this material are called natural 
magnets because they exhibit the properties of magnetism without any special
treatment. The earth is considered to be a huge natural magnet because it
possesses the same properties as smaller magnets. Due to irregularities in 
size, shape and strength, natural magnets have little commercial value.
However, when placed in a magnetic field, a piece of steel becomes a magnet.
By proper treatment called ageing, it can be made to keep it's magnetism
almost indefinitly. When properly magnetized and treated, a piece of steel
is called a permanent magnet.

        Almost any kind of steel and certain types of ceramics can be made
into a permanent magnet, but some alloys can be more strongly magnetized
than ordinary steel. The most popular magnetic alloys for permanent magnets
are alloys of pure iron, aluminum, nickel and cobalt, called Alnico.
By combining different percentages of these metals, various magnetic qualities
are set up in the alloys when they are processed in a magnetic field. Permanent
magnets made of alnico are used commonly in television, radar and other
electronic instruments. Ceramic magnets are also used to a wide extent in
these instruments.

        When in contact with a permanent magnet, a piece of iron becomes
magnetized as strongly as a piece of steel. However, when the permanent
magnet is removed, the iron loses practically all of it's magnetism. Therefore
a piece of iron, especially soft iron, is called a Temporary Magnet.
Because they do not occur in a natural state, all forms of magnetized steel
and iron are considered as Artificial Magnets.

Magnetic Poles:
        
        When a bar magnet is suspended, it rotates to a generally north and
south direction, with the same end always pointing to the north. No matter
which way it is pointed, when released, the magnet comes to rest pointing
approximately north and south. At each end of the bar magnet, there is a 
concentration of magnetic force. This concentration of force is known as a
Magnetic Pole. The presence of the poles can be demonstrated by dipping
the ends of a magnet into a pile of iron filings, when withdrawn filings
cling to each end demonstrating the concentration of force at each end. The
poles of a bar magnet are named after the direction in which they point when
the magnet is suspended. The end that points northward is the North Pole
and the that points south is the South Pole.

Magnetic Fields:

        A magnet produces some surprising effects. When placed close to one
end of a strong magnet, a small piece of iron actually jumps to reach the 
magnet. The fact that the iron jumps shows clearly that the magnetic force
extends for some distance around the magnet. This area of influence in the
space around the magnet is known as the Magnetic Field. The unknown
force that causes the field is referred to as the Magnetomotive force
(mmf) of the magnet.

        For convenience, a magnetic field is considered as being made up of
manetic lines of force, or simply Magnetic Lines. However, the single
line of force is seldom considered in analyzing  magnetic fields. Instead, it
is more common to use the term Magnetic Flux, which refers to the total
number of magnetic lines that make up a magnetic field.

        A simple but effective method of making a magnetic field "visible" is
to place a piece of glass over a bar magnet and then sprinkle iron filings
on the glass. The glass offers no opposition to the field, even though it is
not a good conductor of magnetic lines. Thus, the filings are affected by the
field, and align themselves in a pattern like that of the field around the 
magnet.

        In addition to providing a convenient means of explaining the action
of a magnetic field, the theoretical lines of force also provide a means of
measurement. In practicle work, magnetic fields have a comparatively large
number of lines and are of all shapes and sizes. Therefore, they usually are
described as having a certain number of lines per square centimeter, which
means the number of lines that pass through each square centimeter of a surface
that is placed squarely across the magnetic field.

        Because a given number of lines of force may be spread over a large
field or compressed into a relatively small field, it is necessary to know
both the number of lines that make up the flux and the size of the area
through which the lines of force pass. The number of lines passing through
a given area is known as the density of the field or the Flux Density.

        The standard unit of measurement of the number of magnetic lines of
force or magnetic flux is the Weber, abbreviated Wb. One weber is equal
to 100,000,000 or 10 to the 8th power lines of force. The symbol used to 
represent magnetic flux is the Greek letter Phi. As previously mentioned, the
lines of force passing through a given area is also important. The standard
unit of measurement of the lines of force for a given area of the flux density
is the Tesla. A tesla is equal to a weber per square meter; that is,
10 to the 8th power magnetic lines of force passing through an area one meter
by one meter. The capitol letter B is often used to represent flux
density. 

Attraction and Repulsion:

        The earlier suspended bar magnet can be used to demonstrate the
attraction and repulsion of the magnet. Suspend a bar magnet by a string
and end marked N will point to the north. Take another bar magnet with the
ends marked N (north) and S (south), hold the north end of this magnet near
the north end of the suspended magnet and the suspended magnet will move
away, place the south end near the north end and they will be attracted to
each other. Like magnetic poles Repel; Unlike magnetic poles Attract

Magnetic Induction:

        Certain materials can be magnetized while under the influence of a
magnetic field, but lose their magnetism as soon as the field is removed.
Materials of this type are temporary magnets, and one of the materials most
commonly used in temporary magnets is iron. Ordinary tacks can become
temporary magnets if they are brought under the influence of a magnetic
field. A tack placed at the end of a magnet becomes at magnet itself, and
another tack placed at the end of it becomes another magnet and a third,
each tack with it's own north and south pole, north connected to south to
north to south all down the line of tacks. This process of setting up
magnetism in an object that is under the influence of a magnetic field is
called Magnetic Induction. If the original magnet is taken away from
the first tack, the magnetic poles at the end of each tack disappear, and
there is no longer a force to hold them together, the entire string falls 
apart.

