History and magnetics of compass adjusting

zyxwvutsrqpo zyxwvutsrqpo IEEE TRANSACTIONS ON MAGNETICS, VOL. 24, NO. 6 , NOVEMBER 1988 2883 HISTORY AND MAGNETICSOF COMPASS ADJUSTING G. W. Barbert and A. S. Arrott, Department of Ph sics Simon Fraser University, Burnaby, B. C., Canada V.&l IS6 A brief review of the developments in the compass and its use in response to the change to iron ships in the 19th century is given in commemoration of the 100th anniversary of the adoption of Kelvin's compass by Her Majesty's Navy. Even in the days of "Iron men and wooden ships", it was known that the brass cannons1 should be placed near the compass and the iron ones further away and that it made a difference how one stowed the anchor chain. Major marine disasters in the 19th century occurred almost daily. When things went wrong what better to blame than that animate object, the compass, the computer of those days. As wood gave way to iron, first by plating and then by replacement, it became increasingly important that compass errors on each heading of the ship be either predictable or minimized through compensation. This problem attracted the attention of Poisson, Gauss, Sir George Biddell Airy, who as Astronomer Royal fought a battle for fifty years with the Royal Navy on how it should be done,2 and William Thomson, later Lord Kelvin, who eventually carried the day, as much with politics as with science, in gaining Royal Navy acceptance of his approach to the problem. The argument could be expressed as "Software versus Hxdware." Either measure the deviations and then use the brain to calculate the corrections for each course, or compensate the magnetism of the ship by the appropriate placement of magnetic materials in the vicinity of the compass. As in all good arguments, the proponents of both methods were partially right and in the end both were done. Compensation, followed by measurement of the residual deviations on major headings, results in a cume of dezJintions valid for the magnetic latitude where adjusted. soft magnets. It all sounds simple enough, but it took fifty years to convince the Royal Navy.7 It wasn't quite as simple as Airy thought. The Royal Navy approach was to find the position on the ship where all of these effects were a minimum, carefully measure the deviations, and provide the mathematical analysis necessary to predict the corrections under all circumstances. The foundation for the Royal Navy approach was built by Archibald Smith, Q.C. Though recognized as the best mathematics student of his time at Cambridge, he saw no possibility of earning a living at his first love. He could count the chairs in mathematics in the United Kingdom; they were filled and would remain so for a long time. He went into law and raised a large family. "His most loved recreation from the labours of Lincoln's Inn was always a cruise in the West Highlands."* He combined his boyhood love of the sea and his mathematical talents. Working most evenings until 2 am for the rest of his life he made major contributions to understanding the magnetism of ships. His writing shows the clarity of a legal mind describing mathematics applied to a practical problem.9 It appears that he was the first to recognize that the response to the earth's field of the ship's magnetism did not depend upon the permeability of iron as long as it was high. Shape was to a very good approximation the main effect. He calculated the demagnetizing fields of spheres, ellipsoids, long rods and flat plates with an acknowledgement to Thomson.'o He figured out how to make harmonic analysis practical aboard ship. zyxwvutsrqponm zyxwvu zyxwvut zyxwvu zyxwvu zyxw Captain Matthew Flinders in 1801 had already reached the conclusion that the world would be a better place for the compass if there were no vertical component to the Earth's field. In his voyage to Australia he "was struck by the fact of the north end of the compass being drawn to the ship's head in northern, and to the stern in southern latitudes. He, with great sagacity, compared it to the effect produced by a vertical rod of soft iron, and corrected it by introducing such a rod abaft the compass."3 Later Kelvin would name such a bar in his correction system in honor of Flinders.4 Poisson was the first to recognize the tensor nature of the problem.5 The field at the compass is determined by the response of a magnetizable body of rather arbitrary shape to the earths magnetic field. In 1824, the soft iron effects were dominant.3 The major effect would be second harmonic in the angle of the ship's heading with respect to the direction of the horizontal component of the earth's field. If one placed anchor