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37. State clearly how that part of the heeling error due to the permanent part of the magnetism of the ship varies as the ship changes her position on the globe, and what is the reason of this?
That part of the heeling error due to the permanent part of the ship's magnetism decreases as the ship approaches the magnetic equator where it is least but does not vanish ; it increases again as the ship goes towards the south magnetic pole, and has the same name in both hemispheres. The reason for this is that although the sub-permanent magnetism is of the same amount in all latitudes and both hemispheres, the earth's horizontal force which directs the compass needle is greatest on the magnetic equator and least at the magnetic poles, therefore, the sub-permanent magnetism would have least effect where the horizontal directive force is greatest, and greatest effect where the horizontal directive force is least.
38. State clearly how that part of the heeling error due to the induction in transverse iron (which was horizontal when ship was upright) and iron vertical to the ship's deck, varies as the ship changes her position on the globe.
That part of the heeling error due to induction in vertical iron and transverse iron which when the ship was upright caused no heeling error varies as the tangent of the dip. It is therefore greatest in high latitudes, decreasing to zero on the magnetic equator where it is nil and increases again as the ship recedes from the magnetic equator into the southern hemisphere where it is of an opposite name.
39. Your steering compass having a large error, show by “ Beall's Compass Devia scope” how you would correct it by compensating magnets and soft iron (as usually practised by compass adjusters in the Mercantile Marine) in order to reduce the error within manageable limits. Show also how the heeling error can be compensated.
This question is given for examination in practical compass adjustment on the deviascope. In order to prepare for such examination, and also to obtain much valuable information on the magnetism of iron ships, the “ Handbook to the Deviascope" should be obtained.
scope" should be obtained. It is sold (price Is., and Supplement 3d.) by J. D. Potter, 145, Minorics, E. I.
40. As the co-efficient B (capable of being corrected) usually consists of two parts, one due to the permanent magnetism of the ship, and the other to vertical induction in soft iron, how should each of the two parts, strictly speaking, be corrected when compensating the compass?
The part due to permanent magnetism should be corrected by magnets, the part due to vertical induction by an upright bar of iron on the opposite side to the disturbing force.
41. If the whole of co-efficient B be corrected by a permanent magnet, as is usually done, what is likely to ensue as the ship changes her magnetic latitude ?
If the whole B is corrected by magnets, + B or — B will probably appear as the ship goes to the northward or southward, because the part of B depending on induction will increase as the ship goes from, and diminish as the ship goes towards the equator, and is of opposite name in the southern hemisphere.
42. Provided the needles of your compass are not so long and powerful, and so near, as to cause the soft iron correctors to become magnetised by induction, would the co-efficient D if properly compensated be likely to remain so in all magnetic latitudes and both hemispheres. If so, state the reason why.
If the correctors are not affected by the compass needles, the correction of the D will remain perfect in all magnetic latitudes. Because the D is caused by induction in the horizontal, soft iron of the ship from the earth's horizontal force, and the value of soft iron as a corrector depends upon the same force. Therefore, the disturbing force and the correcting force vary together, and, if once made equal, they remain equal. If D is not corrected it should always be of the same amount for the same reason.
43. State at what distance, as a general rule, the magnets and soft iron correctors should be placed from the compass needles, and what will be the consequence if they are placed too near the needles.
Magnets of the length generally used should not be placed nearer than twice the length of the magnets from the centre of the compass. Soft iron correctors should not be nearer than i į the times length of the needles from the centre of the compass.
If the magnets are placed too near the compass needles, and the semicircular deviation is exactly corrected on the cardinal points, that correction would not be perfect on other points. The error would be greatest on the quadrantal points.
If the quadrantal correctors are too near, the D would be found to be over-corrected when the ship goes to a place of less horizontal force, and under-corrected if she goes to a place of greater horizontal force.
If the iron bar for correcting the induced part of B is too near, either end of the needle will be drawn towards the bar when in its vicinity, and so vitiate the correction.
It is especially necessary to consider the distance of the correctors from the centre of the compass when the compass is suspended in such a manner as to allow the needles to alter their position relatively to the correctors.
