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Re: [Phys-L] magnetic circuits ... was: change in inductance with iron core

This is another example of changing physics culture. Back in the 50’s, in addition to dc circuits, etc., we were taught magnetic circuits. I distinctly remember E.A. Wms saying cutting the iron was like cutting a wire in an e-circuit. (not quite, of course) So I was surprised by Carl's question. So much more to learn, so mag. circuitry dropped. [The Text was Harnwell “Principles of Electricity and Electromagnetism”

The Lab. was projects; The shop built a box Tightly fitting lid. I installed two 931 MPTs “looking" at a plastic scintillator W/PMMA light pipes (flat and curved machined, polymethyl silane gel, etc. W/6AK5’s distributed amps. (I think 100MHz BW ) fed a Ross dual triode coincidence. I inserted varying lengths of RG62-U cable to find the resolution.

My friend Ian made a clock using a quartz oscillator and jillions of tubes to divide down to one second. Did very well agains WWV. I don’t remember the other two or three others.

bc still remembers MMF, reluctance, etc.

On 2021/May/04, at 17:02, John Denker via Phys-l <> wrote:

On 5/4/21 3:36 PM, 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?

Interesting topic. Most people have very little intuition about
how magnetic circuits work. I know a guy who earned a living
as a consultant designing such things.