iron has high permeability but low retentivity; it is easily magnetized, but it loses
magnetism easily. Hard steel has low permeability but high retentivity; it is more difficult
to magnetize but retains magnetism for a long period.
2-13. MAGNETIC FIELDS
The region about a magnet in which its magnetic force is detectable is the
magnetic field. A magnetic field can be demonstrated by putting a piece of paper over a
bar magnet and sprinkling iron filings over the paper. If the paper is gently tapped, the
filings arrange themselves in a pattern (figure 2-7) following the magnetic lines of force
that make up the magnetic field. The concentration of lines of force determines the
strength of the magnetic field at that point. The strength of a magnetic field is always
greatest at the poles of the magnet.
Figure 2-7. Iron filings on a transparent paper show the
magnetic field around a magnet.
2-14. CHARACTERISTICS OF LINES OF FORCE
a. Lines of force are assumed to leave the north pole of a magnet and enter the
south pole of a magnet (figure 2-6). Each line of force leaving the magnet at the North
Pole returns to the magnet at the South Pole. Lines of force in a magnetic field repel
each other; therefore, the lines tend to spread out and away from the surface of the
magnet (figure 2-8A).
b. The tendency of magnetic materials to draw magnetic lines of force
themselves explains magnetic permeability. The more permeable a substance, the
more lines of force it will concentrate within itself (figure 2-8B) because it offers less
resistance to magnetic lines of force than the surrounding air.