Phone magnetometer: how your phone senses magnetic fields, and what it can and cannot find

What does a phone magnetometer measure?

Android’s developer documentation calls it the geomagnetic field sensor, and says it lets apps monitor changes in the Earth’s magnetic field. It reports the field strength along three axes, x, y and z, in microtesla (μT).

Three axes matter because a magnetic field has a direction as well as a strength. Turn the phone and the share of the field along each axis changes, even though the field itself has not.

How strong is the Earth’s magnetic field?

The US National Centers for Environmental Information (NCEI), part of NOAA, says the Earth’s magnetic field intensity is roughly between 25,000 and 65,000 nanotesla (nT), which is 0.25 to 0.65 gauss. In the units a phone uses, that is about 25 to 65 μT, depending on where you are on the planet.

NCEI also explains that the field has a direction: declination is the angle between magnetic north and true north, and inclination is the angle between the horizontal and the field. So even in an empty room, your phone is always picking up a background field.

Why do magnets and steel change the reading?

A magnet has its own field, and close to the phone it can easily stand out against the Earth’s. Iron and steel also distort the field nearby. That is why the reading changes as you bring a fridge magnet, a steel tool or even a phone case clasp close to the sensor.

Android’s documentation mentions this too. Alongside the normal reading, it offers an uncalibrated version that leaves out “hard iron calibration”, and reports an estimate of the iron bias on each axis. Hard iron bias usually comes from magnetised parts close to the sensor, which can include parts of the phone itself. That explanation is ours, not Android’s, but it is why the normal reading has a correction applied.

Why does a magnetometer app need a baseline?

Because the background field is never zero, a game or visualiser usually starts by measuring what is normal where you are. Changes from that baseline are what show up as a magnet getting closer. Recalibrating when you move to a new spot, or after the phone has been near something magnetic, keeps the comparison fair.

Can a phone magnetometer find metal or treasure?

Not in the way people hope. It responds to magnetic fields, so it reacts to magnets and to iron and steel close to the phone. It is not a gold or treasure detector, and it only notices things near the phone.

Tips for a magnet hunt

• Set a fresh baseline before each round, with no magnets nearby.
• Keep the phone still while you hide the magnet. Turning the phone changes the reading on each axis.
• Take off magnetic cases and move away from steel desks or radiators if the readings look odd.
• Sweep slowly. Move the phone gradually and watch the reading rise as you get closer.

Questions people ask

Where Magnet Hunt helps, and where it does not

Magnet Hunt is our Android hide and seek game for magnets. One player looks away while the others hide a magnet; the seeker taps Ready to set a baseline, then sweeps the phone around and follows a hot and cold meter, clicks and vibration. It compares the field against the baseline in a way that ignores turning the phone, so only getting closer counts. A Test your magnet screen shows how far away your magnet shows up, and a field view shows the live reading. It works offline with no account.

Free: Classic Chase for 2 players, the field view and Test your magnet. Pro, a one time unlock: up to 8 players, the Timed Blitz, Blindfold and Triple Hide modes, a field graph, direction needle and XYZ axes, three more hunt sounds and four more looks. There is no subscription.

What it is good for: a fun way to see an invisible field, and a quick hide and seek game with a fridge magnet.

What it will not do: it is not a metal, gold or treasure detector. Iron, steel and magnets near the phone move the reading.

Questions or bugs: Email us.

Sources

1. Position sensorsAndroid Developers · developer.android.com
2. Geomagnetism frequently asked questionsNOAA National Centers for Environmental Information · ncei.noaa.gov