Magnetic 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 become 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 Ohm's Law (E=IR ; I=E/R ; R=E/I), while Power is
related to voltage, current, and resistance by Joule's Law
(P=IE ; P=I2R ; P=E2/R).
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
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 Centimeter-Gram-Second,
denoting the root measures upon which the whole system is
based. The other was originally known as the mks
system, which stood for Meter-Kilogram-Second,
which was later revised into another system, called rmks,
standing for Rationalized Meter-Kilogram-Second.
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
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 cross-sectional 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).