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Experimental measurements of relative magnetic permeability

  1. Jul 17, 2013 #1
    I am trying to measure the relative permeability of a few materials, but the numbers I'm getting don't quite make sense. Maybe someone here can figure out what might be going wrong.

    My setup is as follows. I'm making solenoids using 30 AWG magnet wire wrapped around ferrite, steel and wood cores. I'm doing a single layer of wire and I get about 3600 turns per meter, however the solenoids themselves are about 1"L x 1/4"D. I have a hall effect sensor, the Allegro A1324 which has a sensitivity of 5mv/G. I'm placing the end of the solenoid directly against the sensor and measuring the voltage deviation with respect to current in the solenoid.

    With the equation B=kμnI where n=N/L~=3600 I'm solving for the relative permeability k. So k=B/(μnI). At 2 amps I get readings of 201, 170, and 15 G for ferrite, steel, and wood respectively. Then solving for the relative permeability I get 2.2, 1.9 and 0.17. Wood should have something close to 1, so I know something is wrong. And the steel/ferrite should be at least 200.

    Any ideas? Thanks
  2. jcsd
  3. Jul 19, 2013 #2


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    Staff: Mentor

    Welcome to Physics Forums. :smile:

    Quite possibly your ampere-turns per meter was excessive, and the magnetic materials were driven into saturation. Repeat using much smaller currents.
  4. Jul 19, 2013 #3
    What value do you get if there is no core in the solenoid?
  5. Jul 19, 2013 #4
    Thanks for your reply. I'm in the linear region, I've verified experimentally. I'm not sure where saturation will occur, but the relationship between B and I is very linear from 0-2A for these solenoids. Also, 3600 turns per meter is a relatively small value as far as I'm aware.

    @ Nasu, I used the wood to mimic an air core. The wire is flimsy and would not hold the shape on its own. Wood and air should both be very close to 1.

    I did find another equation for non-ideal solenoids where the length and radius are taken into account. The results did not improve much.
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