Compass Work


Magnetic Compass


Construction of a liquid card magnetic compass


Keeping the card practically horizontal in all latitudes

The weight of the card and magnets is supported partly by the buoyancy and partly by an iridium point fitting into a sapphire bearing.

The point of support is above the centre of gravity of the card, so that the card remains substantially horizontal in all latitudes.



Composition of the liquid and explains how allowance is made for changes in volume of the liquid

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

·        Low freezing point about -30°C

·        Small coefficient of expansion

·        Does not discolour the card

·        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. This liquid has a small coefficient of expansion, but since its volume changes slightly with change of temperature, a flexible corrugated plate or expansion chamber is fitted to the bowl.


Removing Air Bubbles

Air bubbles often appear in a compass bowl. If the bubble is quite small it is preferred that it be left alone. However if the bubble is too large then the frictionless movement of the card would be affected, thus the need to remove the air bubble or at least to reduce it to quite a small one.

To remove the air bubble, first the gimbals are locked with the swivel plates provided. Next the bowl is inverted and the filling hole is unscrewed. A syringe is filled with distilled water and slowly the water is injected into the bowl. An estimate of the bubble size would give an indication of the extra fluid that has to be injected, if the same is not possible then the bowl has to be tilted in the inverted condition such that the highest point is the filling hole, this would drive the air space (bubble to the filling hole. Once the water is injected to satisfaction the filling hole is screwed back, and the bowl returned to normal position, and the gimbals unlocked.

Checking that the card is turning freely on its pivot.

The compass card may be tested for pivot friction by deflecting the compass cards a few degrees. If the pivot is in good condition the card should return to and settle in its original position with no sign of sticking or jerkiness.

Marking of the lubber line and its purpose

LUBBER’S ( LUBBER (clumsy fellow) LINE: The mark on the binnacle, which is brought to meet the desired point on the compass card.

The lubber’s line marks the direction of the fore and aft line of the ship.

So called because a ‘real’ seaman can do without it.

Binnacle and the arrangement of correcting devices provided

The binnacle is a cylindrical container made of teakwood. 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 an electric bulb. Light from this bulb passes upwards through a small slot at the bottom of the compass bowl to illuminate the compass card from below.

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

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

‘Deviation’ and how it is named

Deviation is defined as deflection right or left of the magnetic meridian.

On a lifeboat (fibreglass) the lifeboat compass magnet is affected only by the Earth’s magnetic field. As such the LB’s course would be the magnetic course – affected only by the variation caused by the earth’s magnetism.

And the North of the LB compass would be the Magnetic North.

For a ship’s magnetic compass however the compass magnet is affected by the Earth’s magnetic field as well as by the magnetic field caused by the magnetism of the ship’s hull. The ship’s hull becomes a huge magnet due to the repeated high current used in fabricating the ship.

Thus the ship’s magnetic compass would show a different North than either the True North or the Magnetic North – this is called compass North. The difference between the Magnetic North and the Compass North is known as ‘Deviation’. If the compass North is west of the Magnetic North then the deviation is termed ‘West’ and if the compass north is to the East of the Magnetic North then the deviation is termed as ‘East’. is termed as ‘East’.

Care in the placing of portable items of magnetic material, including spare corrector magnets, or electrical equipment in the vicinity of compasses

Since the magnetic compass is affected by magnetic fields, all Ferro magnetic material if brought close to a magnetic compass will slowly be magnetised (over a period of time), this material may later exert a magnetic field on the compass magnet.

Similarly any electrical wiring will produce a electrical field which will exert its influence on the magnet of the compass. This is the reason all electrical/electronic bridge equipment like Radar’s electrical/electronic bridge equipment like Radar’s etc come with a ‘compass safe distance’ marked on them.

Need for regular checking of the compass error

Due to change in latitude, course and speed the compass error should be taken at regular intervals.



Compass error should be checked after a major alteration of course

After alteration of course the gyro as well as the magnetic compass error should be taken. This since both are affected differently, errors on different headings should be taken.

Regular comparisons of standard compass, steering compass and gyrocompass should be made

Comparisons between magnetic compass and gyrocompass should be made once a watch and after every alteration of course.

All the repeaters should be compared with the Master gyrocompass and the Magnetic compass should be compared with the Master Gyrocompass.

So that while taking bearings the error if any may be applied.

Since the gyro is based on the electrical current fed to the gyroscope any earthing of the supply would result in the gyro spinning less than at optimum speed and thus errors may creep in.

 Also the synchro motors for the repeaters may sometimes slow down resulting in errors of the repeaters.

The sphere does not give any warning before it slows down so it is always better to compare before the watch and after every alteration to a new course.

Gyro Compass may be used to approximate the error of the magnetic compass

Since the gyro compass basically works on the principle of the torque required to tilt a spinning wheel from its position of rest and that the earth’s tilt does not vary sufficiently in a short time period, the gyro compass is more stable and is less affected by local magnetism as well as the ships inherent magnetism. Whereas the magnetic compass is affected by the twin factors of variation and deviation. Variation being sufficiently constant over a short time interval change in distance the deviation is the major factor in causing error on various headings. Thus a magnetic compass may be compared with a gyro compass to arrive at a approximate of the error.