Recently, I had one of the most fascinating “aha moments” I’ve experienced in eddy current testing in a long time.
I was experimenting with several magnets while observing the impedance plane response of an eddy current probe.
Some behaved roughly as expected.
But one weak kitchen-style refrigerator magnet produced a signal that rotated dramatically downward into the conductivity-dominated region of the impedance plane — even LOWER than copper.
In fact, some people looking at the response guessed:
“Is that silver?”
Now THAT should make any eddy current technician stop and think.
How can a weak refrigerator magnet behave more like silver than steel?
The answer may lie in one of Dr. Friedrich Förster’s deepest concepts:
Effective Permeability
Most People Think “Magnetic” Means One Thing
In everyday life, we simplify materials into two categories:
- magnetic
- nonmagnetic
And many eddy current technicians subconsciously carry that same simplification into the impedance plane.
If ferrite is normalized upward:
- magnetic materials should go “up”
- nonferrous materials should go “down”
Simple, right?
Well…
not exactly.
Because the impedance plane is not a “magnet detector.”
It is a map of electromagnetic interaction.
The Coil Does Not Care What the Material “Is”
This is one of the deepest concepts in eddy current testing.
The probe does not ask:
“Is this object magnetic?”
Instead, it asks:
“How does this material behave inside my changing AC electromagnetic field?”
That behavior depends on:
- conductivity
- permeability
- frequency
- geometry
- coupling
- magnetic saturation
- demagnetization effects
- incremental permeability
- magnetic losses
Dr. Förster tied these interactions into what he called effective permeability.
So What Is “Effective Permeability”?
In simple terms:
Effective permeability is how “magnetic” a material ACTS to the eddy current coil under actual test conditions.
Not its textbook permeability.
Not its static magnetic strength.
Not how strongly it sticks to steel.
But how it dynamically responds to the coil’s changing AC field.
That distinction changes everything.
Why the Refrigerator Magnet Matters
At first, I assumed the magnet behaving below copper on the impedance plane made absolutely no sense.
But then I realized something important:
This was not a strong soft-iron magnet.
It was a weak kitchen-style refrigerator magnet — probably around 100 gauss or so.
That means it may actually be:
- ferrite ceramic
- bonded ferrite powder
- flexible magnetic polymer
- a high-resistivity magnetic composite
And those materials behave VERY differently from soft magnetic steel.
Many ferrite materials are intentionally designed to:
- suppress eddy currents
- exhibit high electrical resistance
- operate in AC magnetic fields
- reduce magnetic losses at higher frequencies
In other words:
A refrigerator magnet may possess a STATIC magnetic field…
while responding very differently to the probe’s alternating AC field.
Static Magnetism vs AC Magnetic Behavior
This is where the “aha moment” really happened for me.
A material can:
- appear highly magnetic in daily life
while - exhibiting relatively low effective permeability to an eddy current probe
Why?
Because the probe is not measuring static magnetism.
It is measuring dynamic electromagnetic response.
Many permanent magnets:
- resist changes in magnetization
- operate near magnetic saturation
- exhibit lower incremental permeability
- experience strong phase lag effects
- produce unusual impedance-plane behavior
As a result, conductivity-related behavior can begin dominating the response.
And the signal may rotate dramatically downward into the conductivity-dominated region of the impedance plane.
Exactly what I observed.
Förster Was Thinking Far Beyond “Magnetic vs Nonmagnetic”
One of the remarkable things about Förster’s work is that he never treated permeability as a simple fixed material constant.
His papers repeatedly connect effective permeability to:
- frequency ratio
- conductivity
- geometry
- fill factor
- dimensional effects
- coupling conditions
That means the impedance plane is not simply separating:
- magnetic materials
from - nonmagnetic materials
It is mapping the effective electromagnetic interaction between the material and the AC test field.
That is a much deeper idea.
The Impedance Plane Is a Living Physics Map
The more you study eddy current testing, the more you realize:
The impedance plane is not random.
It is a visual map of competing electromagnetic phenomena.
Once technicians begin understanding concepts like:
- effective permeability
- magnetic saturation
- incremental permeability
- demagnetization
- conductivity loading
- phase behavior
…the signals stop looking like mysterious squiggles on a screen.
The data begins telling a story.
And sometimes…
that story includes a refrigerator magnet behaving more like silver than steel.
That’s the kind of moment that transforms someone from merely operating an eddy current instrument…
…into truly understanding one.
For more advanced discussions on Dr. Förster, impedance plane theory, eddy current history, and electromagnetic testing concepts, visit: