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Re: [Phys-L] change in inductance with iron core

-I followed up with a test, as promised, with an amusing result. I selected a hank of speaker wire which measured 407 microhenries in the air-cored condition.I inserted a nail (actually a 3/8 in bolt) in the core and noted the inductance indication dropped to 397 microhenries, and closing this magnetic circuit showed no change.
I measured the test frequency which was 250 kilohertz and realized any increase in permeability was being swamped by the eddy currents (so-called) in these bolts. Even a 60 hertz magnetic field is usually confined with thin insulated laminations, and  higher frequencies call for iron powder or ferrite cores. A much better way of improving inductance is the ferrite "pot" core which provides a central post and a completely enclosed magnetic path. The core splits in two to insert a coil, and its ends exit in side  ports.       Portable radios featured ferrite rods and such rods would do much better than the nails I proposedfor improving permeability!  <g>
On Tuesday, May 4, 2021, 05:50:21 PM CDT, Brian Whatcott <> wrote:

I'd guess you are providing an open (magnetic) circuit.I'll try this with three nails in contact, and let you know.Brian W

    On Tuesday, May 4, 2021, 05:36:52 PM CDT, Carl Mungan via Phys-l <> wrote: 

I’m wondering about the following. Some multimeters can measure inductance. I have tried with a few coils lying around and I find that with nothing (ie. air) in the core, I get one value of the inductance, say maybe 0.1 H for things like Pasco coils or even some larger coils. If I now insert iron rods or laminated iron bars into them, the inductance increases by about a factor of 10, maybe a bit more or maybe a bit less.

My question is: Since the relative permeability of iron (even at low applied fields) can easily be 10 000 or more, why am I not seeing substantially bigger increases in the inductance when I insert these iron cores?

Carl E. Mungan, Professor of Physics  410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363 ;

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