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. _A body charged with electricity causes or “induces” charges on neighboring bodies._ The process in this case is called _electrostatic induction_. A magnet induces magnetism in neighboring masses of iron or other magnetic materials by the process of _magnetic induction_. Again, a moving magnet induces electric currents in neighboring conductors by the process of _electromagnetic induction_. =Faraday’s Discovery.=--All dynamos of whatever form, are based upon the discovery made by Faraday[11] in 1831, which may be stated as follows: _Electric currents are generated in conductors by moving them in a magnetic field, so as to cut magnetic lines of force._ =Ques. What does the expression “cut lines of force” mean?= Ans. A conductor, forming part of an electric circuit, _cuts lines of force_ when it moves across a magnetic field in such manner as to _alter_ the number of magnetic lines of force which are embraced by the circuit. It is important to clearly understand the meaning of this expression, which will be later explained in more detail. [Illustration: FIG. 126.--Faraday’s dynamo which embodies his discovery in 1831 of _electromagnetic induction_, the principle upon which all dynamos work, as well as induction coils, transformers, and other electrical apparatus.]