The amount of secular change in the earth’s magnetic field which occurs in 1 year. magnetic annual variation; the small systematic temporal variation in the earth’s magnetic field which occurs after the trend for secular change has been removed from the average monthly values.



Related Terms

ISOGONIC CHART

A chart showing magnetic variation with isogonic lines and the annual rate of change in variation with isoporic lines.

EQUIVALENT ANNUAL RATE

A measure used in setting up a monthly rate on a comparable basis for each of the months regardless of their variation in working days, or for making the rate comparable with an annual rate regardless of the variation in working days during each month.

ELECTROMAGNET

A temporary magnet made of a ferromagnetic core wound with insulated wire through which is passed an electric current. The current flow through the windings causes a magnetic field to build that will attract magnetic material.

DEGAUSSING

Neutralization of the strength of the magnetic field of a vessel, using electric coils permanently installed in the vessel.

ARMATURE REACTION

The effect caused by the interaction of the magnetic field of the current carrying conductors in the armature with the main magnetic field. The two magnetic fields combined cause a distortion in the overall magnetic field that results in a shift in the neutral plane in the direction of armature rotation.

ANOMALISTIC MONTH

The average period of revolution of the moon from perigee to perigee, a period of 27 days, 13 hours, 18 minutes, and 33.2 seconds in 1900. The secular variation does not exceed a few hundredths of a second per century. anomalistic period. The interval between two successive passes of a satellite through perigee. Also called PERIGEE-TO-PERIGEE PERIOD RADIAL PERIOD. See also ORBITAL PERIOD.

ELASTIC HYSTERESIS

Phenomenon exhibited by some solids in which the deformation of the solid depends not only on the stress applied to the solid but also on the previous history of this stress; analogous to magnetic hysteresis, with magnetic field strength and magnetic induction replaced by stress and strain respectively.

VIBRATING NEEDLE

A magnetic needle used in compass adjustment to find the relative intensity of the horizontal components of the earth’s magnetic field and the magnetic field at the compass location. Also called HORIZONTAL FORCE INSTRUMENT.

GROUND MAGNETIC SURVEY

A determination of the magnetic field at the surface of the earth by means of ground-based instruments.

LOCAL MAGNETIC DISTURBANCE

An anomaly of the magnetic field of the earth, extending over a relatively small area, due to local magnetic influences. Also called LOCAL ATTRACTION, MAGNETIC ANOMALY.

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