Compass Work
Magnetism of the Earth and the Ship’s Deviation
Theory of
magnetism as applied to ferromagnetic materials
Ferromagnetic materials are those in which each
molecule has a substantial magnetic moment. The molecular fields interact and
the crystalline structure of the materials is such that groups of molecules
become aligned over regions, which are called domains. If a bar of such
material is subjected to an impressed/inducing field, the domains tend to
realign themselves with the field.
The degree of alignment depends upon the structure of
the material and the strength of the inducing field. When the maximum alignment
has occurred the material is said to be magnetically saturated and further
increases in the inducing field will evoke no further contribution from the
molecular fields.
Ferromagnetism is a strong effect and permeabilities are much greater than 1.
Above a certain
temperature, thermal agitation of the molecules is sufficient to prevent the
formation of domains and ferromagnetic materials at normal temperatures may be
made to exhibit ferromagnetic properties if cooled sufficiently.
This is the temperature to which a Flinders bar and or
spherical bar and or spherical correctors are raised and then cooled slowly to
get rid of any magnetism induced in them.
Any piece of metal on becoming magnetized will develop
regions of concentrated magnetism called poles.
Any such magnet will have at least two poles of
opposite polarity.
Magnetic force (flux) lines connect one pole of such a
magnet with the other pole.
The number of such lines per unit area represents the
intensity of the magnetic field in that area.
If two such magnetic bars or magnets are placed close
to each other, the like poles will repel each other and the unlike poles will
attract each other. The force between magnetic poles being
directly proportional to the strength of the poles and inversely proportional
to the distance between them.
A unit pole is that which is associated with a
magnetic flux of 1 weber (f).
Magnetic moment (M) is the product of the pole
strength and the length of a magnet, thus,
M = 2 L f (Where L = half length of magnet)
Unit field strength is that which exerts a force of
one
The field stength (H) at a
point distant ‘d’ metres from a magnetic pole of
strength ‘f’ units is
given by:
H = (f / d2)
x 106 / 16
The field strength at a point end on to a short bar
magnet at a distance of ‘d’ from its centre is:
H = (2M / d3) x (106 / 16)
And the field strength at a point broadside on to a short bar magnet at a distance ‘d’ is:
H = (M / d3) x (106 / 16)
Thus we see the effect of a corrector magnet of
constant magnetic moment in a binnacle varies inversely as the cube of its
distance from the compass needles irrespective of whether the magnet is end on
or broadside on to the compass.
Magnetism can be either permanent or induced.
A bar having permanent magnetism will retain its
magnetism when it is removed from the magnetizing field.
Whether or not a bar will retain its magnetism on
removal from the magnetizing field will depend on:
· The
strength of that field,
· The degree
of hardness of the iron (retentivity), and also
· Upon the
amount of physical stress applied to the bar while in the magnetizing field.
The harder the iron, the more permanent will be the
magnetism acquired
Magnetic induction and differences between ‘hard’ and
‘soft’ iron
Soft Iron: A bar of ferromagnetic material placed in a
magnetic field becomes induced with magnetism. If the material is easily magnetised, but loses most of its magnetism when removed
from the inducing field, it is said to be magnetically soft. Such materials
usually, but not necessarily, have high have high permeabilities
and are mechanically soft.
Hard Iron: is a ferromagnetic material which is not
easily magnetised by an inducing field, but which
retains a substantial proportion of its magnetism when the inducing field is
removed. Such material usually, but not necessarily, has lower permeability and
is mechanically hard.
A uniformly magnetised bar
has a pole approximately one twelfth of the length from each end and gives rise
to a magnetic field.
Intensity
of magnetisation:
This is the Flux density established within a material
due to its own magnetism.
It is related to pole strength and magnetic moment in
uniform bar magnets.
Thus, the pole strength (_) is the total flux within a
magnet and the flux density is therefore this quantity divided by the cross
sectional area (A) of the magnet.
Permeability
Is the ratio between the induction and the strength of
the field in which the object lies or the number of lines of force per square
cm inside the object divided by the number of lines of force per square cm
outside the object.
It is also the ratio between the force that would be
exerted on a unit pole inside and the force that would be exerted on a unit
pole outside.
It is therefore the number of gauss produced by 1 oersted.
Susceptibility
Magnetic susceptibility: This is the ratio of the
intensity of magnetisation in a material (J) to the
flux density of an inducing field of strength.
Absolute susceptibility is sometimes used instead of
the relative susceptibility as defined above. It compares the intensity of
magnetization with the strength of the inducing field rather than with its flux
density.
Terrestrial Magnetism
Consider the earth as a huge magnet surrounded by
magnetic flux lines. connecting
its two magnetic poles.
These magnetic poles are near, but not coincidental
with, the earth’s geographic poles.
