by changing the position of small compensating
magnets in the compass case. However, it is
usually not possible to remove all the deviation
on all headings. You must determine the residual
deviation for each compass installation and record
it on a deviation card. The card shows the
actual deviation on various headings or, more
frequently, the compass headings for various
magnetic headings. You can accomplish this
using a process known as compass swinging.
COMPASS ERROR
The net result of both variation and deviation
is the compass error. If variation and deviation
have the same name (east or west), you add to
get compass error. If they have different names,
subtract the smaller from the larger. Give the
difference given the name of the larger. (See fig.
7-10. ) You can label variation and deviation plus
(+) if east, and minus () if west. In this case the
compass error is the algebraic sum of the two.
Figure 7-10.-Effect of compass error.
Example 1.
small angles near the equator to very large angles
near the poles. You can measure the angles with
a dip needle, which is a magnetic needle free to
turn about a horizontal axis. At San Francisco
the dip angle is about 62. A line connecting all
locations having equal dip angles is an isoclinic
line.
The total intensity of the magnetic field is
along the dip angle. However you can show it as
two components--vertical and horizontal. (See
Example 2.
Only the horizontal component is effective as
a directive force for a magnetic compass (wet
compass). It loses its effectiveness near the
magnetic poles because of the weak horizontal
component there. The vertical component causes
errors in a magnetic compass during aircraft
maneuvers that tilt the compass card east or west.
MAGNETIC DIP
If an aircraft heading east increases its speed, or
when heading west decreases its speed, the
compass card tilts. Also, when turning east from
At the magnetic poles, the direction of the
a north or south heading, the floating compass
earth's magnetic field is vertical (perpendicular
to the earth's surface). Along the aclinic line
card will tilt. In both cases, the east side of the
(sometimes called the magnetic equator), roughly
card sinks and the west side rises. The vertical
half way between the poles, the field's direction
component of the earth's field causes the
compass card to rotate to the east when in the
is horizontal (parallel to the earth's surface). The
northern hemisphere. It will cause the card to
difference between the direction of the earth's
rotate west when in the southern hemisphere. The
field and the horizontal at any location is the
magnetic dip. The magnetic dip varies from very
amount of error is zero at the aclinic line, and it
