Someone did an application to detect metal using the Android G1. It’s a clever use of the built-in compass that detects the magnetic field of the Earth. It can serve as a handheld metal detector like those they use in airports to see if someone is hiding metal. The G1 is more smartypanties than other smartphones, but it’s not magic perhaps, it’s just a clever way of using the compass to detect the iron core of the earth’s magnetic field, which is what happens to the phone.”]
Source: https://gizmodo.com/app-converts-android-g1-into-airport-security-like-hand-5235681