Your phone has sensor of changing magnetic field so it can be used as precise metal detector for checking if the object is made from iron steel gold or other type of metal! Application works perfectly even with older phone as almost every device with Android has magnetic field sensor. In some devices the sensor is located on the bottom of your phone so please check amount of µT (micro Testla) by touching objects with the bottom of your device.

The magnetic field level in nature is around 49 µT. If any metal is near the value of magnetic field will increase and you will see those values on the graph in the app.