Magnetic Declination.
The angular deviation of the magnetic needle, causing it to rest at an
angle with the true meridian; the variation of the compass. (See
Magnetic Elements.)
Magnetic Density.
The intensity of magnetization expressed in lines of force per stated
area of cross-section in a plane at right angles to the lines of force.
Magnetic Dip.
The inclination from the horizontal assumed by a magnetic needle free to
move in the vertical plane. (See Magnetic Elements.) The angle of dip or
inclination is entirely a function of the earth, not of the needle.
Magnetic Discontinuity.
A break or gap in a magnetic circuit. To make a complete circuit the
iron or other core must be continuous. If the armature of a horseshoe
magnet is in contact with both poles the continuity is complete. If the
armature is not in contact magnetic continuity gives place to
discontinuity. It is an attribute of a paramagnetic substance only, and
is identical for permanent magnets, or for electro-magnets.
Magnetic Elements.
The qualities of the terrestrial magnetism at any place as expressed in
its action upon the magnetic needle. Three data are involved.
I. The Declination or Variation.
II. The Inclination or Dip.
III. The Force or Intensity.
I. The Declination is the variation expressed in angular degrees of the
magnetic needle from the true north and south, or is the angle which the
plane of the magnetic meridian makes with that of the geographical
meridian. It is expressed as east or west variation according to the
position of the north pole; east when the north pole of the needle is to
the east of the true meridian, and vice versa. Declination is different
for different places; it is at present west in Europe and Africa, and
east in Asia and the greater part of North and South America. The
declination is subject to (a) secular, (b) annual and (c) diurnal
variations. These are classed as regular; others due to magnetic storms
are transitory and are classed as irregular, (a) Secular variations. The
following table shows the secular variations during some three hundred
years at Paris. These changes are termed secular, because they require
centuries for their completion.