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Units of Measurement
If the burden of two systems of measurement for common
quantities (English vs. metric) throws your mind into confusion, this is not the
place for you! Due to an early lack of standardization in the science of
magnetism, we have been plagued with no less than three complete systems of
measurement for magnetic quantities.
First, we need to get acquainted with the various
quantities associated with magnetism. There are quite a few more quantities to
be dealt with in magnetic systems than for electrical systems. With electricity,
the basic quantities are Voltage (E), Current (I), Resistance (R), and Power
(P). The first three are related to one another by the first Ohm's Law equation
(E=IR), while Power is related to voltage and current by another (P=IE). All
other Ohm's Law equations can be derived algebraically from these two.
With magnetism, we have the following quantities to deal
with:
 Magnetomotive Force  The quantity of magnetic
field force, or "push." Analogous to electric voltage (electromotive
force).
 Field Flux  The quantity of total field effect,
or "substance" of the field. Analogous to electric current.
 Field Intensity  The amount of field force (mmf)
distributed over the length of the electromagnet. Sometimes referred to as Magnetizing
Force.
 Flux Density  The amount of magnetic field flux
concentrated in a given area.
 Reluctance  The opposition to magnetic field flux
through a given volume of space or material. Analogous to electrical resistance.
 Permeability  The specific measure of a
material's acceptance of magnetic flux, analogous to the specific resistance of
a conductive material (ρ), except inverse (greater permeability means
easier passage of magnetic flux, whereas greater specific resistance means more
difficult passage of electric current).
But wait . . . the fun is just beginning! Not only do we
have more quantities to keep track of with magnetism than with electricity, but
we have several different systems of unit measurement for each of these
quantities. As with common quantities of length, weight, volume, and
temperature, we have both English and metric systems. However, there is actually
more than one metric system of units, and multiple metric systems are used in
magnetic field measurements! One is called the cgs, which stands for CentimeterGramSecond,
denoting the root measures upon which the whole system is based. The other was
originally known as the mks system, which stood for MeterKilogramSecond,
which was later revised into another system, called rmks, standing for Rationalized
MeterKilogramSecond. This ended up being adopted as an
international standard and renamed SI (Systeme International).
And yes, the µ symbol is really the same as the metric
prefix "micro." I find this especially confusing, using the exact same
alphabetical character to symbolize both a specific quantity and a general
metric prefix!
As you might have guessed already, the relationship
between field force, field flux, and reluctance is much the same as that between
the electrical quantities of electromotive force (E), current (I), and
resistance (R). This provides something akin to an Ohm's Law for magnetic
circuits:
And, given that permeability is inversely analogous to
specific resistance, the equation for finding the reluctance of a magnetic
material is very similar to that for finding the resistance of a conductor:
In either case, a longer piece of material provides a
greater opposition, all other factors being equal. Also, a larger
crosssectional area makes for less opposition, all other factors being equal.
The major caveat here is that the reluctance of a material
to magnetic flux actually changes with the concentration of flux going
through it. This makes the "Ohm's Law" for magnetic circuits nonlinear
and far more difficult to work with than the electrical version of Ohm's Law. It
would be analogous to having a resistor that changed resistance as the current
through it varied (a circuit composed of varistors instead of resistors).
Lessons In Electric Circuits copyright (C)
20002011 Tony R. Kuphaldt, under the terms and conditions of the Design
Science License.
