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 Newton on a pole of strength 1 weber. The unit of field strength is the ampere per metre. Normally field strength is denoted by ‘H’ but for magnetism ‘H’ is reserved for the horizontal componenet of the Earth’s field.

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.


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.


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 earths 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.