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# Magnetic path length of the core for rod inductor?

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#### carpenter

##### Full Member level 6
if I have a ferrite rod with a diameter of 3 mm, length 20 mm.
On the rod is a coil with a length of 5 mm.
How is magnetic path length?

The term is only meaningful for closed magnetic pathes. What's the problem behind your question?

As I remember each magnetic path in closed. Path may be composed of ferrous material and non ferrous air part. Problem is how to calculate magnetic resistance in magnetic circuit carpenter has. I don't remember how.

Each magnetic circuit is closed, but otherwise we ended up at the same considerations.
I wanted to use it for measuring of magnetic permeability any as https://meettechniek.info/passive/magnetic-permeability.html

Describe the problm.

I have rod, possibly a mixture of iron and nickel.
It's ferromagnetic, in which I only know Curie temperature and I need to measure of magnetic permeability.
Using me available instrumentation :-D

The reluctance (magnetic resistance) of air is hundreds of times that of the ferrite, and thus will dominate -by a large margin- on a rod core.

The way I've seen it used, is to use plots or graphs created by the magnetic manufacturer, to estimate the equivalent it based on geometry.

Of course a good FEA software may be also used to calculate it.

With the air gap you're right, in the meantime, it also occurred to me (slow lines).
From the producers any support or data about the material can not wait, they are not public.
The magnetic permeability is materil constant and as such it can only be measured, unfortunately, I do not know how.
More precisely, I do not know how to measure it at the ROD core

You can't measure the permeability in an open path configuration without calculating the field geometry. But even if you have a rather accurate calculation, it will only give a very rough estimation of the permeabilty because most of the total reluctance is created in-air.

In a similar topology, I found that increasing core µr from 100 to 1000 raised the inductance only by 10%. There may be more sensitive differential measurement methods using multiple coils.

FvM's point is a good one.

The rod core topology, with its immense air gap, makes the equivalent inductance relatively immune to the core-material's permeability.

As a matter of fact, if I remember properly, fringing effects have a significantly higher effect than the core itself.

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