44. Is it necessary that the magnets used for compensating co-efficients B and C should be placed on the deck ? If not, state where they may also be placed, and the rules to be observed in placing them into position.
It is not necessary that the magnets for correcting the semi-circular deviation should be placed on the deck. The magnets for correcting the B must be horizontal, fore-and-aft, and the middle of the magnets in the same vertical athwartship plane as the centre of the compass. The magnets for correcting the C must be horizontal, athwartships, and the middle of the magnets in the same vertical fore-and-aft plane as the centre of the compass. If these conditions are fulfilled the magnets may be placed inside or on the outside of the binnacle, or in any convenient place. In wheel houses it is sometimes convenient to place them on a near bulkhead.
45. Can the compensation of the heeling error be depended upon when the ship changes her latitude ? If not, state the reason.
Compensation of the heeling error cannot be depended on when the ship changes her latitude, because part of it is due to induction in transverse and vertical soft iron. This part decreases as the ship approaches the magnetic equator, where it is zero, and is of contrary name in the two hemispheres. It is impossible that this can be compensated by a fixed magnet, which is the usual practice, except for the place where the ship is, and for places where the induced magnetism will be the same.
SYLLABUS OF EXAMINATION FOR EXTRA MASTER IN THE LAWS
OF THE DEVIATION OF THE COMPASSES OF AN IRON SHIP, AND IN THE MEANS OF COMPENSATING OR CORRECTING IT.
From January 1, 1895, the following Syllabus will be substituted for the one at present in use in the examination of all candidates for the voluntary examination in Compass Deviation and for Extra Masters' Certificates. Candidates will be required to give correct written answers to at least 15 of the questions; these will be marked by a cross by the Examiner. Thes will also be required to prove by “ Beall's Compass Deviascope” (1) their knowledge of the Tentative method of Compass Adjustment, and (2) that they really possess a good knowledge of what they have written, by showing on the Deviascope that they are acquainted with the practical application of the same, or of any other questions in the Syllabus that the Examiner in the course of the examination may think proper to touch upon. Questions 31, 61, 62, 69, 70, 72, and 92, will be marked by the Examiner in all cases. The other questions will be constantly varied.
1. Describe an artificial magnet, and how a steel bar or needle is usually magnetised.
An artificial magnet generally consists of a tempered steel bar, magnetised by the inductive action of another magnet, either natural or artificial The loadstone and the earth itself are natural magnets. A very general method of magnetising a steel bar or a compass needle is by the means of a horse-shoe magnet; the needle to be magnetised is laid on a flat surface, and one of the poles of the horse-shoe magnet when pressed on the needle should be drawn from end to end. The other pole of the horse-shoe magnet should then be brought to the centre of the needle, and drawn several times in the opposite direction. This process should be repeated on the other side of the needle, care being taken that in all cases the same pole of the horseshoe should be drawn to the same end of the needle. Steel bars for correciing compasses, and smaller pieces of steel for compass needles, are also magnetised by drawing each half over the poles of a strong compound magnet or of an electro-magnet.
2. Which end of the compass needle, or a magnet, is commonly termed the red, and which the blue pole ?
That end of the compass needle or magnet which points towards the north when it is suspended so as to move freely in the horizontal planes usually termed the red pole, and the end which points towards the south is termed the blue pole.
3. Which is the red magnetic pole of the earth, and which the blue? and give their geographical positions.
The northern magnetic pole is termed blue, and the southern red. The blue pole is in lat. 70° N. and long. 97° W. The red pole is about 73° S. and
4. What effect has the pole of one magnet of either name on the pole of the same name of another magnet, and what would be the consequence of the pole of one magnet of either name being brought near enough to affect the pole of contrary name, if in the se ca ses both magnets were freely suspended ?
The pole of one magnet of either name will repel the pole of the same name of another magnet. If the pole of one magnet of either name is brought near to the pole of a contrary name of another magnet, the two poles will mutually attract each other.
