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Magnetic compasses are adjusted in order to diminish the effect of deviation. This also increases the force of the earth's magnetic field. The method of adjustment depends upon:
•Dividing the magnetism of the vessel into permanent and induced magnetism (it is not possible to compensate for the semipermanent magnetism).
• The magnetic field of the vessel is divided into longitudinal, athwart ships and vertical magnetism. The magnetism of the vessel is mainly generated during construction by the earth's magnetic field. So vessels built in the Northern hemisphere get a blue pole in their upper works and a red pole in the lower part of the ship. This permanent magnetism will remain constant unless some major event should occur, such as a collision, lightning strike or rebuilding.
The induced magnetism is changed with changes in course and latitude. The horizontal component of the magnetic field of the earth varies with a maximum at the magnetic equator to zero at the magnetic poles. As the magnetic field of the earth interacts with the vessel's hull on different courses so changes occur in the induced magnetism.
Adjustment is carried out by utilising the same types of magnetism: the permanent magnetism is counterbalanced by the use of small permanent magnets (B- and C- corrections). The induced magnetism is counterbalanced by soft iron spheres (D-correctors) and a vertical tube filled with soft iron (a Flinders bar), usually located on the front edge of the binnacle.
Магнитные компасы отрегулированы таким образом, чтобы уменьшить влияние отклонения. Это также увеличивает силу магнитного поля Земли.
Способ регулировки зависит от:
The magnetic field at the compass position is split into two components. The horizontal component affects the compass direction. The vertical component produces a heeling error when the vessel heels
The horizontal component is further divided into fore-and-aft and athwarthships components. These magnetic forces are eliminated by installing permanent magnets in fore-and-aft and athwartships positions
Adjustment of the permanent magnetism requires splitting the magnetic field into fore-and-aft and athwartships components
Heeling error is caused by the vertical component of the vessel's magnetic field. One can visualise this as the magnets below the compass moving away from the compass axle as the vessel heels.
Heeling error is adjusted using small bar magnets in a holder underneath the compass. The vertical distance of the holder to the compass is regulated with a chain.
Before starting the compass adjustment procedure it is important to check the compass neighbourhood for extraneous magnetic objects and to see that any movable magnetic objects are in their normal positions.
If soft iron balls and a Flinders bar have been installed then they should be in their approximate positions. Heeling error is adjusted on an easterly or westerly course with a special instrument (vertical force instrument). Alternatively the adjustment can be carried out as the vessel rolls on a northerly or southerly course until the compass shows minimal oscillations.
The procedure is as follows, with the permanent magnetism being adjusted first:
The compass is adjusted by putting small magnets, of opposite polarity to the ship's magnetism, close to the compass
The magnetism induced in the soft iron spheres counterbalances the magnetism induced in the hull. The diagram shows athwartships adjustment, which is the commonest
The deviation table
To make a thorough examination of deviation the vessel has to describe a complete circle (360°). Deviation is noted down at increments of about 15°. In order to avoid lag effects this process should
be done in both directions of rotation. From the obtained values a smooth deviation curve can be constructed on graph paper. From this, the deviation at each 10° interval can be read off and entered into the deviation table.
For more information about the compass and its adjustment, see: http://msi.nga.mil/MSISiteContent/StaticFiles/HoMCA.pdf
Setting up and calibrating electronic compasses must be done using the main compass (master), not from a slave (daughter).
To deviate an electronic compass it has to be activated. After this the vessel is put into a slow and even 360° turn. When this is achieved, the process will start. During this process the instrument measures the variations of the magnetic field and performs a mathematical adjustment for the permanent and induced magnetism.
The procedure can be divided into four phases: