Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Domestic wiring problems/magnetic fields

  1. Oct 23, 2011 #1
    We have magnetic fields (strong enough to spin a compass needle round 360 degrees) generated along the lengths of our household wiring. I haven't measured the strengths of these fields yet because I am trying to decide what to buy to do this. I presume that I need a gauss meter. I have been advised that ordinary electrical wiring carrying a conventional AC current should not generate a significant magnetic field. Can anybody offer an explanation for why our wiring is different.
  2. jcsd
  3. Oct 23, 2011 #2
    Have you tried shutting off the power...and then checking to see if the magnetic fluctuations are present or not?

    Check out magnetometers here:


    keep in mind that a very weak magnetic field is sufficient to rotate a compass....

    An ac current at 60 Hz is almost certainly too fast to move even a light (repsonsive) compass....likely too much inertia for it to bounce around much. And in addition, the black hot wire and white neutral wire ( "plus" and "minus") carry oppositely moving charges...and tend to cancel.

    Have you tried to test a neighbors house with the same compass? What "problem" do you observe?
  4. Oct 23, 2011 #3
    Thanks Naty1. I've shut down the household electricity supply at the fuse box and the magnetic fields are still present ie situated in line with the power supply cables to plug sockets and light switches. I've checked the wiring in two plug sockets and a light switch and it seems OK. The back boxes to these sockets/switch are not magnetic and do nothing to a compass needle, and when the power is off there is no current at the live, neutral or earth terminals of these sockets/switch.
    One of the sockets is used to supply a computer via a belkin surge protector. When the plug to the surge protector is disconnected from the wall socket we have been shocked if we touch the live and neutral plug pins. It has been suggested that this is discharge from charged capacitors in the surge protector. Does this indicate that the power supply to this particular plug socket is very uneven for the surge protector to be discharging in this way?
    I'll try using the compass in a different location to find out if there is something unusual about it.
  5. Oct 23, 2011 #4


    User Avatar

    Staff: Mentor

    May I ask what the point of your investigation is?
  6. Oct 23, 2011 #5


    User Avatar

    Could you describe the experiment in details?
    Where did you put the compass, and what did you to make it spin full turn?
  7. Oct 23, 2011 #6


    User Avatar
    Science Advisor
    Homework Helper

    It's possible that the charge was stored in the capacitors in the computer's power supply and has nothing to do with the surge protector.

    If I shut down my desktop computer, then disconnect if from the mains, and then switch it on again, there is enough residual charge in the power supply to turn the cooling fans etc for a short time.

    But with the UK 240V mains supply, I'm certainly not going to "test" anything by poking my fingers in the sockets!

    One possible explaination for what you describe is nothing to do with eletricity, but simply that there is some iron or steel supporting the internal structure of your house and affecting your compass when you go near it. It's quite possible to magnetize iron nails etc simply by hammering them into something, if they happen to be aligned in the right direction.
  8. Oct 23, 2011 #7


    User Avatar

    Bought any cheap wiring recently? Maybe made of steel wire instead of copper!?! Or maybe you are picking up metal parts in your stud-walls? 101 things possible here. Doesn't sound like an 'electro-magnetic' effect, though, more a DIY/home improvement issue.
  9. Oct 25, 2011 #8
    xts: Find a compass. Locate a supply wire to a light switch (ours travel up from the switch to the ceiling). Move the compass horizontally across the wall in front of the supply wire, and watch what the needle does. At intervals along the supply wires to our light switches and plug sockets the compass needle turns round 360 degrees. I think AlephZero has the answer ie the wires are probably fixed in the recesses made for them by metal supports or plastic supports held to the wall by steel nails. Its the nails or metal supports that have become magnetic and are causing the compass needle to move at intervals along the lengths of the supply wires.
  10. Oct 25, 2011 #9
    russ_watters: It wasn't so much an investigation as something I found when I was messing around with our compass. Having found these magnetic fields in the walls I was just trying to make sense of them.
  11. Oct 25, 2011 #10
    AlephZero: Our computer is one of the new Macs which doesn't seem to have a power supply unless its built into the computer. I've been using a different wall socket last night and today, and the surge protector plug pins don't stay charged when the plug is pulled out of the wall socket. I suppose therefore the shock problem is to do with the current in the wires that supply the socket I used first.

    I agree that the magnetic fields in the walls are to do with metal wire supports that hold the supply wires in the wall. Why would these become magnetic?
  12. Oct 25, 2011 #11
    cmb: Yes I think you are right.
  13. Oct 28, 2011 #12
    that's what I suspected....so as implied in posts since I was here last, the magnet spinning has nothing to do with 60 hertz household current.

    Does the magnetic spin one partial revolution or so and align in a stable direction?? That implies a permanent magnetic within your walls....
    or does it keep rotating endlessly...suggesting a varying magnetic field??

    This is normal: hot and neutral will normally shock this way...neutral is connected to ground at your electrical box so hot and neutral IS a complete circuit. )Ground is a saftey precaution wire unnecessary for normal operation of appliances.)

    not if I understand what you have written.

    Russ Watters is getting at the heart of the issue....exactly what are you observing?? In other words, defining the problem is the first step of a resolution.
    Last edited: Oct 28, 2011
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook