Retentivity

Some of the ferromagnetic materials stay magnetized better than others. When the substance, like iron, is subjected to the magnetic field as intense as it can handle, say by enclosing it in a wire coil which is carrying a massive current, there may exist some residual magnetism left when the current stops flowing in the coil. Retentivity, also called as remanence, is a measure of how well the substance will memorize the magnetism, and become a permanent magnet.

Retentivity can be expressed as a percentage.  If the flux density in material is x tesla or gauss when it is subjected to greatest possible magnetomotive force, and then it goes down to y tesla or gauss when the current is removed, retentivity is nearly equal to 100(y/x).

As an instance, assume that a metal rod can be magnetized to 135 gauss when it is enclosed by a coil which is carrying an electric current. Assume that this is the maximum possible flux density which the rod can be forced to have. For any of the substance, there is always such a

Table Permeability of some common materials.

maximum; by further increasing the current in wire will not make the rod any more magnetic. Suppose that current is shut off, and 19 gauss still remain in the rod. Then the retentivity, Br, can be given by

Different substances have good retentivity; these are perfect for making permanent magnets. Other materials have poor or bad retentivity. They can work fine as electromagnets, but not as permanent magnets.

At times it is desirable to have a substance having good ferromagnetic properties, but poor retentivity. These are the cases when you want to possess an electromagnet which will operate from direct current, such that it maintains a constant polarity, but which will lose its magnetism when the current is shut off.

If a ferromagnetic substance has poor retentivity, it is easy to make it work as the core for an alternating current electromagnet, as the polarity is easy to switch. If the retentivity is quite high, the material is sluggish and will not work well for alternating current electromagnets.