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If I hold a strong neodymium magnet near my iPhone while the compass app is running, there is no effect whatsoever on the compass bearing. It doesn't matter where I hold the magnet in relation to the phone, nor which way the magnet itself is oriented.

It also doesn't matter if I hold the magnet near the phone before I start the compass app (I thought perhaps that the compass app might take an initial bearing and then use the phone's gyroscopes to calculate offsets from this).

How is this possible?

Clarification: the compass on the phone is working perfectly as a compass i.e. it indicates magnetic north correctly. So the compass is somehow indicating earth's magnetic field while not being susceptible to nearby strong magnet.

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    $\begingroup$ Is your magnet a simple dipole? or is it some kind of array magnet that is intended for sticking papers to a metal surface? If it's the latter, then there might not be much disturbance in the ambient field more than a centimeter or so from the surface of the magnet. $\endgroup$
    – Solomon Slow
    Commented Jul 19, 2019 at 15:30
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    $\begingroup$ P.S.; If the magnet has one face that sticks tight to a flat metal surface, but the opposite face is only weakly attracted or not attracted at all, then it probably is some kind of array magnet. $\endgroup$
    – Solomon Slow
    Commented Jul 19, 2019 at 16:15

3 Answers 3

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iPhone has a Hall effect/magnetometer, not a magnet (https://theoryandpractice.citizenscienceassociation.org/articles/10.5334/cstp.158/) - you can look up the model Alps Electric HSCDTD007

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  • $\begingroup$ Right but shouldn't this be susceptible to the magnetic field from a nearby magnet? $\endgroup$
    – Ronan Cremin
    Commented Jul 19, 2019 at 14:41
  • $\begingroup$ It is a bit difficult to find documentation but I am convinced that these sensors by Alps Electric use AMR (anisotropic magnetoresistance) and not the Hall effect. $\endgroup$
    – user137289
    Commented Jul 19, 2019 at 17:19
  • $\begingroup$ This would be better as a comment. It isn't an answer. $\endgroup$
    – user4552
    Commented Jul 19, 2019 at 17:44
  • $\begingroup$ @Pieter : A similar part is a Hall sensor (mouser.com/ProductDetail/ALPS/…) $\endgroup$
    – akhmeteli
    Commented Jul 20, 2019 at 1:07
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    $\begingroup$ @Pieter : So far we cannot find reliable info on the Alps sensor principle, but at least some iphone compasses are definitely Hall effect (AK8973). $\endgroup$
    – akhmeteli
    Commented Jul 20, 2019 at 10:52
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A phone electronic compass usually uses AMR (anisotropic magnetorestivity), four of those sensors in a Wheatstone bridge for each axis. Typically, they saturate at about 10 gauss (Earth's magnetic field is 0.5 gauss).

After exposed to a strong magnetic field, degaussing is necessary. This is usually triggered by moving the phone in a figure-8-pattern.

So try that first. Then approach carefully from a distance with a magnet.

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  • $\begingroup$ Just to be clear, the compass on the phone is working perfectly as a compass i.e. it indicates magnetic north correctly. So the compass is somehow indicating earth's magnetic fiield while not being susceptible to nearby strong magnet. $\endgroup$
    – Ronan Cremin
    Commented Jul 19, 2019 at 15:15
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    $\begingroup$ @RonanCremin Have you tested with a classic needle compass next to your phone? $\endgroup$
    – user137289
    Commented Jul 19, 2019 at 15:18
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    $\begingroup$ I once stacked a whole bunch of small neodymium magnets end-to-end to make a "bar" magnet about 10cm long, and I tried it on my phone. Turning the magnet caused the reading on my phone's compass to change even from a distance of something like four meters. $\endgroup$
    – Solomon Slow
    Commented Jul 19, 2019 at 16:23
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I dont think the compass inside the phone is running with the help of magnet. Instead they might have done the compass running with compass pointing towards the north,what ever might be the orientation of phones using some preinstruction or setting directions electrically.

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  • $\begingroup$ Phones use a kind of solid state magnetometer for their compasses. They should, I think, measure any magnetic field. $\endgroup$
    – Ronan Cremin
    Commented Jul 19, 2019 at 14:39
  • $\begingroup$ I dont think magnetometers and hall effect are distinct.They on a broad scale are one and the same. $\endgroup$
    – mechanics
    Commented Jul 19, 2019 at 14:56
  • $\begingroup$ @RonanCremin The magnetoresistive sensors cannot measure strong fields but they are better than Hall effect at low fields. $\endgroup$
    – user137289
    Commented Jul 19, 2019 at 15:15
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    $\begingroup$ Re, "...using some preinstruction or setting directions electrically." Those words don't explain anything. In order for a compass to know which way to point, there must be some physical phenomenon that the compass can measure. A magnetic compass senses the orientation of Earth's magnetic field lines. A GPS "compass" can't actually tell you which direction it points, but it can tell which direction it is moving based on very precise measurements of radio signals from satellites. etc. $\endgroup$
    – Solomon Slow
    Commented Jul 19, 2019 at 16:13

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