5. By applying this law to all magnets, natural as well as artificial, describe what would be the result on a magnetic bar or needle, freely suspended, but by weight or by the nature of its mounting constrained to preserve a horizontal position ; and what would be the result, if so mounted, but free to move in every direction, the earth being regarded as a natural magnet ?
If the magnetic bar or needle can only move in a horizontal plane it will take up a position which will coincide with the direction of the magnetic meridian.
If the magnetic bar or needle is free to move in any direction it will take up a position which will coincide with the magnetic meridian, and the north end would dip below the horizon in the north magnetic hemisphere and the south end in the south magnetic hemisphere. At the magnetic equator it would be horizontal and at the magnetic poles vertical.
6. What is the cause of the variation of the compass ?
Variation of the compass is caused by the magnetic poles of the earth not coinciding with the geographical poles.
7. What is meant by the deviation of the compass ?
The deviation of the compass is the angle between the magnetic meridian and the direction of the compass needle and is caused by the iron of the ship, whether used in her construction and equipment, or in her as cargo.
8. What is meant by the term “ local attraction"; under what circumstances have ships' compasses, from recent careful investigation, been found to be affected by it, and name some of the localities in different parts of the world where this disturbance is to be found, and consequently where increa sed vigilance is necessary ?
The term “ local attraction" may be understood to mean any disturbance of the compass by the magnetism of objects external to the ship. The term has especial reference to the disturbance caused by the magnetism of the ground in certain localities. As the effects of magnetism are not cut off by the intervening water, a ship's compasses may be affected in shallow water, though she may be at some distance from the land. The following places are given in the “Admiralty Manual ” where, from well-authenticated observations, ships' compasses have been found to be disturbed :
The approaches to Cossack, in North Australia ; Cape St. Francis Labrador; New Ireland and Bougainville, Solomon Islands; and Tumbora Volcano, Sumbawa Island, near Java ; the coasts of Madagascar, especially near St. Mary's Island ; Iceland and its adjacent waters; Odessa Bay and the shoal south of it ; Isle de Los, West Coast of Africa.
9. What do you understand by the term soft” iron; and what are i properties as regards acquiring and retaining magnetism ?
The term “soft” iron is applied to iron which becomes instantly mas. netised by induction from any magnetic force to which it is exposed, bu which loses that magnetism, or changes its magnetic condition, when thinducing cause is removed or changed.
10. What do you understand by the term “hard” iron ; and what are iis properties as regards acquiring and retaining magnetism ?
The term " hard ” iron is applied to iron which is not easily magnetist by induction, but has the property of retaining the magnetism so acquirt more or less permanently.
II. Describe the meaning of the term " horizontal force" of the earth ; wher is it the greatest, and where the least, and what effect has it in respect to :
1 increa se or decrea se of the directive force of the compass needle ?
The term “horizontal force" means the horizontal component of th earth's magnetic force. The earth's force increases and is more inclined fros the horizontal as the latitude is increased. The angle at which it is inclineis called the dip. The dip and the earth’s total force increase together, bu in such a manner that the horizontal force diminishes as the magnetic po.:: are approached in both hemispheres. At the magnetic poles, the dip bir 90°, there is no horizontal force. If the magnetism in the compass needl. remains the same, the directive force on the compass needle increases decreases in proportion to the horizontal force.
12. Does the magnetic equator coincide with the geographical equator ? Ij * state clearly how it is situated.
The magnetic equator-a term applied to a line on the earth's surfa where its magnetic force is horizontal-does not coincide with t geographical equator. It intersects the latter in about 12° west longitud continuing the line to the eastward, it keeps in north latitude, going as bi. as 10°, and crosses into south latitude in about 170° west longitude. attains its greatest south latitude (149 in about 45° west longitude, ar rejoins the equator in 12° west longitude.
13. Where can the values of the magnetic dip, the earth's horizontal f." and the variation, be found ?
Charts showing the magnetic dip and horizontal force are to be found the “ Admiralty Manual of the Deviation of the Compass.” The variat is accurately shown on the Admiralty variation chart, by curves equal variation drawn to each degree.