Although catalog values or the maximum permeability of head materials such as 4-79 mo-permalloy are about 300 000 µ0., the practical values for initial permeability at 1000 Hz for thin laminations bonded together into a head core are more nearly 5000 µ0 to 10 000 µ0. The initial permeability of a toroid or a "gapless" head may be calculated from physical dimensions and an inductance bridge measurement. The Maxwell bridge with a variable capacitance works especially well.
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