MAGNETIC COMPASS

MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.

Magnetic Compass

Learning Objectives

Basic Theory of the Magnetic Compass

Understand the use and care of ship's compass.

Describe the construction of a liquid card magnetic compass.

State the composition of the liquid used.

Explain how allowance is made for changes in volume of the liquid in

a liquid card compass.

Describe the marking of the lubber line and it's purpose.

Describe the Binnacle of the compass.

Explain why regular comparisons between the Standard compass,

Steering compass and the Gyrocompass is necessary.

Demonstrate taking bearings of celestial bodies and landmarks.

List the care and precautions for a Magnetic compass.

Magnetism of the Earth and the Ship’s Deviation.

Theory of Magnetism.

The theory sates that all magnetic substances consists of magnetic molecules each being a minute magnet. When a substance is unmagnetised these minute magnets are not arranged in any particular direction. In fact it can be proved that they prefer this arrangement rather than to be lined up in a particular direction.

Once these minute magnets are aligned, the mutual attraction of their poles tend to hold them in position after the removal of the external force used to align them. This alignment can be destroyed by physical vibration such as hammering or by heating. The ease with which it can be destroyed depends on whether the substance is magnetically hard or magnetically soft. In the case of ferrous material the terms hard iron and soft iron refer to this particular property.

In pure soft iron the molecules are entirely free and unless under some external magnetic field the iron will be unmagnetised. This is not the case of hard iron. The molecules are not free to move nor are they easily moved. But once lined up in a particular direction they tend to remain in that direction indefinitely. In this case the iron is said to be magnetised permanently (permanent magnets). Permanent magnets apart from the effects of vibration or heating are not truly permanent, but tend gradually to lose their magnetism during the course of time. Magnets made of magnetically hard substances are normally referred to as permanent magnets.

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 known as Curie point, 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.

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 permeabilities and are mechanically soft.

A line drawn through the magnet in the direction of its internal field (joining it poles) is known as the magnetic axis of the magnet. A line at right angles to the magnetic axis midway between the poles is called the neutral axis of the magnet. Magnetic poles exert a force upon one another. Like poles repel and unlike poles attract one another. The force between two poles is dependent upon their distance. The strength of the north and south poles of a magnet are equal.

Magnetic Field of the Earth

The earth as a magnet is obvious from the fact that a freely suspended magnet will come to rest in a direction approximately north and south. In other words the magnet will settle in a direction of the earths field at the place at which the magnet is being used.

It would appear that the earths magnetic field is similar to that of a bar magnet. As a first approximation this is substantially correct. The general magnetic field of the earth is similar to that which could be expected at the surface of a short but strongly magnetised bar magnet were located at the centre.

The above partly explains the fact that the magnetic poles are relatively large areas, due to the spreading out of the lines of force from the magnet. It also gives the reason for the direction of the field being horizontal in the vicinity of the equator. It is most probable that there is such a magnet at the centre of the earth. In actual fact many scientists are investigating the cause of the field. No theory put forward up to the present time has found acceptance.

As far a we are concerned the idea of a magnet at the centre of the earth is useful as it helps us to visualise the general form of the magnetic field, as it is known to be despite the many imperfections. The area termed the North Magnetic Pole is situated approximately 71⁰N, 96⁰W. The South Magnetic Pole is situated in 73⁰S, 156⁰E. These positions are very approximate, but one fact emerges namely, that the South Pole is not diametrically apposite to the North Pole.

Theoretically the maximum strength of the earth’s magnetic field should be at the poles. Actually the field strength in certain other areas in both high north and south latitudes is found to exceed that at the magnetic poles. These are called magnetic foci. In order to determine the direction and force of the earth’s magnetism at any place we require three of four magnetic elements. The four elements are variation, dip, horizontal force and vertical force.

Magnetic Pole

Is the region of a magnetic that exhibits magnetic properties from which the greater part of the magnetic flux emerges or at which it enters. In the case of a bar magnet the longer the bar in comparison with its thickness the more nearly do the poles approach the ends of the magnet.

