I wrote a simple android app that shows the magnetic field(using the magnetic field sensor in the phone) in the current location.
When I place a two inch steel screw on the top of the phone (which is running the app), there is a change in magnetic field for a while. I'm guessing that it's due to the induced voltage in the screw due to the EM radiation, flowing in my room. However, after a few seconds, the magnetic field stops to change and stabilizes to a value. I am not sure why.
My doubt is, since EM is generally alternating radio waves, shouldn't it induce an alternating voltage in the screw and result in the magnetic field changing continuously?
If the induced voltage is too small, then why did the magnetic field reading change for a while?