chains or cannon balls appropriately one could create a canceling second harmonic response. A nice theory if the ship remained horizontal itself, and no one moved the compensating material. Sir George recognized the increasing importance of the hard component.6 Hard magnetism did not change with the heading of the boat, but it could decay with time. Soft magnetism changed with the heading and magnetic latitude, but did not change with time. Changes on the time scale of the maneuvers of the ship, e.g. the magnetic after-effect, would have to be neglected. Hard magnetism could be corrected with permanent magnets; the soft magnetism with When Smith died in 1871, Thomson wrote his obituary for the Proceedings of the Royal Society. In 29 pages Thomson explains the contributions of Smith to navigation including two figures and 35 equations. If the ship's magnetic course is t; the ship's compass course is 5' and the deviation of the compass is b = 5 - C, the deviation is expressed in terms of five coefficients: tan 6 = 1 at + P sin 5 + Qt cos 5 + B sin 25 + C cos 2t; cos t; + a sin t; + B cos 25 + @ sin 25 ' + 30 zyxwvu zyxwvu The coefficients are combinations of the components of the ship's tensor response to the earth's field and of the ship's permanent magnetism. In terms of the compass course and the deviation: sin 6 = Zl cos 6 + W sin + ctt cos 5' + B sin (25'+ 6 ) + C cos (zC+ 8). For small deviations the mathematics simplifies to 6= A + B sin 5' + C cos 5' + D sin 25' + E cos 25' . By the mid 19th century the use of iron had made such an approximation sufficiently inexact that Smith revised his original analysis to permit calculations of deviations with the exact formula by the use of geometrical constructions. Part of the problem of compensation is that the coefficients B and C (predominantly B) include contributions from fields in the plane of the compass induced by the vertical component of the earth's field as well as a hard iron 0018-9464/88/1100-2883$01.0001988 IEEE 2884 zyxwvutsrqponmlkji zyx zyxwvutsr component. These soft iron components are proportional to the tangent of the dip. Unless these effects are compensated for separately, a correction at one magnetic latitude would be invalid at another. Ideally one would balance the hard magnetic components at the magnetic equator and then the soft components at some other latitude. A good compass adjuster has enough experience that he can look at the ship and guess how much of the B coefficient is due to the soft iron. To minimize the effects of inclination of the ship, the vertical field at the compass position is adjusted to equal that of the earth's field alone at that latitude. There were additional problems: the compass needle was not a point dipole, it sensed also the second derivative of the field; the compass needle induced images in nearby magnetic material; ships heel and pitch. The plane of the compass in equilibrium remains horizontal but the ship's tensor does not. As the ship rolls and vibrates, the dynamic response of the compass must be considered, also. These things involve the design of the compass itself. In 1841 Smith solved one mechanical response problem by making the moments of inertia the same parallel and perpendicular to the axis of the compass. This required two needles placed so that their tips made an angle of 60 degrees with the suspension point. Twenty years later he discovered that this also solved the problem of the third harmonic coming from the second derivative of the field of a nearby permanent magnet and the fourth harmonic from the interaction of a long needle with its image in a nearby soft magnetic material.11 It seems very likely that Smith knew about images from Thomson who published a paper in 1845 on the method of images for a dipole in front of a finite permeable plate.12 the trans-Atlantic cable that he purchased the 155 ton yacht Lalla Rookh. Thomson had a taste for the sea and a taste for money. He could combine both in the development of a successful compass system. Success meant convincing Evans and the Royal Navy to abandon their opposition to compensation. Thomson particularly irritated Evans by insisting that no deviation chart was needed and besides if there were a correction to be made the chances would be fiftyfifty that the navigator would get the sign wrong. The biographers of Kelvin stress the scientist against the bureaucracy. In a recent book, Steady as She Goes, Commander A. E. Fanning gives the bureau's side of the story as well as a detailed and entertaining history of the Compass Department of the Admiralty.' Remembering that there is nothing like an adversary to make one do a better