Centuries ago, it was discovered that certain types of
mineral rock possessed unusual properties of attraction to the metal iron. One
particular mineral, called lodestone, or magnetite, is found
mentioned in very old historical records (about 2500 years ago in Europe, and
much earlier in the Far East) as a subject of curiosity. Later, it was employed
in the aid of navigation, as it was found that a piece of this unusual rock
would tend to orient itself in a north-south direction if left free to rotate
(suspended on a string or on a float in water). A scientific study undertaken in
1269 by Peter Peregrinus revealed that steel could be similarly
"charged" with this unusual property after being rubbed against one of
the "poles" of a piece of lodestone.
Unlike electric charges (such as those observed when amber
is rubbed against cloth), magnetic objects possessed two poles of opposite
effect, denoted "north" and "south" after their
self-orientation to the earth. As Peregrinus found, it was impossible to isolate
one of these poles by itself by cutting a piece of lodestone in half: each
resulting piece possessed its own pair of poles:
Like electric charges, there were only two types of poles
to be found: north and south (by analogy, positive and negative). Just as with
electric charges, same poles repel one another, while opposite poles attract.
This force, like that caused by static electricity, extended itself invisibly
over space, and could even pass through objects such as paper and wood with
little effect upon strength.
The philosopher-scientist Rene Descartes noted that this
invisible "field" could be mapped by placing a magnet underneath a
flat piece of cloth or wood and sprinkling iron filings on top. The filings will
align themselves with the magnetic field, "mapping" its shape. The
result shows how the field continues unbroken from one pole of a magnet to the
As with any kind of field (electric, magnetic,
gravitational), the total quantity, or effect, of the field is referred to as a flux,
while the "push" causing the flux to form in space is called a force.
Michael Faraday coined the term "tube" to refer to a string of
magnetic flux in space (the term "line" is more commonly used now).
Indeed, the measurement of magnetic field flux is often defined in terms of the
number of flux lines, although it is doubtful that such fields exist in
individual, discrete lines of constant value.
Modern theories of magnetism maintain that a magnetic
field is produced by an electric charge in motion, and thus it is theorized that
the magnetic field of a so-called "permanent" magnets such as
lodestone is the result of electrons within the atoms of iron spinning uniformly
in the same direction. Whether or not the electrons in a material's atoms are
subject to this kind of uniform spinning is dictated by the atomic structure of
the material (not unlike how electrical conductivity is dictated by the electron
binding in a material's atoms). Thus, only certain types of substances react
with magnetic fields, and even fewer have the ability to permanently sustain a
Iron is one of those types of substances that readily
magnetizes. If a piece of iron is brought near a permanent magnet, the electrons
within the atoms in the iron orient their spins to match the magnetic field
force produced by the permanent magnet, and the iron becomes
"magnetized." The iron will magnetize in such a way as to incorporate
the magnetic flux lines into its shape, which attracts it toward the permanent
magnet, no matter which pole of the permanent magnet is offered to the iron:
The previously unmagnetized iron becomes magnetized as it
is brought closer to the permanent magnet. No matter what pole of the permanent
magnet is extended toward the iron, the iron will magnetize in such a way as to
be attracted toward the magnet:
Referencing the natural magnetic properties of iron (Latin
= "ferrum"), a ferromagnetic material is one that readily
magnetizes (its constituent atoms easily orient their electron spins to conform
to an external magnetic field force). All materials are magnetic to some degree,
and those that are not considered ferromagnetic (easily magnetized) are
classified as either paramagnetic (slightly magnetic) or diamagnetic
(tend to exclude magnetic fields). Of the two, diamagnetic materials are the
strangest. In the presence of an external magnetic field, they actually become
slightly magnetized in the opposite direction, so as to repel the external
If a ferromagnetic material tends to retain its
magnetization after an external field is removed, it is said to have good retentivity.
This, of course, is a necessary quality for a permanent magnet.
(also called Magnetite) is a
naturally-occurring "permanent" magnet mineral. By
"permanent," it is meant that the material maintains a magnetic
field with no external help. The characteristic of any magnetic material to
do so is called retentivity.
materials are easily magnetized.
materials are magnetized with more
- Diamagnetic materials actually tend to repel
external magnetic fields by magnetizing in the opposite direction.
Lessons In Electric Circuits copyright (C)
2000-2011 Tony R. Kuphaldt, under the terms and conditions of the Design