I'm trying to locate good areas in my house to store magnetic media that is sensitive to magnetic fields. I found a spot that's ideal, but it's located about 18" from two electrical devices: a doorbell transformer (located inside a double light switch box) and a lamp containing an LED light bulb.

I downloaded a smartphone app that has a microtesla meter. Relatively speaking, it seems to work decently - if I place my phone near various objects like magnets and electronic devices, the meter will change from the baseline of about 50 microtesla (measured in the middle of a room, 6+ feet from any electrical devices or metal objects).

So I placed my phone near the LED light bulb, and there's really no change on the meter. But when I place it near the transformer, the meter jumps around rapidly from about 100 to 140 microtesla. As I move the phone further away, the microtesla reading continues to rapidly jump up and down, but the value is lower and lower. At 12" away, it's back to the baseline of a steady 50 microtesla.

Given this, I would like to assume that at 18" I have nothing to worry about as far as magnetic fields from either device. But is this a correct assumption to make, or are smartphone sensors only capable of detecting certain magnetic fields (e.g., AC only or DC only)? If that's the case: what is the best way to detect both fields so I can get an idea of the relative strength of a magnetic field (I don't need actual numbers) in a particular area of my house that doesn't involve purchasing expensive meters?

  • $\begingroup$ What kind of magnetic media are you trying to store? A media as sensitive as this seems to be doom to damage. What if after you find that perfect spot, and then, someone passing nearby receives or make a cell phone call?. Cell phones create a “considerable” amount of electromagnetic energy. Are you not just over killing the magnetic problem. $\endgroup$
    – J. Manuel
    Commented Nov 21, 2016 at 10:41
  • $\begingroup$ Definitely stay away from the doorbell transformer. $\endgroup$
    – Hot Licks
    Commented Feb 7, 2018 at 3:36

1 Answer 1


The Hall effect integrated circuit chips used in smartphones can sample data up to a maximum rate of 80 Hz or 100 Hz depending on which chip is used.

The wild fluctuations you which you observed was probably due to the sample rate being comparable with the frequency at which the magnetic field fluctuates.

Different magnetometer apps either set the date capture rate or allow the user to do so.
The "Physics Sensor Toolbox Kit" app for the iPhone allows the user to select the rate and to output the data on a spreadsheet. There are many such apps with "Sensor Kinetic Pro" which produces a graphical output and a clear indication of the data rate which is used.


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