        The reason for magnetic induction is best explained by the molecular
theory. The molecules that make up the tack material are easily turned from
a random state of alignment, all the N's and S's pointed in random directions,
to an organized state of alignment, all the N's northward of the material and
all the S's southward of the material. In this organized state the material
is considered to be a magnet. However, these molecules turn back to their 
original state of a random organization as soon as the original magnet is 
removed. This characteristic distinguishes the temporary magnet from the
permanent magnets. In a material such as hardened steel, the molecules require
a great deal of force to bring them into alignment, but once this alignment
occurs, the steel tends to maintain it's magnetism indefinitly.

Reluctance and Permeability:
        Because most of the molecules of soft iron turn quite easily under
the influence of a magnitizing force, the overall effect is quite strong.
In some other materials, very few or none of the molecules turn because of
the rigid structure of the material. In other materials, the magnetic field
for each molecule is quite small, the total magnetic effect is very weak.

        Although magnetic lines of force pass through all substances under
normal conditions, some materials do not carry them as readily as others. 
This opposition which a substance offers to the passage of magnetic lines
is known as it's Reluctance.

        Reluctance is a property of every material. Just as there is no 
perfect electric conductor, under normal conditions, there is no perfect
magnetic conductor. However, soft iron has a very low reluctance and is a
good conductor of magnetic lines in comparison with most other materials.
Reluctance describes the opposition offered to magnetic lines of force. The
term Permeability describes the ease of passage of magnetic lines of
force. Thus, a material with a high permeability has a low reluctance, and
vice versa.

        Measurement of permeability are taken with air as the reference. Air
has a permeability of one. Iron, for example has a permeability several
thousand times that of air. The Greek letter Mu is often used to represent
permeability.

Magnetic Circuits:

        The paths taken by magnetic lines of force can be thought of in much
the same way as the current paths in an electric circuit. In an electric 
circuit, pressure or voltage overcomes the resistance of the conducting path
and sets up a current flow. In a magnetic circuit, similar conditions exist,
but instead of voltage or electromotive force (emf), there is a: 
Magnetomotive Force(mmf) which causes magnetic lines throughout the
circuit.

        The opposition which the magnetic circuit offers to this flux is 
known as reluctance. Electrically, nearly all materials have different
resistance characteristics, some offering little and others offering high
opposition to an electric current. However, with the exception of the 
magnetic metals, most substances offer nearly equal reluctance. Air and
some other nonmagnetic materials have high reluctance.

        To further compare the magnetic circuit with an electric circuit:
there is an mmf instead of voltage; line of force, or flux lines, instead of
current; and reluctance instead of resistance. The relationship between mmf
flux and reluctance in the magnetic circuit is very similar to the relation
ship between voltage, current and resistance in the electric circuit. Just
as current is equal to voltage divided by resistance, flux is equal to
magnetomotive force divided by reluctance.

Electromagnetism:

        It has been found that an electric current sets up a magnetic field
similar to that produced by a permanent magnet. This action is known as
Electromagnetism and is very important in many devices. A desirable
feature of electromagnetism is that it is possible to control the strength
and polarity of the magnetic field. When current exists in a coil, the coil
has all the magnetic qualities of a permanent magnet and is called an
Electromagnet. If this electromagnet is brought near a permanent
magnet or another electromagnet, the like and unlike poles react exactly
as explained for the permanent magnets. Moreover, an increase of current
in the coil increases the strength of the magnetic field, and a decrease
of current weakens the field.

Ampere-Turns:

        When the number of loops or turns of the coil is increased and the
current remains the same, the strength of the magnetic field increases.
Each loop or turn of the coil sets up it's own magnetic field, which unites
with the fields of the other loops to produce the field around the entire
coil. The more loops, the more magnetic fields unite and reinforce each other
and, as a result, the total magnetic field becomes stronger.

        To compare the magnetic strength of different coils, and to obtain
a basis for measuring the magnetomotive force of an electromagnet, the number
of turns of wire is multiplied by the number of amperes of current carried
by the wire and the result is called Ampere-Turns (NI). The ampere-turn
is the unit for measuring the magnetomotive force of a current-carrying
coil. In a formula, the magnetomotive force in ampere-turns can be expressed
as:
                F = NI
                F = magnetomotive force in ampere-turns
                N = number of turns
                I = current in amperes
For example:
        A coil with 10 turns and a current of 10 amperes has an F of 100
        ampere-turns.

Effect of an iron core:

        Earlier it was stated that iron and steel have low reluctance and
carry magnetic lines of force much more readily than air and certain other
materials. To increase the magnetic field of a coil, it is common practice
to insert a piece of iron through the center of the coil. This piece of iron
is called the core, and it's low reluctance permits passage of many more
magnetic lines of force through it than the surrounding air will carry. It
tends to concentrate the coil's magnetic field.

        The magnetic behavior of a coil carrying electric current can be 
summed up in the following three statements:

        Whenever current is present in a coil of wire, a magnetic field is
        set up in and around the coil, which then exibits all of the
        properties of a magnet.

        The strength of the magnetic field varies with the number of turns
        and the current. With no current, there is no magnetism.

        An iron core, placed inside the coil, permits a large increase in
        the strength of the magnetic field by providing a magnetic circuit
        with less reluctance than air.
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