magnitude and direction for several representative cities
straight, converging lines like the meridians on a globe,
in the Northern and Southern Hemispheres. As you look
but appear more like the isobar lines on a weather map.
at this table, you can see that the vertical component is
By convention, the positive external direction of the
positive in the Northern Hemisphere and negative in the
magnetic field of a bar magnet is from the north pole to
Southern Hemisphere. These component polarities
the south pole. However, lines of force for the earth's
occur because lines of force leave the earth in the
field leave the earth in the Southern Hemisphere and
Southern Hemisphere and reenter in the Northern
reenter in the Northern Hemisphere. For this reason,
Hemisphere. For this reason, the upward field, in the
you can think of the polar region in the Arctic as the
Southern Hemisphere, is assigned a negative value; and
north-geographic, south-magnetic pole. The Antarctic
the downward field, in the Northern Hemisphere, is
polar region is then the south-geographic,
assigned a positive value. There are two areas of
north-magnetic pole.
maximum vertical intensity but opposite polarity--the
north and south magnetic poles. The vertical intensity
Look at figure 10-1. Here, you can see that the
at the magnetic equator is zero since the entire field is
magnetic lines of force form closed loops, arching from
horizontal.
the earth's magnetic core to outer space, and then
reentering the earth in the opposite hemisphere. Since
The vector sum of the H and Z components defines
all lines of magnetic force return to their points of origin,
the magnitude and the direction of the total field at any
they form closed magnetic circuits. It is impossible to
point on the earth's surface.
eliminate the earth's field; however, the effect a ship has
on the earth's magnetic field may be lessened. The
purpose of degaussing is to prevent the ship from
THE SHIP'S MAGNETIC FIELD
distorting the earth's magnetic field. Some highly
The magnetic field of a ship is the vector sum of the
developed techniques are used in degaussing.
ship's permanent magnetic field and the ship's induced
The rest of this chapter explains the fundamentals
magnetic field. The ship's magnetic field may have any
of degaussing and describes the operating principles of
magnitude and be at any angle with respect to the
manual and automatic shipboad degaussing systems.
horizontal axis of the ship.
Learning these knowledge factors will help the EM
stand watch at the degaussing switchboard operate the
PERMANENT MAGNETIZATION
degaussing equipment, and maintain the installed
degaussing system.
The process of building a ship in the earth's
The magnitude and direction of the earth's magnetic
magnetic field develops a certain amount of permanent
field at any point may be resolved into components.
magnetism in the ship. The magnitude of the permanent
These components are the horizontal (H) component
magnetization depends on the earth's magnetic field at
and the vertical (Z) component. Since the earth is
the place where the ship was built, the material used to
spherical, an X and Y component would have little
construct the ship, and the orientation of the ship at the
meaning; therefore, X and Y are combined into one
time of building with respect to the earth's field.
component, the H component.
The ship's permanent magnetization is the source of
The angle of the field with the horizontal,
the ship's permanent magnetic field. This permanent
sometimes called the dip angle, may be easily
magnetic field can be resolved into two factors:
determined by a dip needle. A dip needle is a simple
1. The vertical permanent field component,
two-pivot compass needle held with the needle pivot
designated as Z
axis parallel to the earth's surface. Since a compass
needle always aligns itself parallel to the lines of force
2. The horizontal permanent field component,
of a magnetic field, the dip needle indicates the angle of
designated as H
the earth's field to the horizontal by aligning itself with
The horizontal permanent field component includes
the lines of force entering or leaving the earth at that
the longitudinal permanent field component and the
point. Both direction and strength of the field maybe
athwartship permanent field component. The vertical,
determined by a mine search coil and flux-measuring
longitudinal, and athwartship permanent field
equipment.
components are constant, except for slow changes with
time. They are not affected by continuous changes in
Table 10-1 shows horizontal and vertical
heading or magnetic latitude.
component magnitudes and the resulting total field
10-2
