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Albert Poon 3/10/91 022T SP2 Types of Magnets Generally, the two broad classes of magnets are permanent magnets and electromagnets. An electromagnet is a type of magnets needs currents from an external source to maintain its magnetic field. The current in a wire conductor create as associated magnetic field. And these current and its magnetic field become concentrated in a smaller space. As the result, it create a strong magnetic field. In other words, more current and more turns made a stronger magnetic field. One of the most common application of electromagnets magnetic tape recording. The tape is coated with fine particles of iron oxide. The recording head is a coil that produces a magnetic field in proportion to the current. As it passes through the air gap of the head, the small areas of the coating become magnetized by induction. When you played back the tape, the moving of the magnetic tape produces variation in electric current. Permanent magnets are made of hard magnetic material, cabolt steel. It magnetized by induction in the manufacturing process. A strong field is needed for induction in these material. When they removed the magnetizing field. The residual induction makes the material a permanent magnet. One of the disadvantage of the permanent magnet is when it above the curie temperature, the molecular structure can not be rearrange. The resulting in loss of magnetism that is not recovered after cooling. For instance, when iron is above 800'C, it will loss its ferromagnetic properties. Magnetic Materials. 1 Ferromagnetic materials, like iron, steel, nickel and cobalt. These materials become strongly magnetized in the same direction as the magnetizing field, and also with a high permeability from 50 to 5000. 2 Paramagnetic materials, for example, aluminum and manganese. The permeability is slightly more than 1, they become weakly magnetized in the same direction as the magnetizing field. 3 Diamagnetic materials. These includes copper, zinc, gold and silver. The permeability is less than 1, and they become weakly magnetized , but in the opposition direction from the magnetizing field. 4 Ferrites is a nonmetallic material that have the ferromagnetic properties. Which contains high permeability. But it made of ceramic material. The permeability is from 50 to 3000.