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Re: Test your magnetic compass.
From: John Huth
Date: 2012 Nov 18, 07:21 -0500

4 amps at a meter in an infinite wire is about 8 milliGauss - from my naive calculation - someone can check my math. That's at roughly the 1% level of the earth's field.

For iron and ferromagnetic materials, the permeability ranges from 100 to a few thousand, or larger, depending on various factors. That's a big multiplier.

I think I'd hunt down pieces of iron and the like as culprits before going after ground currents now that I did that calculation.

Still, when I'm inside a building, I have to exercise some care with a compass to keep away from walls, computer keyboards, etc.

On Sat, Nov 17, 2012 at 3:13 PM, Bill Morris <engineer@clear.net.nz> wrote:

My original query of 16 November is close to being answered:
"Can some physicist or power engineer explain how an alternating current can produce a constant magnetic field? "

We postulate leakage currents being rectified at, say, corroded joints and the direct current in the ground or transmission towers setting up a constant magnetic field. Short circuit currents are apparently in the order of thousands of amps, but probably 400 amps is a more usual line current (does anyone know?).
Assuming a 1 percent leakage, a 4 amp ground current could easily occur. We can imagine this concentrated in a water pipe and having an effect on a compass, but at what distance, I wonder?
We have only 33 kV lines near where I live and it has just rained, so I might just take a compass and go in search of one to see if I can detect an effect.

Bill Morris
Pukenui
New Zealand
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