Some substances cause the magnetic lines of flux to get closer together than they are in the air. Some materials can cause the lines of flux to become farther apart than they are in the air. The first kind of material is ferromagnetic, and is of primary importance in magnetism. Ferromagnetic substances are the ones that can be "magnetized." Iron and nickel are examples. Various alloys are even more ferromagnetic than pure iron or pure nickel.
The other kind of material is called diamagnetic. Wax, dry wood, bismuth, and silver are substances that actually decrease the magnetic flux density. No diamagnetic material reduces the strength of a magnetic field by anywhere near the factor that ferromagnetic substances can increase it.
Permeability is measured on a scale relative to a vacuum, or free space. Free space is assigned permeability 1. If you have a coil of wire with an air core, and a current is forced through the wire, then the flux in the coil core is at a certain density, just about the same as it would be in a vacuum. Therefore, the permeability of pure air is about equal to 1. If you place an iron core in the coil, the flux density increases by a factor of about 60 to several thousand times. Therefore, the permeability of iron can range from 60 (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 as 1,000,000 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 as possible, you could use dry wood or wax for the core material. But usually, diamagnetic substances are used to keep magnetic objects apart, while minimizing the interaction between them.
Diamagnetic metals have the useful property that they conduct electric current very well, but magnetic current very poorly. They can be used for electrostatic shielding, a means of allowing magnetic fields to pass through while blocking electric fields.
Table 8-1. Permeability of some common materials.
|Aluminum||Slightly more than 1|
|Bismuth||Slightly less than 1|
|Silver||Slightly less than 1|
|Wax||Slightly less than 1|
|Wood, dry||Slightly less than 1v|
Certain 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 enclosing it in a wire coil carrying a massive current, there will be some residual magnetism left when the current stops flowing in the coil. Retentivity, also sometimes called remanence, is a measure of how well the substance will "memorize" the magnetism, and thereby become a permanent magnet.
Retentivity is expressed as a percentage. If the flux density in the material is x tesla or gauss when it is subjected to the greatest possible magnetomotive force, and then goes down 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 is enclosed by a coil carrying an electric current. Imagine that this is the maximum possible flux density that the rod can be forced to have. For any substance, there is always such a maximum; further increasing the current in the wire will not make the rod any more magnetic. Now suppose that the current is shut off, and 19 gauss remain in the rod. Then the retentivity, Br, is
Br = 100(19/135) = 100 # 0.14 = 14 percent
Various different substances have good retentivity; these are excellent for making permanent magnets. Other materials have poor retentivity. They might work well as electromagnets, 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 will operate from dc, so that it maintains a constant polarity, but that will lose its magnetism when the current is shut off.
If a ferromagnetic substance has poor retentivity, it's easy to make it work as the core for an ac electromagnet, because the polarity is easy to switch. If the retentivity is very high, the material is "sluggish" and will not work well for ac electromagnets.