All ships that are to be fitted with a shipboard
the ship is south of the magnetic equator. Hence, the
vertical induced magnetic field changes with magnetic
degaussing installation and some ships that do not
require degaussing installations, are depermed.
latitude and to some extent, when the ship rolls or
Deperming is essentially a large-scale version of
pitches. The vertical induced magnetic field does not
demagnetizing a watch. The purpose is to reduce
change with heading because a change of heading does
not change the orientation of the ship with respect to the
permanent magnetization and bring all ships of the same
vertical component of the earth's magnetic field.
class into a standard condition so the permanent
magnetization, which remains after deperming, is about
The longitudinal induced magnetic field changes
the same.
when either the magnetic latitude or the heading changes
and when the ship pitches. If a ship is heading in a
INDUCED MAGNETIZATION
northerly geographical direction, the horizontal
component of the earth's magnetic field induces a north
After a ship is built, its existence in the earth's
pole in the bow and a south pole in the stem (fig. 10-2,
magnetic field causes a certain amount of magnetism to
view A). In other words, the horizontal component of
be induced into it. The ship's induced magnetization
the earth's field induces a longitudinal or fore-and aft
depends on the strength of the earth's magnetic field and
component of magnetization. The stronger the
on the heading of the ship with respect to the inducing
horizontal component of the earth's magnetic field, the
(earth's) field.
greater the longitudinal component of magnetization. If
the ship starts at the south magnetic pole and steams
Like the ship's permanent magnetization, the ship's
toward the north magnetic pole, the magnitude of the
induced magnetization is the source of the ship's
longitudinal component of induced magnetization starts
magnetic field. This induced magnetic field can be
at zero at the south magnetic pole, increases to a
resolved into the following components:
maximum at the magnetic equator, and decreases to zero
1. The vertical induced field component.
at the north magnetic pole. Hence, for a constant
heading, the longitudinal component of induced
2. The horizontal induced field component. The
magnetization changes magnitude as the ship moves to
horizontal induced field component also
a different latitude.
includes the longitudinal induced field
component and the athwartship induced field
If, at a given magnetic latitude, the ship changes
component.
heading from north to east, the longitudinal component
The magnitude of the vertical induced
of the induced magnetic field changes from a maximum
magnetization depends on the magnetic latitude. The
on the north heading to zero on the east heading. When
vertical induced magnetic field is maximum at the
the ship changes heading from east to south, the
magnetic poles and zero at the magnetic equator. The
longitudinal component increases from zero on the east
vertical induced magnetization is directed down when
heading to a maximum on the south heading. On
the ship is north of the magnetic equator and up when
southerly headings, a north pole is induced at the stem
Figure 10-2.--Effect of the earth's magnetic field upon a ship.
10-4
