Permeability

Chapter Permeability

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • PermeabilitySome substances cause the magnetic lines of flux to get closer together than they are inthe air. Some materials can cause the lines of flux to become farther apart than they arein the air.The first kind of material is ferromagnetic, and is of primary importance in mag-netism. Ferromagnetic substances are the ones that can be “magnetized.” Iron andnickel are examples. Various alloys are even more ferromagnetic than pure iron or purenickel.The other kind of material is called diamagnetic. Wax, dry wood, bismuth, and sil-ver are substances that actually decrease the magnetic flux density. No diamagneticmaterial reduces the strength of a magnetic field by anywhere near the factor that fer-romagnetic substances can increase it.Permeability is measured on a scale relative to a vacuum, or free space. Free spaceis assigned permeability 1. If you have a coil of wire with an air core, and a current isforced through the wire, then the flux in the coil core is at a certain density, just aboutthe same as it would be in a vacuum. Therefore, the permeability of pure air is aboutequal to 1. If you place an iron core in the coil, the flux density increases by a factor ofabout 60 to several thousand times. Therefore, the permeability of iron can range from60 (impure) to as much as 8,000 (highly refined).If you use certain permalloys as the core material in electromagnets, you can in-crease the flux density, and therefore the local strength of the field, by as much as1,000,000 times. Such substances thus have permeability as great as 1,000,000.If for some reason you feel compelled to make an electromagnet that is as weak aspossible, you could use dry wood or wax for the core material. But usually, diamagneticsubstances are used to keep magnetic objects apart, while minimizing the interactionbetween them.Diamagnetic metals have the useful property that they conduct electric currentvery well, but magnetic current very poorly. They can be used for electrostatic shield-ing, a means of allowing magnetic fields to pass through while blocking electric fields.Table 8-1 gives the permeability ratings for some common materials.RetentivityCertain ferromagnetic materials stay magnetized better than others. When a substance,such as iron, is subjected to a magnetic field as intense as it can handle, say by enclos-ing it in a wire coil carrying a massive current, there will be some residual magnetismleft when the current stops flowing in the coil. Retentivity, also sometimes called re-manence, is a measure of how well the substance will “memorize” the magnetism, andthereby become a permanent magnet.Retentivity is expressed as a percentage. If the flux density in the material is x teslaor gauss when it is subjected to the greatest possible magnetomotive force, and then goesdown to y tesla or gauss when the current is removed, the retentivity is equal to 100(y/x).As an example, suppose that a metal rod can be magnetized to 135 gauss when it isenclosed by a coil carrying an electric current. Imagine that this is the maximum possibleflux density that the rod can be forced to have. For any substance, there is always such a142 Magnetism