Magnetic Equator

A line joining all positions on the earth’s surface where the direction of the magnetic field is horizontal is called the magnetic equator.

Angle of Dip

The vertical angle contained between the horizontal and the direction of the earth’s magnetic field at any given place is called the Angle of Dip. Dip is conventionally considered positive when the north end of a freely suspended magnetised needle dips below the horizontal, and negative when the south end dips below the horizontal.  Thus all angles of dip north of the equator will be positive and all angles of dip south of the magnetic equator will be negative.

REGULATIONS (Concerning Safety of Navigation)

In Singapore context, the Merchant Shipping (Safety Convention) Regulations apply to all Singapore flagged vessels engaged in international voyages.

The following regulation refers to navigational aids.

Regulation 12 is quite lengthy and Ws entire interpretation is beyond the scope of this module, however it's salient features concerning Magnetic Compass is being reproduced below.

Ships of 500 GRT and upward need to be equipped with .........

** A Standard Magnetic Compass with a reflector for the use of the helmsman. If without the reflector, than another compass for steering.

COMPASSES (An Introduction)

A Compass is an instrument designed to seek a certain direction (preferably North, in shipboard applications) and to hold that direction permanently.

Magnetic compasses depend for it's directional properties on the magnetism of the earth. Their role in present day navigation is substantially reduced, but because of the compass independence from power failures, it continues to remain an essential element in the ship's overall navigational equipment. In fact, it is legally required to be carried, (Remember Reg. 12 - Shipborne Navigational Equipment) and it's error checked and logged.

Magnetic compasses suffer from:

Magnetic Variation

Magnetic Deviation

But with careful maintenance, service, correction and care, the instrument can be a good back up during emergencies.

The re-entry of Transmitting Magnetic Compass (TMC) and Flux Gate Magnetic Compass is likely to re-kindle interest in the Magnetic Compasses. Understanding the functioning of a TMC and a flux-gate compass is beyond the scope of this module but another type of compass namely the Gyrocompass would be discussed in the next module.

THE MAGNETIC COMPASS

A Magnetic compass is usually fitted on the upper bridge, (also known as the monkey island), more or less on the centre line of the ship. This is referred to as a Standard Compass because it is a primary means of indicating direction on a ship.

There are two (2) basic types:

(a) The dry card Compass

(b) The wet card Compass

The basic compass (whether dry or wet) consists of a card with cardinal graduations, suspended inside a bowl. The suspension should provide a frictionless support.

The directive element in these types of compasses consists of needle magnets attached to the card. Modern compasses use ring magnet as a directive element.

The compass card is enclosed in a cylindrical brass bowl having a transparent top glass. The top glass is retained in position by a brass "verge ring", which is secured to the brass bowl by brass screws along the circumference. A rubber washer between the verge ring and the top glass ensures water-tightness.

The dry card compass is too sensitive for steering purposes, especially in bad weather and even small disturbances causes the dry card to oscillate. This type is not very popular and hence we will not discuss the same in this module.

In the wet card compass, the oscillations are damped, without loss of accuracy, by immersing the card in liquid. The card therefore does not oscillate but has “a dead beat" movement.

THE LIQUID (in the compass bowl)

The bowl is filled with a mixture of distilled water and pure ethyl alcohol thereby making the mixture to have the following properties

(a)                Low freezing point about -30˚C

(b)               Small coefficient of expansion

(c)        Does not discolour the card

(c)                Low relative density about 0.93

The top of the bowl is of transparent glass. The bottom is of frosted glass to diffuse the light coming from an electric bulb below.

ALLOWANCE FOR EXPANSION

Increase and decrease of atmospheric temperature expands and contracts the liquid inside the bowl. Different methods are adopted to cope with this problem. Two types of arrangements are as follows:

(1)        Fitting of corrugated chambers

(2)        Fitting of nut-and-screw expansion chambers

In this module, we shall try and understand the first arrangement i.e. using a small accordion-like expansion chamber attached to tile howl.

