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Ability to retain magnetism after removal of the magnetizing force.

Related Terms

HARD IRON

Iron or steel which is not readily magnetized by induction, but which retains a high percentage of the magnetism acquired

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.

BLUE MAGNETISM

The magnetism displayed by the south-seeking end of a freely suspended magnet. This i

MAGNETISM

The phenomena associated with magnetic fields and their effects upon magnetic materials, notably iron and steel. The magnetism of the north-seeking end of a freely suspended magnet is called red magnetism; the magnetism of the south-seeking end is called blue magnetism. Magnetism acquired by a piece of magnetic material while it is in a magnetic field is called induced magnetism. Permanent magnetism is retained for long periods without appreciable reduction, unless the magnet is subjected to a demagnetizing force. The magnetism in the intermediate iron of a ship which tends to change as the result of vibration, aging, or cruising in the same direction for a long period but does not alter immediately so as to be properly termed induced magnetism is called sub permanent magnetism. Magnetism which remains after removal of the magne- tizing force may be called residual magnetism. The magnetism of the earth is called terrestrial magnetism or geomagnetism.

MAGNETIC RETENTIVITY

The ability to retain magnetism after removal of the magnetizing force.

ELECTROMAGNETISM

1. Magnetism produced by an electric current. 2. The science dealing with the physical relations between electricity and magnetism.

GAUSSIN ERROR

Deviation of a magnetic compass due to transient magnetism caused by eddy currents set up by a changing number of lines of force through soft iron as the ship changes heading. Due to these eddy currents, the induced magnetism on a given heading does not arrive at its normal value until about 2 minutes after change to the heading. This error should not be confused with RETENTIVE ERROR.

PARAMETER

1. A quantity which remains constant within the limits of a given case or situation. 2. One of the components into which a craft’s magnetic field is assumed to be resolved for the purpose of compass adjustment. The field caused by permanent magnetism is resolved into orthogonal components or parameters: Parameter P, Parameter Q, and Parameter R. The field caused by induced magnetism is resolved into that magnetism induced in 9 imaginary soft iron bars or rods. With respect to the axis of a craft, these parameters lie in a fore-and-aft direction, an athwart ships direction, and in a vertical direction.

QUADRANTAL DEVIATION

Deviation which changes its sign (E or W) approximately each 90° change of heading. It is caused by induced magnetism in horizontal soft iron.

RETENTIVE ERROR

Deviation of a magnetic compass due to the tendency of a vessel’s structure to retain some of the induced magnetic effects for short periods of time. For example, a vessel on a northerly course for several days, especially if pounding in heavy seas, will tend to retain some fore-and-aft magnetism gained through induction. Although this effect is not large and generally decays within a few hours, it may cause incorrect observations or adjustments, if neglected. This error should not be confused with GAUSSIN ERROR.

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