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If you place a piece of ferromagnetic material such as iron, nickel, or steel inside a coil carrying a large electric current, the number of flux lines per unit cross-sectional area, a factor known as the magnetic flux density, can become extremely large. When that happens, you get a magnetic field so strong that you must apply tremendous force to break free from it.

Some specialized metal alloys make the flux density hundreds or even thou-sands of times greater than it is in free space, so their permeability values range in the hundreds or in the thousands—vastly larger than 1 (μ >> 1). Some materials don’t compress the lines of flux very much; they have permeability values only a little larger than 1 (μ > 1). A few materials can slightly dilate the flux lines com-pared to their density in free space, so they have permeability values a little bit smaller than 1 (μ < 1). Table MAG2-1 lists some common substances and their approximate permeability values.