TMC

Transmitting Magnetic Compass


Related Terms

OEHMAN'S SURVEY INSTRUMENT

A drillhole surveying apparatus that makes a photographic record of the compass and clinometer readings.

KEYHOLE SAW

A fine compass saw with a blade 1116 inches (2841 centimeters) long.

HORIZONTAL PENDULUM

A pendulum that moves in a horizontal plane, such as a compass diaphragm and the air load.

GYRO ERROR

The error in the reading of the gyrocompass, expressed in degrees east or west to indicate the direction in which the axis of the compass is offset from true north.

HALVING

The process of adjusting magnetic compass correctors so as to remove half of the deviation on the opposite cardinal or adjacent intercardinal headings to those on which adjustment was originally made when all deviation was removed. This is done to equalize the error on opposite headings.

GYROCOMPASS

A compass having one or more gyroscopes as the directive element, and which is northseeking. Its operation depends upon four natural phenomena, namely gyroscopic inertia, gyroscopic precession, the earth's rotation, and gravity. When such a compass controls remote indicators, called GYRO REPEATERS, it is called a master gyrocompass.

HEELING ERROR

The change in the deviation of a magnetic compass when a craft heels, due to the change i

INDUCED MAGNETISM

The magnetism acquired by soft iron while it is in a magnetic field. Soft iron will lose its induced magnetism when it is removed from a magnetic field. The strength and polarity of the induced magnetism will alter immediately as its magnetic latitude, or its orientation in a magnetic field, is changed. The induced mag- netism has an immediate effect upon the magnetic compass as the magnetic latitude or heading of a craft changes.

LUBBER'S LINE

The vertical line inside a compass case indicating the direction of the ship's head.

INVERTED COMPASS

A marine magnetic compass designed and installed for observation from below the compass card. Frequently used as a telltale compass. Also called HANGING COMPASS, OVERHEAD COMPASS.

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