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I'm goofing around with the Android's API(not really important, I don't think). Anyway, I noticed in one particular corner of my room I'm getting a consistent reading of >100 mT on the phone's magnometer. It only seems to be occurring in this one object, but it has been consistently present for at least 2 weeks now. Everywhere else the readings seem typical, high near electronics, matching expected ambience elsewhere. Of note, the reading seems to be right next to or colocating a stainless steel book case. I've verified there are no plugged in electronics on this shelf. I've done my absolute best to isolate any potential "charging" of the metal shelf and to the best of my knowledge the shelf should be at ground.

I'm finding the readings to produce localized maxima near the meetings of the horizontal and vertical sections of the "book case", all maximum readings >150 mT, with localized minima at the vertical mid points. The orientations of the maxima do not seem to be aligned, though I have not fully mapped out their directions, only noting they're "not the same direction" and >150 mT.

At least by hand, I can detect no attraction to iron anywhere on the shelf itself. A comment indicated I should probably be detecting noticeable forces at these levels. The field attenuates to ambient at about ~50 cm, observationally the vertical separation of the shelves.

The horizontal shelves seem to be insulated from the vertical structure.

I'm curious. I've never seen or encountered something like this.

Questions; I don't think an EM standing wave could "magnetize" a metal shelf in such a way, but maybe?

As pointed out, this is a pretty high field that seems to have a span of the case, on the meter by meter scale. I'm struggling to come up with an explanation for what I'm seeing and really just want to better understand what/how could be causing this.

If necessary, I could provide an illustration better demonstrating my observations, but the description should be good enough.

Dale
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3 Answers3

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Wiring in walls/floors near the shelf could generate AC magnetic fields. Even if the shelf isn’t powered, nearby live wires (e.g., in outlets, lighting) might induce currents in the metal shelf, creating a localized field.

If you want to test this, turn off the power supply to the home and check the reading again.

Chetan patil
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100 mT is a lot. It might be some kind of interference rather than actual filed. First thing to do would be to verify this by another means - for example with a magnetic compass. It aligns in earth magnetic field which is about 0.1mT so 100 mT should have a very strong effect.

In fact such a field is strong enough to pull on or apply torque on everyday magnetic objects (most hand tools, screws, nails etc.). In labs that use superconducting magnets you typically mark so called "50 Gauss line" - border of area inside which the stray field of the magnet is above 5mT. This roughly coincides with where you start to feel the mechanical pull of the magnet on magnetic tools.

And yes, you can use the direction of the field to find the source. It is a reasonable bet that the source will look something like magnetic dipole - you can then use the known pattern of dipole field to narrow down the possible locations of the source. But it is usually easier to use just the magnitude - bigger means you are getting closer.

Karel
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The magnetometers in phones are typically not designed for "strong" fields. They need to see, if a manget comes close and they need to measure the much smaller earth magnetic field.

So relativly strong fields are usually way above the calibrated range. Numbers are not to be trusted.

That beeing said: Steel can become magnetic. For example: Non magnetic tools can become magnetic, when they are subjected to mechanical stress. Maybe the maxima are places where there were mechanical stresses, like screw points, dents from forming etc?

pyrochlor
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