FWD FWD N S Geographic pole Magnetic pole Figure 10-78.  The earth and its magnetic field.  Figure 10-77.  Total air temperature (TAT) probes.  diaphragm expands and contracts to adjust the volume of  the space inside the case so it remains full.  [Figure 10-79]  The entire assembly is enclosed in a sealed case that is filled  with a liquid similar to kerosene. This dampens vibration and  oscillation of the moving float assembly and decreases friction. On the front of the case, a glass face allows the numerical  compass card to be referenced against a vertical lubber line.  The magnetic heading of the aircraft is read by noting the  graduation on which the lubber line falls. Thus, direction  in any of 360° can be read off the dial as the magnetic float  compass card assembly holds its alignment with magnetic  north, while the aircraft changes direction.  There are accuracy issues associated with using a magnetic  compass. The main magnets of a compass align not only  with the earth’s magnetic field, they actually align with the  composite field made up of all magnetic influences around  them, meaning local electromagnetic influence from metallic  structures near the compass and operation of the aircraft’s  electrical system. This is called magnetic deviation. It causes  a magnet’s alignment with the earth’s magnetic field to be  altered. Compensating screws are turned, which move small  permanent magnets in the compass case to correct for this  The liquid that fills the compass case expands and contracts  as altitude changes and temperature fluctuates. A bellows  N 33 30 27 N-S E-W Float Filler hole Outer case Compensating magnet Instrument lamp Bellows expansion unit Lubber line Compass card Lens Sensing magnet Compensating screws Jewel post Jewel spring Jewel Pivot Figure 10-79.  The parts of a typical magnetic compass.  10-44