Since the north-seeking end of a compass needle is
conventionally called the North Pole, or positive pole, it must therefore be
attracted to a South Pole, or negative pole.
The flux lines enter the surface of the earth at
different angles to the horizontal, at different magnetic attitudes.
This
angle is called the angle of magnetic dip, ‘q’, and increases from 0°, at the
magnetic equator, to 90° at the magnetic poles.
The direction of the earth’s
total field T at any point lies in the plane of the magnetic meridian and is
inclined to the horizontal by the angle of dip.
The total magnetic field is generally considered as
having two components:
· H, the
horizontal component; and
· Z, the
vertical component.
These components change as the angle ‘q’, changes,
such that:
H is maximum at the magnetic
equator and decreases in the direction of either pole;
Z is zero at the magnetic equator and increases in the
direction of either pole
Magnetic variation
Since the magnetic poles of the earth do not coincide
with the geographic poles, a compass needle in line with the earth’s magnetic
field will not indicate true north, but magnetic north.
The angular difference between the true meridian
(great circle connecting the geographic poles) and the magnetic meridian
(direction of the lines of magnetic flux) is called VARIATION
This VARIATION has different values at different
locations on the earth.
These values of magnetic variation may be found on a
Variation Chart, on pilot charts, and, on the compass rose of navigational
charts.
The VARIATION for most given areas undergoes an annual
change, the Lines joining through places on a chart
having the same value of variation are called Isogonic Lines.
Lines drawn through places where the variation is zero
are called Agonic Lines.
A compass needle which is constrained to the
horizontal can respond only to the horizontal components H of the earth’s total
field and the field due to the ship’s magnetism
Here
is a representation of the earth’s total field T as resolved into horizontal H
and vertical Z components:
Ship’s Magnetism
A ship under construction or major repair will acquire
permanent magnetism due to hammering and vibration while being stationary in
the earth’s magnetic field.
After launching, the ship will lose some of this
original magnetism as a result of vibration and pounding in varying magnetic
fields, and will eventually reach a more or less stable magnetic condition.
The magnetism, which remains, is the permanent
magnetism of the ship.
The fact that a ship has permanent magnetism does not
mean that it cannot also acquire induced magnetism when placed in the earth’s
magnetic field.
The magnetism induced in any given piece of soft iron
is a function of:
The field intensity,
The alignment of the soft iron in that field, and
The physical properties and
dimensions of the iron.
This induced magnetism may add to, or subtract from,
the permanent magnetism already present in the ship, depending on how the ship
is aligned in the magnetic field.
The softer the iron, the more readily it will be
magnetized by the earth’s magnetic field, and the more readily it will give up
its magnetism when removed from that field.
The magnetism in the various structures of a ship,
which tends to change as a result of sailing, vibration, or ageing, but which
does not alter immediately so as to be properly termed induced magnetism, is
called sub- permanent magnetism.
This magnetism, at any instant, is part of the ship’s
permanent magnetism, and consequently must be corrected by permanent magnet
correctors.
It is the principal cause of deviation changes on a magnetic
compass.
Thus when we refer to permanent magnetism the
reference is to the apparent permanent magnetism, which includes the existing permanent, and sub-permanent magnetism. and
sub-permanent magnetism.
A ship, then, has a combination of permanent,
sub-permanent, and induced magnetism
Therefore, the ship’s apparent permanent magnetic
condition is subject to change from excessive shocks, welding, and vibration.
The ship’s induced magnetism will vary with the
earth’s magnetic field strength and with the alignment of the ship in that
field.
Magnetic Adjustment Magnetic Adjustment
A rod of soft iron, in a plane parallel to the earth’s
horizontal magnetic field, H, will have a north pole induced in the end toward
the north geographic pole and a south pole induced in the end toward the south
geographic pole.
This same rod in a horizontal plane, but at right
angles to the horizontal earth’s field, would have no magnetism induced in it,
because its alignment in the magnetic field is such that there will be no
tendency toward linear magnetisation, and the rod is
of negligible cross section.
Should the rod be aligned in some horizontal direction
between those headings which create maximum and zero induction, it would be
induced by an amount which is a function of the angle of alignment.
If a similar rod is placed in a vertical position in
northern latitudes so as to be aligned with the vertical earth’s field Z, it
will have a south pole induced at the upper end and a north pole induced at the
lower end.
These polarities of vertical induced magnetisation will be reversed in southern latitudes. The
amount of horizontal or vertical induction in such rods, or in ships whose
construction is equivalent to combinations of such rods, will vary with the
intensity of H and Z, heading and heel of the ship.
The magnetic compass must be corrected for the
vessel’s permanent and induced magnetism so that its operation approximates
that of a completely nonmagnetic vessel. Ship’s magnetic conditions create
magnetic compass deviations and sectors of sluggishness and unsteadiness.