Use as a metal detector: pass the Android over a ferrous metal object and observe the needle behaviour.
For example, to test the app, hold your phone near a loudspeaker. Some metals, like Aluminium and Stainless Steel, are not magnetic.
The accuracy of the meter is determined by phone hardware.
Q: What do the speeds mean?
A: Fast, medium, slow refer to how quickly the meter needle moves in response to a change in the magnetic field.
Q: What is "zeroed"?
A: Zeroed and unzeroed are for looking at the difference between the value when the button was pressed and now ... For example, if you are in unzeroed mode, and the meter reads 50, then you press the button the meter will now read zero, effectively subtracting 50 from the reading.
Q: What are "coarse" and "fine"?
A: Coarse and fine refer to the meter scale ... One reads from zero to 50 microTesla, the other to 500 microTesla. The Earth''s magnetic field is around 30 microTesla.