207. Magnetic Induction.—The action of magnetic lines of force in magnetizing iron when they pass through it, is called Magnetic Induction. This may now be defined as the production of magnetism in a body by placing it within a magnetic field. Freely suspended magnets place themselves parallel to the lines of force in a magnetic field, therefore when an iron rod is placed in a weak field, or one with few lines of force, the iron is but slightly magnetized; that is, but few molecules are brought into line. Increasing the strength of the magnetizing field, gives stronger magnetization to the iron up to a certain point. After this, stronger fields give no increase in magnetizing effect. When iron exhibits its greatest magnetization it is said to be saturated.
Fig. 183.—Effect of a piece of iron in a magnetic field.
208. Permeability.—If a piece of iron is placed between the poles of a horseshoe magnet, the "field" obtained by sprinkling iron filings upon a sheet of paper over the magnet resembles that shown in Fig. 183. The lines in the space between the poles of the magnet seem to crowd in to the piece of iron. The property of the iron by which it tends to concentrate and increase the number of lines of force of a magnetic field is called permeability. Soft iron shows high permeability. Marked differences in behavior are shown by different kinds of iron and steel when placed in a magnetic field. Very pure iron, or soft iron, is strongly magnetized by a magnetic field of medium strength. Its magnetism, however, is quickly lost when the magnetizing field is removed. This indicates that soft-iron molecules are easily swung into line, but also disarrange themselves as easily when removed from a magnetizing force. Soft-iron magnets having high permeability quickly lose their magnetism. They are therefore called temporary magnets. On the other hand a hardened steel bar is difficult to magnetize, but when once magnetized retains its magnetism permanently, unless some action weakens the magnet. Such magnets are called permanent magnets.
Note.—The term "line of force" as used in this text means the same as "line of induction" as used in more advanced texts.
Important Topics
1. Molecular theory of magnetism, saturation, permeability.
2. Magnetic fields and lines of force.
3. Six facts concerning magnetic fields.