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Permeability μ measured at very low fields at the

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permeability μ 0 , measured at very low fields at the toe of the hysteresis loop. It is a familiar phenomenon that some ferromagnetic materials when magnetized by an external field do not return to a completely unmagnetized state when removed from that field. In fact, these materials must be subjected to a reversed field of a certain intensity to demagnetize them. Other ways to demagnetize are to heat the material to a characteristic temperature called the curie point (above which ferromagnetic ordering of atomic moments is thermally destroyed) or to work the material mechanically to reduce the magnetization. If an external field varied in a controlled manner is applied to a completely demagnetized (virgin) specimen and if the magnetic induction in the specimen is measured, the magnetization curve of the material may be determined. Figure 17 shows a representative hysteresis loop for a ferromagnetic material. As shown in Fig. 17a, starting at the origin O with the specimen in the unmagnetized condition and increasing the magnetizing force H in small increments, the flux density B in the material increases quite rapidly at first and then more slowly until it reaches a point beyond which any increase in the magnetic field intensity does not increase the flux density. This is shown by the dashed curve OA. In this condition the specimen is said to be magnetically saturated. When the magnetic field intensity is gradually reduced to zero, the curve AB results (Fig. 17b). The amount of magnetism that the steel retains at point B is called residual magnetism or remanence and is represented by B r . When the magnetizing current is reversed and gradually increased in magnitude, the flux continues to diminish. The flux does not become zero until point C is reached, at which time the magnetic field intensity is represented by OC (see Fig. 17c), which graphically 337 Electromagnetic Techniques for Material Identification P ART 4. Hysteresis Loop Characteristics 1 F IGURE 15. Magnetization or BH curve showing relation between flux density B and magnetic field intensity H in ferromagnetic materials. The intensity of magnetization at saturation is I s . Flux density B (relative scale) I s Magnetic field intensity H (relative scale) F IGURE 16. Variation of magnetic permeability with flux density. Magnetic permeability μ (relative scale) Flux density B (relative scale) μ max μ 0
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designates the coercive force H c in the material. As the reversed field is increased beyond C, point D is reached (Fig. 17d). At this point, the specimen is again saturated but in the opposite polarity. The magnetic field intensity is now decreased to zero and the DE line is formed and retains reversed polarity residual magnetism B r in the specimen. Again increasing the magnetic field intensity in the original direction completes the curve EFA. Now the cycle is complete and the hysteresis curve (ABCDEFA) is called the hysteresis loop . In alternating current applications, the ferromagnetic material goes through this cycle for every reversal
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  • Fall '19
  • Magnetism, Magnetic Field, Electrical conductivity

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