This arrangement is similar to the attachment of corrugated bellows in an aneroid barometer. The chamber increases or decreases in volume whenever the liquid inside the bowl expands or contracts due to variations in atmospheric temperature.

THE LUBBER LINE

Forward, inside part of tile bowl, there is usually a small projection with a line marked on it, This line is called the "lubber line", and it represents the direction of the ship's head.

The compass is fixed on the centre line of tile ship, with the lubber line aligned towards forward.

The reading of the compass card, which is in line with the lubber line, is the compass course of the ship at that time.

THE BINNACLE (of a magnetic compass)

The binnacle is a cylindrical container made of teak wood. No magnetic material is used in the construction. The compass bowl is slung inside the top portion of the binnacle. The middle portion is accessible by a door and contains -in electric bulb. Light from this bulb passes upwards through a slot. Key issues bottom of the compass bowl to illuminates the compass card from below.

A mechanical shutter can control the intensity of the light. The number of magnets in the bucket, the bucket's position with reference to the compass card and the number of hard iron magnets depends on the disturbing forces. A qualified “compass adjustor” can calculate this force after conducting certain tests.

Once tile compass has been adjusted, the magnets should not be disturbed and the doors giving access to tile corrector magnets should be kept locked.

Quadrennial Correctors, these are two "Soft iron" spheres, which are fitted in brackets, one on either side of the binnacle. The brackets have a sliding way or slots so that the distance between the spheres can be altered as desired during compass adjustment.

Flinders bar, this is a soft iron corrector, diameter about 7.5 cms, inserted in a 60 cm long brass case, fitted vertically either on the forward or aft part of the binnacle. The position forward or aft depends on where the superstructure is more.

The Helmet, the top of the binnacle is provided with a large brass helmet. This protects the compass bowl from direct sunlight, rain, spray, dew, frost etc. during non-use.

COMPARING COMPASSES (why is it necessary)

The needles of a magnetic compass do not point to the north pole but to a point about 1600 kilometres away (called the magnetic north pole). Mariners using a compass of this type have to make an allowance in their steering, and this allowance differs not only from place to place but also from year to year (because the magnetic poles slowly alter their positions). This is variation. Further, the magnetic compass is affected by the magnetism of the ship itself and this error is the deviation.

The solution to the above problem is through the use of a marine Gyro Compass, which is basically a Point-magnetic compass capable of being made to point true north and using the directional property of a free gyroscope.

Marine gyro Compasses are more complicated than the magnetic compass, reliable and accurate. The directional signals from the gyro Compass can be inputted into an automatic steering system thus allowing the ship to be steered in the required direction without continuous human effort.

In spite of all the above advantages, the basic disadvantage of the gyro systems is the requirement of electric power and that too a 3-phase power supply which is not possible from a back-up battery. This disadvantage creates a need for the carrying of a magnetic compass by all ships as a reliable safety measure in case of an electrical failure.

Since the power failure could be sudden and unexpected, it becomes necessary to compare the magnetic compass and gyro Compass and check the error and the deviation. This is done at least once during the navigational watch as well as after every course change. In case of gyro or power failure, the ship can continue to be navigated since the errors are known.

In case the ship is installed with a Transmitting Magnetic Compass (TMC), the auto steering can be re-connected to the magnetic compass via the transmitting system and the ship can continue on her voyage on the autopilot.

PRECAUTIONS, CARE AND MAINTENANCE

(1)        The doors giving access to the corrector magnets should always be kept locked.

(2)        The wooden parts of the binnacle should be varnished and not painted, as paint may cause the doors to jam.

(3)        The soft iron spheres and their brackets should be painted. This prevents rust.

(4)        All magnetic materials like aerials, electrical wires and equipment etc. should be well away from the compass.

(5)        The binnacle light should be switched off during daytime.

(6)        The helmet should always protect the azimuth mirror and the compass card from sprays, direct sunlight, rain etc. except when bearings are being actually taken.