Knoxville journal. [volume] (Knoxville, Iowa) 1874-1986, 1875-08-19
Terrestrial Magnetism.

ia unxcrfeigrd steel needle - suspended
e,x- n resser Of gravity, or placed upon
,.asns = viii take a deierminsfe direction
s~x-s-.s ! rsasns of the horizon which Is
rsr. r,ea-sy north and south The force
wAsrA nrs.as,xeS this direction Is called
,.-~~s.-isa magnetism. Ii Is one OF the
.r,r.aes N manifestation OF the nafurai
s,urysS Of electricity. since magnetism
Ssseff Is esnly particular form OF electric
i.v The magnetic force CT our globe Is
r..sn;l.sss-s AZ it, surface by three Glasses
~ pherosnsnA. namely. the deeiinanon OF
he rnarnesized needle, is inclination,
az. the intensity with which the force
srts The deciination is the angle thai J.
formed with tire direction of the meridian
az The Mace by the direction OF the magnet
ixesi needle placed upon vertical pivot;
The inclination Is the angle that Is formed
with the horizon in the magnetic meridian
f,y the direction of magneilzed needle
sustained BIZ its center Of gravity, around
which ii i able TO turn freely In yerticai
plane. These three Observation-Disclosures
inclination and iniensity-not only vary
from one plsce to another, but in the same
nlsce with time They also manifest
irregular and accidental variations, deslg-
mated under the name of disturbances, the
existence Of which Is connected Win lhe
ureSence of some natural phenomena,
such in particular as that OF the aurora
borealis. Ii is well established that me
forces which act upon the niagnetIzed
needle emanate directly from the terres-
trial globe, and we are naturally led to re-
gard The earth as great magnet, and as
having one pole situated to the north OIL us,
attracting the north pole OF needle IN
that direction.
III we suspend magnetic needle By It's
center OF gravity, SO that 1t may move
freely either in vertical or horizontal
plane. the extremity which turns toward
the north will incline below the horizon,
making at New York an angle with the
horizon Of about 72 deg. Hence we con-
cludc that, II the earth be great magnet,
giving direction to the needle, Its pole
must be situated, not ON the north hor-
lion, but almost vertically beneath us.
If the earth is really magnet, the mag
netism of soft iron ought to be decom-
posed by it, in the same manner as is done
by bar magnet, and such is the fact II
bar Of soft iron is held in the direction
which magnetic needle assumes when
freely suspended, its lower endimmediate-
" becomes north pole and its upper
a small magnetic needle near each end OF
the bar. On inverting the bar it will be
found that its poles have Immediately
changed, the lower end being again a
north pole, and the upper one south
pole. If the bar is held horizontally,
pointing east and west, no such effect
takes place.
A similar but slightly diminished eff
feet il produced on bar of iron suspend
ed in a vertical position; and iron rods
which have remained long in vertical
position frequently acquire permanent
magnetism
When bar of iron is rendered mag
netic by the influence of terrestrial mag
netism, stroke of a hammer will some
times fix the magnetism, and the poles
will not be reversed when the bar is in
verted But if several blows with the
hammer be struck when in the inverted
position its magnetism may be destroyed
or its poles be reversed.
The action exerted by the earth upon
magnetic needle is simply to give direction
| 10 the needle, for the weight of a needle is
not increased by its magnetism. Hence it
is concluded that the attraction of the
| earth for one pole of the needle is exactly
equal 10 its repulsion for the other. II a
magnetic needle be placed upon cork
| floating on water, ii will soon adjust itself
to the magnetic meridian; but it has no
tendency to travel either toward the north
or south
Although magnetic needle, when fully
suspended, generally points nearly north
and south, II il found In almost all parts
Of the world that the north pole OF the
needle deviates few degrees from the as
| Economical meridian. This deviation is
called the magnetic declination. The de-
| clination is said to be east or west, accord
| ing as the north pole of the needle de
viatss TO the east or west of the true me-
| ridian. The declination of the needle is
| very different at different places on the
earth's surface. There are places where
the declination is 10 deg, 20 deg, 30 deg.
and even 80 deg. west and there are places
| where the deelination is as much to the
east AT most places on the earth's sur
| face, the dipping needle will not rest in a
horizontal line, one pole pointing down
ward and the other upward. This dip
varies at different places from 0 deg. to
| 70 deg, and observations 10 determine its
amount have been made in almost every
part OF the world. In order 10 represent
| all these observations conveniently upon
chart, line Is drawn connecting all those
places where the dip is the same. A line
connecting all those places where the nee.
die rests horizontally is called the magnet
ic equator This line exhibits numerous
sinuosities in It's course around the globe,
but does not depart much from great cir
Gle. IL crosses the terrestrial equator near
the western coast of Africa, attains its
greatest southern latitude in South Ameri-
ca, where 1t is Is deg south of the geo.
graphical equator, crosses the equator
again near the meridian OF New Zealand,
and attains north latitude of IL deg. near
the southern part of Hindostan.
As we travel northward from the mag.
netic equator, the north end of the needle
inclines downward and the dip continual
ly increases, at the rate of about deg. for
| deg. OF latitude, until we reach the north
magnetic pole, where the needle stands
vertically, In latitude 10 deg. min. N,
longitude 90 deg. As min. w.
As we travel southwark from the mag
netic equator, the south end of the needle
inclines downward, and this dip continu-
ally increases until we reach the south
magnetic pole.
That terrestrial magnetism is not pro.