It is a simple metal sniffer app which uses magnetic field sensor.
so you need a smart phone with built in emf magnetic sensor.
Smart phone metal detector in your pocket!
You can use this app anywhere.

* How to Use:
Just move it around electromagnetic fields.
If the result value exceed a 200 μT(microtesla), the phone will vibrate.
That means this app detect nearby ferromagnetic materials.

Note:
Some phones will give different sensor readings based on the hardware used by the manufacturer.