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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?

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There are 2 types of magnetic fields coming from phone:

  1. DC, from smartphone circuitry
  2. AC, from chip transmission (for each chip, if they are different,e.g Wi-Fi, Bluetooth, GSM. You will see different radiation pattern)

It's better to say that voltage induced due to emission of your smartphone, electronics emission is DC. When you put screw you disturb system balance, so it takes some time to stabilize.

Also, your smartphone emits every 30 seconds small id burst to BTS generating a magnetic field in its location, this time it passes an alternating current through the screw. Same goes for transmission, you will see huge fluctuations of EM field.

This kind of sensors used primary for navigation. DC or "static" lines of magnetic flux naturally emanate from the earth and provide a sensor with the ability to indicate "Magnetic North". There is standard calculation model and Google used it too.

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  • $\begingroup$ It might be interesting to read this question about NFC too, since it also uses magnetic fields physics.stackexchange.com/questions/44037/… $\endgroup$ – sigrlami Dec 10 '13 at 10:51
  • $\begingroup$ 1. I am a newbie, could you explain, how the screw is disturbing the system balance, because if the system emission is just DC, then I am not sure if it can induce voltage in a non-moving object that can fluctuate the magnetic sensor. Also, how is the system stabilizing again? 2. Whenever BTS is emitted from the phone, Should one see a drastic change in the mag sensor reading? If so, I am not detecting any. If not why? $\endgroup$ – BaluRaman Dec 10 '13 at 11:54

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