The gradual variation in the magnetic element which occurs over a period of years. 
The gradual variation in the magnetic element which occurs over a period of years. 
Related Terms 

ISOMAGNETICOf or pertaining to lines connecting points of equality in some magnetic element 
SHIELDA metal housing around an electrical or magnetic element to eliminate or reduce the effect of its electric or magnetic field, or to reduce the effect of an exterior field on the element. 
ISOPORIC CHARTA chart with lines connecting points of equal annual rate of change of any magnetic element 
MAGNETIC ELEMENT

DIRECTIVE FORCEThe force tending to cause the directive element of a compass to line up with the reference direction. Also, the value of this force. Of a magnetic compass, it is the intensity of the horizon tal componen 
CRYOTRONA switch that operates at very low temperatures at which its components are superconducting; when current is sent through a control element to produce a magnetic field, a gate element changes from a superconductive zeroresistance state to its normal resistive state. 
ANOMALISTIC MONTHThe 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 PERIGEETOPERIGEE PERIOD RADIAL PERIOD. See also ORBITAL PERIOD. 
ISOPLETH

ANOMALY

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