By using data from the phone internal compass,
This app can measure the magnetic field generated from metals
Or high electromagnetic emission from High Voltage line ,Transformer ,
Electric motors,and some home appliance such as microwave oven.
The units of measure is Tesla -µT : Nikola Tesla (10-Jul-1856 - 07-Jan-1943)
inventor of the Tesla coil and developer of inductor motor working by alternate current.
The normal earth emissions is around 20-40 µT.
High magnetic field can reach 200 µT (e.g. near to speakers).
In very high magnetic field (e.g. near electric motors ) it can be over 300 µT
such high magnetic field ,can easy damage credit ,mechanical watch .
If the phone has been exposed to e high magnetic field , the sensor need to be recalibrate by just shake the phone until the data reader become normal range.
A alarm can be set by the slider, over that value the value become red and the phone will vibrate.
So: stay away from strong magnetic field : find it and avoid it.