Not only is an uncorrected compass subject to large deviations, but there will be sectors in which the compass may sluggishly turn with the ship and other sectors in which the compass is too unsteady to use. Correction of compass errors is generally achieved by applying correctors so as to reduce the deviations of the compass for all headings of the ship. Correction could be achieved, by applying correctors so as to equalize the directive forces across the compass position for all headings of the ship. The deviation method is more generally used because it utilizes the compass itself to indicate results, rather than some additional instrument for measuring the intensity of magnetic fields.
Occasionally, the permanent magnetic effects at the location of the compass are so large that they overcome the earth's directive force, H. This condition will not only create sluggish and unsteady sectors, but may even freeze the compass to one reading or to one quadrant, regardless of the heading of the ship. Should the compass be so frozen, the polarity of the magnetism which must be attracting the compass needles is indicated; hence, correction may be effected simply by the application of permanent magnet correctors in suitable quantity to neutralize this magnetism. Whenever such adjustments are made, it would be well to have the ship placed on a heading such that the unfreezing of the compass needles will be immediately evident. For example, a ship whose compass is frozen to a north reading would require fore-and-aft B corrector magnets with the red ends forward in order to neutralize the existing blue pole that attracted the compass. If made on an east heading, such an adjustment would be practically complete when the compass card was freed so as to indicate an east heading.
I have listed several reasons for correcting the errors of the magnetic compass:
(1) It is easier to use a magnetic compass if the deviations are small.
(2) Although a common belief is that it does not matter what the deviations are, as long as they are known. this is in error inasmuch as conditions of sluggishness and unsteadiness accompany large deviations and consequently make the compass operationally unsatisfactory. This is the result of unequal directive forces on the compass as the ship swings in heading.
(3) Even if the deviations are known. if they are large they will be subject to change with heel and latitude changes of the ship.
(2) Although a common belief is that it does not matter what the deviations are, as long as they are known. this is in error inasmuch as conditions of sluggishness and unsteadiness accompany large deviations and consequently make the compass operationally unsatisfactory. This is the result of unequal directive forces on the compass as the ship swings in heading.
(3) Even if the deviations are known. if they are large they will be subject to change with heel and latitude changes of the ship.
The A error is more generally caused by the miscalculation of azimuths or by physical misalignments, rather than magnetic effects of asymmetrical arrangements of horizontal soft iron. If the physical alignments are checked at dockside, and if care is exercised in making all calculations, the A error will be insignificant Where an azimuth or bearing circle is used on a standard compass to determine deviations, any observed A error will be solely magnetic A error. This results from the fact that such readings are taken on the face of the compass card itself rather than at the lubber's line of the compass. On a steering compass where deviations are obtained by a comparison of the compass lubber's line reading with the ship's magnetic heading as determined by pelorus or gyro, any observed A error may be a combination of magnetic A and mechanical A (misalignment). These facts explain the procedure wherein only mechanical A is corrected on the standard compass by realignment of the binnacle, and both mechanical A and magnetic A errors are corrected on the steering compass by realignment of the binnacle. On the standard compass, the mechanical A error may be isolated from the magnetic A error by making the following observations simultaneously:
(1) Record a curve of deviations by using an azimuth (or bearing) circle. An A error found will be solely magnetic A.
(2) Record a curve of deviations by comparison of the compass lubber's line reading with the ship's magnetic heading, determined by pelorus or by gyro. Any A error found will be a combination of mechanical A and magnetic A.
The mechanical A on the standard compass is then found by subtracting the A found in the first instance from the total A found in the second instance, and is corrected by rotating the binnacle in the proper direction by that amount It is neither convenient nor necessary to isolate the two types of A on the steering compass and all A found by using the pelorus or gyro may be removed by rotating the binnacle in the proper direction by that amount.
(1) Record a curve of deviations by using an azimuth (or bearing) circle. An A error found will be solely magnetic A.
(2) Record a curve of deviations by comparison of the compass lubber's line reading with the ship's magnetic heading, determined by pelorus or by gyro. Any A error found will be a combination of mechanical A and magnetic A.
The mechanical A on the standard compass is then found by subtracting the A found in the first instance from the total A found in the second instance, and is corrected by rotating the binnacle in the proper direction by that amount It is neither convenient nor necessary to isolate the two types of A on the steering compass and all A found by using the pelorus or gyro may be removed by rotating the binnacle in the proper direction by that amount.
The B error results from two different causes, the fore-and-aft permanent magnetic field across the compass, and a resultant asymmetrical vertical induced effect forward or aft of the compass. The former is corrected by the use of fore-and aft B magnets, and the latter is corrected by the use of the Flinders bar forward or aft of the compass. Inasmuch as the Flinders bar setting has been made at dockside, any B error remaining is corrected by the use of fore-and-aft B magnets.
The C error has two causes, namely: the athwartship permanent magnetic field across the compass, and a resultant asymmetrical vertical induced effect athwartship of the compass. The former is corrected by the use of athwartship C magnets, and the latter by the use of the Flinders bar to port or starboard of the compass; but, inasmuch as this vertical induced effect is very rare, the C error is corrected by athwartship C magnets only.
The D error is due only to induction in the symmetrical arrangements of horizontal soft iron, and requires correction by spheres, generally athwartship of the compass.
The existence of E error of appreciable magnitude is rare, since it is caused by induction in the asymmetrical arrangements of horizontal soft iron. When this error is appreciable it may be corrected by slewing the spheres.
The existence of E error of appreciable magnitude is rare, since it is caused by induction in the asymmetrical arrangements of horizontal soft iron. When this error is appreciable it may be corrected by slewing the spheres.
For the most part certain errors are rare, and others have been corrected by adjustments at dockside. For most ships, there remain only three errors to be corrected at sea, namely the B, C,and D errors. These are corrected by the use of fore-and-aft B magnets, athwartship C magnets, aud quadrantal spheres.
