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Units of Measurement


If you find converting between the English and Metric systems difficult, then you will be overwhelmed at the converting systems in the magnet field. Because of an early lack to standardize the science of magnetism, there are more than 3 different measurement systems for magnets.
First, you must get acquainted with the various quantities associated with magnetism. There are quite a few quantities to be dealt with in magnetic systems so we will show the comparisons with Electricity to help give you an easier understanding.
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 voltage times current (P=IE OR P=I2R). All other Ohm's Law equations can be derived algebraically from these two.

With magnetism, we have the following quantities to deal with:

  • Magneto motive 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 it isn’t over. Now we have several different systems of measurement for each of these quantities. As with the common quantities: length, weight, volume, and temperature; we have the English and Metric system. However, in the magnet field, there are more than one metric system of units, and multiple metric systems. One is called the cgs, which stands for Centimeter-Gram-Second, denoting the root measures upon which the whole system is based. The other is originally known as the mks system, which is Meter-Kilogram-Second, which was then later revised into another unit called rmks, standing for Rationalized Meter-Kilogram-Second. This ended up being adopted as an international standard and renamed SI (Systeme International).


The µ symbol is the same as the metric prefix "micro." This is especially confusing, because it uses an exact same character to symbolize 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 similar to 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, the longer the material the greater opposition, all other factors being equal. Also, a larger cross-sectional area makes for less opposition, all other factors being equal.
The major caution 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 related to having a resistor that changed resistance as the current that passed through it varied, like a circuit composed of varistors instead of resistors.

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