Permanent magnets

Chapter Permanent magnets

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

  • maximum; further increasing the current in the wire will not make the rod any more mag-netic. Now suppose that the current is shut off, and 19 gauss remain in the rod. Then theretentivity, Br, isBr = 100(19/135) = 100 0.14 = 14 percentVarious different substances have good retentivity; these are excellent for makingpermanent magnets. Other materials have poor retentivity. They might work well aselectromagnets, but not as permanent magnets.Sometimes it is desirable to have a substance with good ferromagnetic properties,but poor retentivity. This is the case when you want to have an electromagnet that willoperate from dc, so that it maintains a constant polarity, but that will lose its magnetismwhen the current is shut off.If a ferromagnetic substance has poor retentivity, it’s easy to make it work as thecore for an ac electromagnet, because the polarity is easy to switch. If the retentivity isvery high, the material is “sluggish” and will not work well for ac electromagnets.Permanent magnetsAny ferromagnetic material, or substance whose atoms can be permanently aligned,can be made into a permanent magnet. These are the magnets you probably playedwith as a child. Some alloys can be made into stronger magnets than others.One alloy that is especially suited to making strong magnets is alnico. This wordderives from the metals that comprise it: aluminum, nickel and cobalt. Other elementsare often added, including copper and titanium. But any piece of iron or steel can bemagnetized, at least to some extent. You might have used a screwdriver, for example,Permanent magnets143Table 8-1. Permeability of some common materials.SubstancePermeability (approx.)AluminumSlightly more than 1BismuthSlightly less than 1Cobalt60-70Ferrite100-3000Free space1Iron60-100Iron, refined3000-8000Nickel50-60Permalloy3000-30,000SilverSlightly less than 1Steel300-600Super permalloys100,000-1,000,000WaxSlightly less than 1Wood, drySlightly less than 1