Faraday's cage supposed to doesn't work for low frequency

[Marcelo P]

Dear all,
I’d like to ask your help concerning some doubts about electromagnetic waves, Faraday’s cage, and so on.
I read recently that an instrument with a transmitter that emits a low frequency electromagnetic field (around 15 to 22 Hz) located inside a metal pipeline could communicate with another one located outside the metal pipeline. It seems that electromagnetic field with that range of low frequency could pass through metal (steel) pipelines…
Unfortunately the text didn’t give any information about how does it work…So, it drives me confusing because I think about the Faraday’s cage and it couldn’t work… I don’t know how to explain that.
How could the electromagnetic field pass through metal pipeline even for low frequency? Could someone please help me?
Thank you very much!

 

[tech99]

Dear all,
I’d like to ask your help concerning some doubts about electromagnetic waves, Faraday’s cage, and so on.
I read recently that an instrument with a transmitter that emits a low frequency electromagnetic field (around 15 to 22 Hz) located inside a metal pipeline could communicate with another one located outside the metal pipeline. It seems that electromagnetic field with that range of low frequency could pass through metal (steel) pipelines…
Unfortunately the text didn’t give any information about how does it work…So, it drives me confusing because I think about the Faraday’s cage and it couldn’t work… I don’t know how to explain that.
How could the electromagnetic field pass through metal pipeline even for low frequency? Could someone please help me?
Thank you very much!

We would expect the electric field from the central wire to be stopped by the metal pipe. The magnetic field of the wire would be able to pass through a non magnetic metal pipe at low frequencies. This is because magnetic shielding using non magnetic conductors relies on eddy currents, which are more strongly induced at high frequencies. If the pipe is steel and therefore magnetic, it is possible that if leakage is occurring, the permeability of the steel is not high enough. However, I know that steel pipes are occasionally used to shield telephone wires from lightning ground currents, so I am surprised if it is not working. It seems more likely to me that radiation is occurring due to a ground loop, either intentional or unintentional.