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I Induced EMF in a half vs. whole ferrite toroid

  1. Apr 14, 2017 #1
    Hello all -

    I've stumbled upon what seems to be a pretty basic magnetic materials question that's got me stumped. Maybe one of you can help me figure this out.

    I did the following experiment (see figure below):
    • Put a ferrite toroid around a current carrying wire carrying 12 Amps RMS at 120V, 60 Hz.
    • Toroid has a wire coil around it that's wrapped around a small angular section of the toroid
    • Measured 90 mV RMS potential difference between the coil terminals on the voltmeter, which is about what I expected based on what I learned about this problem in college physics
    • Cut the toroid in half, and put the half-toroid with the coil around it right next to the current carrying wire, but in this case measured nearly 0 mV RMS potential difference between the coil terminals
    Can anyone explain why I measured nearly zero mV when the toroid was cut in half? Even though the toroid is cut in half, it's still made of ferromagnetic material, so I would have expected the field strength to be boosted inside the half-toroid (by a factor of α, where α is the ferrite's relative permeability), and therefore would have expected to see a much higher induced voltage in the coil. I would have expected to see more like 50 mV RMS in the coil in this case, but not zero.

    Thanks in advance for any insight you can provide!!


    Last edited: Apr 14, 2017
  2. jcsd
  3. Apr 15, 2017 #2


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    do some reading up of current transformers and how they work
    here is a link or two to start you off



    they will explain why you cannot have a broken or only a section od a core

    Last edited: Apr 15, 2017
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