=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. Exercises 1. Name an object whose usefulness depends upon its retentivity. Explain. 2. How do you explain the retentivity of hard steel? 3. Are the molecules of a piece of iron magnetized at all times? Explain. 4. When a piece of iron is magnetized by induction does any magnetism enter the iron from the magnet? Does the magnet lose as the iron gains magnetism? Explain. 5. Have all magnets been produced by induction? Explain. 6. Why will tapping a piece of iron when in a magnetic field increase the amount it will be magnetized? 7. Express in your own words the theory of magnetism. 8. Place two bar magnets in a line 5 cm. apart, _unlike_ poles adjacent; obtain the magnetic field with iron filings. Sketch it. 9. Repeat Exercise No. 8 using _like_ poles. Describe the appearance of a field that gives attraction; of a field that gives repulsion. (3) THE EARTH'S MAGNETISM