| Substance | Energy dissipated (ergs) |
| Very soft annealed iron | 9,300 |
| Less soft annealed iron | 16,300 |
| Hard drawn steel wire | 60,000 |
| Annealed steel wire | 70,000 |
| Same steel glass hard | 76,000 |
| Piano steel wire annealed | 94,000 |
| Piano steel wire normal temper | 116,000 |
| Piano steel wire glass hard | 117,000 |
Approximately 28 foot pounds of energy are converted into heat in making a double reversal of strong magnetization in a cubic foot of iron.
Residual Magnetism.—When a mass 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, possesses 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 process, such as hammering, rolling, stamping, etc., and cast iron, possess a very large amount of residual magnetism.
Residual magnetism in iron is of great importance in the working of the self-exciting dynamo, and is, indeed, the essential principle of this class of machine.
That is, without residual magnetism in the field magnet core, the dynamo when started would not generate any current unless it received an initial excitation from an external source.
CHAPTER X
ELECTROMAGNETIC INDUCTION
The word induction, introduced by Faraday, has various meanings so far as it relates to electricity. It signifies, in general, phenomena produced in bodies by the influence of other bodies, having no necessary material connection with them.