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Chapter 14: MAGNETISM AND ELECTROMAGNETISM

# 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=I^{2}R ; P=E^{2}/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 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 **C**entimeter-**G**ram-**S**econd, denoting the root measures upon which the whole system is based. The other was originally known as the *mks* system, which stood for **M**eter-**K**ilogram-**S**econd, which was later revised into another system, called *rmks*, standing for **R**ationalized **M**eter-**K**ilogram-**S**econd. This ended up being adopted as an international standard and renamed *SI* (**S**ysteme **I**nternational).

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 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 *var*istors instead of *res*istors).