job, it may be fair to say that the Royal Navy deserves some of the credit for Lord Kelvin's achievements. Aside from the influence he had on Smith, Thomson's first contribution was to improve compass dynamics by making the period of the compass card greater than that of the ship. He used smaller needles, decreased the friction at the pivot, and improved the gimbals. He took into account that the poles are not quite at the ends of the needles. He designed a binnacle to hold the compass and the corrector magnets, hard and soft. He developed optical aids and procedures for the systematic compensation of the ship's magnetism, even in a Scottish fog. He then proceeded to exploit these achievements. zyxwvutsr While the Royal Navy rejected Airy's methods of compensation, the world of commerce readily adopted it. An approximate hardware approach satisfied the owners of mercantile ships. Presumably the quality of the software in the Royal Navy was superior to that in the commercial fleets. In 1855 Capt. Frederick John Evans was appointed Superintendent of the Compass Department of Her Majesty's Navy. At the age of 40, he had already spent 27 years at sea. He and Smith would coauthor a series of experimental and theoretical papers on the behavior of the compass. He would also hold back the Royal Navy in the use of magnetic compensation. It was not that he was against some compensation; he thought it would be dangerous if it were to be relied upon too heavily. When Thomson went to Cambridge from Glasgow at the age of 16, Smith befriended his younger fellow townsman. They remained friends for life. When a chair became open at Glasgow, Thomson's main worry was that Smith might give up his law practice. Thomson campaigned for it, Smith did not.13 Some biographers of Thomson seem to have underplayed Smith's influence on Thomson's later achievements in the design and use of the compass.14 His letter to Thomson in 1851 on the mathematics of the compass response, is recorded in Thomson's diary.15 Thomson learned much about the compass on writing Smiths obituary. About the same time he undertook to write a series of popular articles on the subject.16 But there was a five year gap between the first and second article during which he realized that how to design a suitable compass and to compensate it for the ship's magnetism was a problem worthy of his talents. This also corresponds to the time when he came to sufficient wealth through his companies and the laying of Commander May16 says of Kelvin, "He attempted to corner the compass trade of this country and his effect upon it was nearly as serious as would have been that of the inventor of the pneumatic tyre on the motor car trade, if he had prevented these tyres being used on any vehicle not made by himself." For this purpose he used the courts. May is equally hard on Evans. He describes him as "pig-headed and selfopinionated." To overcome the resistance of the Compass Department, Thomson used political pressure through his wide circle of friends and admirers, including Naval officers. In 1889 Her Majesty's Navy adopted the Kelvin compass, binnacle, and methods of compensation. The German navy had done so long before. Kelvin was greatly and appropriately appreciated in his time and since. A hundred years later, the compass remains a reliable guide to mariners everywhere. When all else fails, the compass continues to point, provided that it has recieved simple care and maintenance. In some ways Evans was right; overconfidence in the compensated magnetic compass continues to be a danger, greatly increased by the tendency of designers to place ever more powerful magnetic items inside prescribed safe distance limits. Speakers, meters, current carriers, hard and soft magnetic objects, when too close to the compass, produce higher harmonics which give unacceptably large deviations after compensation. Still worse are objects that move, e.g. vehicles on open car ferries, containers, booms that swing or the hand held microphone that can be laid along side the compass. Changes in the ship's magnetism can result from loading with electromagnets, proximity of another ship, residual magnetisation of a dry dock, fire, sand blasting, and collision. Iron ships provide magnetic shielding of current carriers below deck. Aluminum ships do not. In a recently designed aluminum cutter, the compass deviations tracked the generator speed. The subject of compass adjusting has been dismissed as a trivial exercise by both Airy, who said it could be done zyxw zyx zyxwvutsrq zyxw 2885 before breakfast, and Kelvin, who said it could be learned by any com etent sailor in three days. Denne, who wrote the manual' on the subject, is more realistic. He says that "by short periods of fairly regular study at sea the whole of the subject could be easily mastered in the course of a few voyages." Y Historically, the compass has one red pole and one blue. The texts on the subject are generally in color. With the emergence of false color representations of magnetic fields using computer graphics it might be fitting to return to this convention. The red one seeks the north and the blue one seeks the south. Or in other words the north pole of the earth is blue. The conference presentation included demonstrations of what is involved in the compass and its compensation. 