Lines of Force
To further describe and work with magnet phenomena, lines are used to represent the force existing in the area surrounding a magnet (refer to fig. 1-14). These lines, called MAGNETIC LINES OF FORCE, do not actually exist but are imaginary lines used to illustrate and describe the pattern of the magnetic field. The magnetic lines of force are assumed to emanate from the north pole of a magnet, pass through surrounding space, and enter the south pole. The lines of force then travel inside the magnet from the south pole to the north pole, thus completing a closed loop.
When two magnetic poles are brought close together, the mutual attraction or repulsion of the poles produces a more complicated pattern than that of a single magnet. These magnetic lines of force can be plotted by placing a compass at various points throughout the magnetic field, or they can be roughly illustrated by the use of iron filings as before. A diagram of magnetic poles placed close together is shown in figure 1-15.
Although magnetic lines of force are imaginary, a simplified version of many magnetic phenomena can be explained by assuming the magnetic lines to have certain real properties. The lines of force can be compared to rubber bands which stretch outward when a force is exerted upon them and contract when the force is removed. The characteristics of magnetic lines of force can be described as follows:
MAGNETIC FLUX. The total number of magnetic lines of force leaving or entering the pole of a magnet is called MAGNETIC FLUX. The number of flux lines per unit area is known as FLUX DENSITY.
FIELD INTENSITY. The intensity of a magnetic field is directly
related to the magnetic force exerted by the field.
It has been previously stated that all substances that are attracted
by a magnet are capable of becoming magnetized. The fact that a material is attracted by a
magnet indicates the material must itself be a magnet at the time of attraction.
If another nail is brought in contact with the end of the first nail, it would be magnetized by induction. This process could be repeated until the strength of the magnetic flux weakens as distance from the bar magnet increases. However, as soon as the first iron nail is pulled away from the bar magnet, all the nails will fall. The reason being that each nail becomes a temporary magnet, and as soon as the magnetizing force is removed, their domains once again assume a random distribution.
Magnetic induction will always produce a pole polarity on the
material being magnetized opposite that of the adjacent pole of the magnetizing force. It
is sometimes possible to bring a weak north pole of a magnet near a strong magnet north
pole and note attraction between the poles. The weak magnet, when placed within the
magnetic field of the strong magnet, has its magnetic polarity reversed by the field of
the stronger magnet. Therefore, it is attracted to the opposite pole. For this reason, you
must keep a very weak magnet, such as a compass needle, away from a strong magnet.