Someone left their cell phone here, it was ringing like crazy. I stuck it in a metal pot with a metal lid to shut it up, it still rang. I later put it in a safe, it still rang, but so muffled as to not be annoying. (Admittedly the safe has a gasket on the door--it provides fire, flood and walking out with the guests protection, it wouldn't defeat a serious burglar.)

How is the signal reaching the phone???

  • $\begingroup$ See the graph and the explanation at wikipedia's page on the skin effect. Use the graph in the example (grey line is aluminum) to estimate the attenuation due to a pot of thickness 2mm (rather thick). Also, put a camera inside and check how many bars the cell phone has. More, disassemble the safe and measure the thickness of the steel layer. $\endgroup$
    – Hector
    Commented Feb 16, 2015 at 5:00
  • $\begingroup$ I don't understand how skin depth is relevant--whatever the resistance of the metal aren't metals basically opaque to RF energy? $\endgroup$ Commented Feb 16, 2015 at 6:38
  • $\begingroup$ Extrapolating the graph a couple decades (to get about 800MHz), the copper δ is about 0.001mm. That is a millimeter of copper attenuates the signal about 10^-435. Therefore, the signal could not have passed through the pot. $\endgroup$
    – Hector
    Commented Feb 16, 2015 at 17:10
  • $\begingroup$ Put the phone in a microwave oven and close the door. These devices have pretty effective shielding so the phone should stop ringing. And if it does still ring, try giving it 800W for 20 seconds. That should stop it ringing and, for reasons I can't yet explain, seems to turn the phone's case into a permanent Faraday cage. $\endgroup$ Commented Aug 25, 2021 at 22:37

1 Answer 1


The pot may have been steel (not that good a conductor) and thin. The lid may not have fit well, leaving gaps. Cell phone signals are short wavelength, meaning a small gap will not completely block them. Cell phones are good at picking up weak signals.

Next time, just turn it off? Or answer it and tell the poor guy where his phone is?

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    $\begingroup$ main point, answer the damn thing. I always ring my phone if I misplace it, and persist, particularly in the garden. $\endgroup$
    – anna v
    Commented Feb 16, 2015 at 5:23
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    $\begingroup$ The answer is: "the lid may not have fit well." as an experimental physicist I put my cell phone in a stainless steel pot with a good lid (the one that allows for very low heat cooking). The phone rang. I put aluminum foil and then put the lid in its place, the phone was mute. $\endgroup$
    – anna v
    Commented Feb 16, 2015 at 5:30
  • $\begingroup$ We had told him where it was, he just couldn't get back for it immediately. And the keypad is damaged, I couldn't figure out how to turn it off. $\endgroup$ Commented Feb 16, 2015 at 6:34
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    $\begingroup$ @LorenPechtel Fine, you should have said so in your narrative. Anyway my test showed that it is the crack between pot and lid that allows for the EM signal to enter. It could be that in this case, small space between lid and pot, the lid could act as secondary antenna, with surface charges. The thin aluminum foil allowing for complete cover of the crack stops this. The foil was a square piece laid over the pot and extending all the way around , and then the lid was settled. $\endgroup$
    – anna v
    Commented Feb 16, 2015 at 7:11
  • $\begingroup$ Anna's experimental result is what I expect. A single sheet of Al foil wrapped around a mobile is enough to block the signal - a demo I use in lectures. Note that pans are often coated with a dielectric film that would allow the signal through even if the lid appears to fit well. $\endgroup$
    – ProfRob
    Commented Feb 16, 2015 at 9:02

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