Magnetic Compass

TM 55-1905-223-24-14

e. Manual Level Mode. The manual level position of the mode selector switch connects the output of the tilt/azimuth

switch to the input of the servoamplifier. Leveling is done in the manner described in the start mode. Leveling is

accomplished by offsetting the phantom fork and vertical ring in azimuth with respect to the rigid gyrosphere, thereby

forcing the stops on the vertical ring to deflect the springs attached to the gyrosphere. The pressure of the springs

creates a vertical torque which causes the gyro to precess about the horizontal axis. When the gyro is caged in this

mode and the tilt/azimuth switch is used to introduce a large tilt, the horizontal torque developed by the pressure of the

cager spring precesses the compass in azimuth. This is used to slew the compass to a desired heading when the gyro is

up to speed.

Automatic Level Mode. This mode is used to rapidly level the gyro while it is still running after being dumped or

heavily tilted. Tilt of the gyro axis is detected by an electrolytic level located on top of the vertical ring which

contains electrodes immersed in an electrolytic fluid. Leveling is accomplished by using the signal output of the

level to offset the phantom fork and apply a torque to the gyro in-the same manner as that described in the

manual level mode for the tilt/azimuth signal. The level signal is routed to the tilt indicating circuit to provide a

visual display of gyro tilt on the level meter. The automatic leveling system provides a course level only to within

4 to 5 minutes because the clearance between the stops on the vertical ring and the springs on the gyrosphere

allows a degree dead spot to exist with no torque developed. The manual level mode must be used for

precise leveling.

g. Transmission System. The gyro compass, primarily a direct reading compass, is also equipped with synchro

transmitters and step transmitters to transmit the compass heading indication to external repeaters.

1-9. Magnetic Compass. When a compass is mounted on a vessel, its magnets align themselves with the magnetic

field in which they exist. If there are no local influences (objects of magnetic material or electrical currents), this

alignment will be parallel to the horizontal component of the earth's magnetic field. The compass card will maintain this

alignment regardless of the vessel's heading.

a. Compass Card. As the compass card is attached to the magnets, the 0mark on the card always points in the

direction of magnetic north, and the vessel's compass heading is indicated by the alignment of the compass card and the

lubber's line. If there are no local disturbing influences and no deviation, then this is also the magnetic heading. When a

compass is installed, great care must be taken to align the lubber's line exactly parallel to the center line of the vessel.

The compass bowl and lubber's line are constrained to turn with the vessel; thus, the direction of the lubber's line from

the center of the compass always represents the direction of the vessel's head. Since the 0mark on the card is always

toward the magnetic north, the direction indicated on the compass card opposite the lubber's line is the vessel's heading.

As the vessel turns, the-lubber's line turns with it, while the compass card remains aligned with north, so that the heading

at any moment is indicated at the lubber's line. The lubber's line, not the compass card, turns.

1-35

Index