Table of Inclination or Dip at London
Year. Inclination.
1576 71° 50'
1600 72°
1676 73° 30'
1723 74° 42'
1773 72° 19'
1780 72° 8'
1790 71° 33'
1800 70° 35'
1821 70° 31'
1828 69° 47'
1838 69° 17'
1854 68° 31'
1859 68° 21'
1874 67° 43'
1876 67° 39'
1878 67° 36'
1880 67° 35'
1881 67° 35'
III. Force or Intensity is the directive force of the earth. It varies
with the squares of the number of oscillations the magnetic needle will
make if caused to oscillate from a determined initial range. The
intensity is supposed to be subject to secular change. According to
Gauss the total magnetic intensity of the earth is equal to that which
would be exerted if in each cubic yard there were eight bar magnets,
each weighing one pound. This is, of course, a rough way of expressing
the degree of intensity. Intensity is least near the magnetic equator
and greatest near the magnetic poles; the places of maximum intensity
are termed the magnetic foci. It varies with the time of day and
possibly with changes in altitude.
Magnetic Elongation.
The elongation a bar of iron or steel undergoes when magnetized. By
magnetization it becomes a little longer and thinner, there being no
perceptible change in volume. The change is accompanied by a slight
sound--the magnetic tick. An exceedingly delicate adjustment of
apparatus is required for its observation.
Magnetic Equator.
A locus of the earth's surface where the magnet has no tendency to dip.
It is, approximately speaking, a line equally distant from the magnetic
poles, and is called also the aclinic line. It is not a great circle of
the earth.