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Circular magnet

  1. Aug 25, 2015 #1
    Hi Guys :)
    There were donut shaped ferrite magnets used in core memory of the 50s - 70s that stored bits by being electronically magnetised in a clockwise, or counter clockwise direction.

    If you had a larger donut shaped magnet that was magnetised in a clockwise or counter clockwise direction how does it have a north or south pole?

    Would there be any way to tell,
    say with metal or other magnets,
    which direction the donut shaped magnet was magnetised?
    Thanks, Art.
  2. jcsd
  3. Aug 25, 2015 #2


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    Staff: Mentor

    It does not.
    Take a magnet with known orientation, put it close to the ring, its alignment then depends on the orientation of the magnetization. Better: make an air gap in the magnet, measure the field there.
  4. Aug 25, 2015 #3


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    Staff: Mentor

    You can perform a "destructive read" of the core with a test coil, much like those memory bits were read with core memories. Then if necessary, you write it back to its original state.


  5. Aug 25, 2015 #4


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    Gold Member

    A toroid has no north- or south-pole, just a magnetic field inside. Neither has a solenoid core a north- or south-pole inside, just a magnetic field. Actually a pole is created when the magnetic field crosses a discontinuity in the magnetic permeability, say at the ends of a solenoid core, and because a toroid core has no ends/discontinuities, it has no poles.
    Well, you could wind some coil around the toroid and supply this coil with a sine-voltage. Due to the magnetizing curve, the dB/dH will be smaller when the direction of the current tries to magnitize the toroid even more. Thus the shape of the current will not be symmetric: The amplitude of the current will be larger in the small dB/dH direction due to a smaller self-induction in this direction.

  6. Aug 26, 2015 #5
    Hi Guys,
    Thanks for the replies :) but I’m polluting the thread by introducing a practical application.
    For core memory it’s square (rectangular loop) ferrite with hysteresis quality:
    but forget about that :D I was meaning to disqualify electronic means of reading the state,
    and pretend the viewpoint with the tools of an inquisitive a child with a larger donut shaped permanent magnet,
    and perhaps the same child would have other magnets, and other basic material, and simple mechanical skills.
    All of the magnets would have strong latency from whenever they were magnetised in the first place.

    mfb, your first answer, take another magnet, say a bar magnet, put it close to the ring... In what orientation?
    If you point the bar magnet north pole somewhere around the outer edge?
    If the ring has no north or south pole, how would the ring magnet ever repel or attract the north pole of the bar magnet?
    Cheers, Art.
  7. Aug 26, 2015 #6


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    Staff: Mentor

    The magnet has a stray field which is circular as well. It does not have poles, but if you are close to the ring it has a "north" and a "south" direction along the ring. Your magnet will try to align to get the same direction as the ring.
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