Retentivity:
Certain ferromagnetic materials remain magnetized better than others. Whenever a substance like iron is subjected to a magnetic field as intense as it can handle, say, by surrounding it in a wire coil carrying a high current, there will be few residual magnetism left whenever the current stops flowing in the coil. Retentivity, sometimes also termed as remanence, is a measure of how better a substance can "memorize" a magnetic field imposed on it and thereby become a permanent magnet.
Retentivity is stated as a percentage. When the maximum possible flux density in a material is x teslas or gauss and then goes down to y teslas or gauss whenever the current is eliminated, the retentivity Br of that material is given by the formula shown below:
Br = 100y/x
What is meant by greatest possible flux density in the foregoing definition? This is a shrewd question. In real world, whenever you make an electromagnet with a core material, there is a boundary to the flux density that can be produced in that core. Since the current in the coil rises, the flux density within the core goes up in proportion-for awhile. Beyond a certain point, though, the flux density level off and further rises in current do not generate any further increase in the flux density. This situation is known as core saturation. Whenever we determine retentivity for a material, we are referring to the ratio of flux density whenever it is saturated and the flux density whenever there is no magneto-motive force acting on it.
As an illustration, assume that a metal rod can be magnetized to 135 G whenever it is enclosed by a coil carrying an electric current. Visualize that this is the maximum possible flux density which the rod can be forced to have. For any substance, there is always a maximum; further raising the current in the wire will not make the rod any more magnetic. Now assume that the current is shut off and that 19 G stay in the rod. Then the retentivity Br is
Br =100 x 19/135 = 100 x 0.14 = 14 percent
Certain ferromagnetic substances have fine retentivity and are excellent for making permanent magnets. The other ferromagnetic materials have poor retentivity. They can work better as the cores of electromagnets, though they do not make good permanent magnets. At times it is desirable to have a substance with excellent ferromagnetic properties though poor retentivity. This is the situation whenever you want to have an electromagnet which will operate from dc therefore it keeps a constant polarity though that will lose its magnetism whenever the current is shut off.
When a ferromagnetic substance has bad retentivity, it is easy to make it work as the core for an ac electromagnet since the polarity is easy to switch. Though, when the retentivity is very high, the material is "magnetically sluggish" and has trouble obeying the current reversals in the coil. This kind of stuff does not function better as the core of an ac electromagnet.