Residual Magnetism

—1. The magnetism retained by the core of an electromagnet after the circuit has been broken. When a ma of iron has once been magnetized, it becomes a difficult matter to entirely remove all traces when the magnetizing agent has been removed, and, as a general rule, a small amount of magnetism is permanently retained by the iron. This is known as residual magnetism, and it varies in amount with the quality of the iron. Well annealed, pure, wrought iron, as a rule, po e es very little residual magnetism, while, on the other hand, wrought iron, which contains a large percentage of impurities, or which has been subjected to some hardening proce , such as hammering, rolling, stamping, etc., and cast iron. po e a very large amount of residual magnetism. This property of residual magnetism in iron is of great importance in the working of the self-exciting dynamo, and is, indeed, the e ential principle of this cla of machines.; 2. The smail residue of magnetism retained Py soft iron after the removal ot the magnetizing orce. 3. The comparative large amount of magnetism held by a ma of hard steel, resulting in the so called permanent magnet.