Angus' piece in the newsletter prompts me to suggest one specific cause: most modern smartphones carry, either by default or after being specifically enabled, a feature called NFC (Near Field Communication); this is what enables things like Apple Pay, Tap and Pay on buses, secure pairing of speakers and headsets etc.
This isn't the place for a detailed description of the technology, but the key thing to note is that it uses magnetic induction (like a transformer) rather than a "normal" radio field. The NFC field is very strong within a sphere of about 10 cm round the antenna, which is usually near the back of the phone (often it forms part of the battery), and falls off very quickly after that. In some NFC modes the phone "listens" for a signal, in others it is normally transmitting and looking for a receiving device.
In the second case, even a fairly brief direct contact between your phone and a weak magnet such as a compass would be enough to demagnetise the compass (I don't think it would normally reverse the polarity but the compass just wouldn't settle). The solution is to turn off NFC when you aren't using it (Settings - Wireless & network - More settings on an Android phone) and particularly when you are planning on putting it close to a weak magnet such as a compass or a hotel room key (bank cards are designed to resist this level of magnetic field).
Those passionate to learn the full details should read my book "Near Field Communications Technology and Applications" (Cambridge Univ Press, 2015)!
SusieAmann
Hi Mike
When you say 'pairing of speakers & headsets' do you mean Bluetooth? In other words should we turn bluetooth off when in proximity to compasses?
Could you also shed light on where to turn off on Android? Under 'more settings' I have options for plane mode, tethering & mobile hotspot, VPN, mobile networks and emergency broadcasts - none of these is obviously connected to what you refer to.
Thanks!
MikeHendry
Susie - several manufacturers, including Sony, have a method for quickly pairing devices by tapping them together; if you have to enter a code to pair using Bluetooth then that's not using NFC. But if you aren't using Bluetooth then it's a good idea to turn that off as well as it uses battery and is yet another source of radio interference.
If NFC isn't listed under More Settings then it probably hasn't been activated on your phone, so no need to worry ...
OwenDay
Good to know, thanks Mike!
MikeHendry
Ian - the reason they are different is that the NFC antenna is designed to maximise the magnetic component (like a transformer) while the transceiver maximises the electrical component of the electromagnetic wave. A transformer, even an incredibly powerful one, isn't a very good radio transmitter, nor is there a very strong magnetic field round a radio transmitter - it falls off very quickly. There could be some distortion of the magnetic field if you bring the two into contact, but I doubt it would be very serious and certainly not long-lasting.
But now imagine turning on an electromagnet (or just a strong magnet) and bringing the compass up to that - remember the school experiments with magnets and iron-filings? Reverse the polarity 13.56 million times a second and a weak magnet will be so confused it forgets where it started.