Scrutable

A compass can reverse its polarity and point the opposite way from what you expect. It used to be rare, but now it is becoming common.

A few years ago, my wife ran a compass exercise for her guide troupe. Various of the guides kept on going off in completely wrong directions. It turned out she told them that the red end of the needle points north. But one of the compasses she had, the red end pointed south. I wasn't there, and this was not identified until after the debacle. I purchased that compass in New Zealand, so I simply assumed that was the reason. I said I never assumed the red end pointed north, maybe it depended on the brand. I always assess a compass before using it to know which end is north.

Today we were doing another navigation exercise with the guides, and I picked up one of our tried and tested red-is-north compasses, to show some guides how to orientate a map, and it pointed the wrong way. "Compass never lies," came a voice from another helper, in a "poor workman blames his tools" kind of way. But I picked up another compass and demonstrated to her they pointed in opposite directions.

I looked it up when I got home, and learned the sentence I started with. Dangerous accidents have occurred of late when people have taken compasses out of a bag in an emergency, and unable to realise they are pointing the wrong way, in fogs, at sea, they have gone in quite the opposite direction unawares.

And the most likely culprit is the mobile phone, though magnetic handbag clips and the like can also do the trick. A few days in a pocket in close proximity to a mobile phone, or magnetic bag clip, is enough to reverse the polarity of a compass.

Have you come across a compass reversing its polarity? Have you checked your compasses recently to see if they still point as you expect? You might be surprised. And avoid having them in extended proximity to your mobile phone or other magnetic device when transporting or using them.

A friend did have that taking some Explorers (including two of my kids) on a night hike over the Kinder Plateau.

He spotted the issue and did have a spare compass. And yes it probably was a mobile phone that did it.

How would a cell phone reverse a compass needle?

Is it RF heating of the needle leading to it remagnetizing with a random polarity?

dyqik wrote: ↑

Sun Apr 16, 2023 10:29 pm

How would a cell phone reverse a compass needle?

Is it RF heating of the needle leading to it remagnetizing with a random polarity?

It's arguably possible that a compass needle held close to a phone speaker could be degaussed (in that the rapidly alternating speaker magnetic field would be applied faster than the needle would have had time to align). Whether this would be able to 'precisely reverse' the NS alignment is moot.

Gfamily wrote: ↑

Sun Apr 16, 2023 10:53 pm

dyqik wrote: ↑

Sun Apr 16, 2023 10:29 pm

How would a cell phone reverse a compass needle?

Is it RF heating of the needle leading to it remagnetizing with a random polarity?

It's arguably possible that a compass needle held close to a phone speaker could be degaussed (in that the rapidly alternating speaker magnetic field would be applied faster than the needle would have had time to align). Whether this would be able to 'precisely reverse' the NS alignment is moot.

Surely phone speakers use piezoelectric crystals not mag coils?

VACCINES.

Grumble wrote: ↑

Sun Apr 16, 2023 11:02 pm

Surely phone speakers use piezoelectric crystals not mag coils?

Possibly, but my name's not ...

lpm wrote: ↑

Sun Apr 16, 2023 11:03 pm

VACCINES.

someone thinks it's down to the mRSA used

I blame it on Brexit.

To be serious again for a moment, it is much more likely to be due to a magnetic fastener than a phone. It could be that a phone has a case with a screen cover that sticks magnetically. My IPad has built-in magnets along the side for reasons, maybe some phones do as well. Maybe a rucksack has magnetic clips. I think the electronics in a phone including its antenna are too weak to permanently affect compass.

dyqik wrote: ↑

Sun Apr 16, 2023 10:29 pm

How would a cell phone reverse a compass needle?

Is it RF heating of the needle leading to it remagnetizing with a random polarity?

I suspect that "reverse" is inaccurate, but smartphones have a lot of circuitry carrying both AC and DC (power rails etc) and can draw enough power to get hot sometimes..

As an aside, when I was involved in designing fire detectors coming up to 2000, a lot of the EMI resilience standards were being reworked as the then-current generation of mobile phones meant that the expected field strengths were sometimes exceeded at certain frequencies.

jaap wrote: ↑

Mon Apr 17, 2023 6:54 am

To be serious again for a moment, it is much more likely to be due to a magnetic fastener than a phone. It could be that a phone has a case with a screen cover that sticks magnetically. My IPad has built-in magnets along the side for reasons, maybe some phones do as well. Maybe a rucksack has magnetic clips. I think the electronics in a phone including its antenna are too weak to permanently affect compass.

My simplistic view is that a compass needle would line up along a static magnetic field, so you wouldn't get any reversal effect.

Gfamily wrote: ↑

Mon Apr 17, 2023 7:05 am

jaap wrote: ↑

Mon Apr 17, 2023 6:54 am

To be serious again for a moment, it is much more likely to be due to a magnetic fastener than a phone. It could be that a phone has a case with a screen cover that sticks magnetically. My IPad has built-in magnets along the side for reasons, maybe some phones do as well. Maybe a rucksack has magnetic clips. I think the electronics in a phone including its antenna are too weak to permanently affect compass.

My simplistic view is that a compass needle would line up along a static magnetic field, so you wouldn't get any reversal effect.

Compass needles often get hung up is the compass isn't being held horizontal, and get stuck in a position.

But this is partly why I suggested RF heating - a compass needle is about the right size to act as a quarter wave dipole antenna at 2.4 GHz for Bluetooth or WiFi (12.5 mm wavelength, 31mm long), a bit longer for the 1.8 and 1.9 GHz cell phone bands. The needle is also pretty well thermally isolated - in a small air pocket, suspended on a point.

If you heat a magnet above its Curie point and then cool it randomly orientated in the Earth's magnetic field, half the time it'll end up reversed (and probably weaker). The Curie point of a iron needle is 770°C, but other materials might be much lower.

But I guess that doesn't make a lot of sense.

I remember having a phone case which clipped closed using magnets. Rendered the phone's internal compass completely useless.

It's also possible that I've had them strip on credit cards wiped by magnets on bags.

We had a famously magnetic corner post on the railings at a re-entrant corner on the school field. It buggered up generations of geography lessons on surveying. Each new generation of PE teachers geography teachers had to relearn that lesson anew.