9 A partial list containing 35 scientific publications of Archibald Smith are found in Catalogue of Scientific Papers 1800-1863 , (Royal Society, London 1871) After 1849 they are all on the subject of ship's magnetism. Ref. 3 above and ref. 10 below are not included in the list. lo E J. Evans and A. Smith, Character of the Armour-Plated Ships of the Royal Navy, The section On the effect of the compass of particular masses of soft iron in a ship, starts with a footnote "I beg to express my obligations to Professor W. Thomson for much of what is contained in this part of the paper, and at the same time to express my hope that he may be induced to complete the promised Treatise on the Mathematical Theory of Magnetism, part of which was published in the Phil. Trans. 1851.-A.S." Smith's plea went unanswered, for the Treatise remained in manuscript for twenty two years. (ref. 13 p107.) 11 Archibald Smith and Frederick John Evans, On the Effect produced on the Deviations of the Compass by the Length and Arrangement of the Compass-Needles; and on a New Mode of Correcting the Quadrantal Deviation, Phil. Trans. 161-181 (1861) l2 William Thomson, Phil. Trans. 243 (1845) l 3 H. I. Sharlin, Lord Kelvin, the Dynamic Victorian, (Pennsylvania State University Press, University Park 1979) 73-79 7fS.P Thompson, The Life of William Thomson: Baron Kelvin of Largs, 2 vols. (Macmillan, London, 1910) William Thomson Diary no. 3,17 May 1851 (inserted between pp 149 and 151) cited by Sharlin (ref 13) p 109 16 WE. May , Lord Kelvin and His Compass, J. Navigation, 32,122-134 (1979) 17 W. Denne, Magnetic Conipass Deviation and Correction, rown, Son and Ferguson, Ltd. Glasgow, 1968), p 2 G.A.A. Grant and J. Klinkert, The Ship's Compass (Routledge, Kegan Paul Ltd.London, 1970); J. Klinkert, Compass-Wise, (Brown, Son and Ferguson, Ltd. Glasgow, 1976) I9F.G. Merrifield, Ship Magnetism and the Magnetic Compass, (Pergamon Press, Oxford, 1963) 20 Armin Wittstein, Julius Klaproth 's Schreiben an Alexander von Huniboldt uber die Erfindung des Kompasses, (T.O. Weigel, Leipzig, 1885) 21 Alexander Russell, Lord Kelvin, (Blackie and Son Ltd, London, 1938) p 79 zyxwvutsrq zyxwvutsrqp It is hoped that this review of the subject may stimulate some reading of the literature cited. It would not be fit to conclude without mentioning the standard works in the field by Klinkertls and Merrifield's. A remarkable early history of the compass is contained in a letter from Julius Klaproth to Alexander von Humbolt in French in 1834, later translated into German by Wittsteinzo. Among many curiosities, Klaproth notes the use of the compass in German mines in the 15th century. Nothing above was intended to detract from the debt that is owed to Thomson. In his eulogy to Lord Kelvinzl, Alexander Russell quotes J.A. Ewing, one of the pioneers of magnetism, as saying that a blue-jacket was once heard to remark "I don't know who this Thomson may be, but every sailor ought to pray for him every night." +business address: Captain George W. Barber, Compass Adjuster 312 Dartmoor Dr., Coquitlam, B.C., Canada zyxwv zyxwvutsrqp 1 W. E J. Morzer Bruyns, J. Navigation 29, 420 (1976) 2 Charles H. Cotter, The Early History of Ship Magnetism: the Airy-Scoresby Controversy, Annals of Science, 34, 589-599 (1977), also George Biddell Airy and his Mechanical Correction of the Magnetic Conipass, ibid, 33, 263-274 (1976) Archibald Smith, O n the Deuiation of the Compass in Iron Ships, Notices of the Proceedings at the meetings of the members of the Royal Institution of Great Britain. IV, 518 1866. eekly Evening Meeting, Friday, February 9, 1866) E e e also Charles H. Cotter, Matthew Flinders and Ship Magnetism, J. Navigation, 39, 123 (1976) 5 Reference to Poisson's work and an English translation are in ref. 2. 6 G. B. Airy, Account of Experiments on Iron-built Ships, instituted for the purpose of discouering a correction for the deviation of the Compass produced by the Iron of the Ships, Phil. Trans. Royal Society, 1839, pp167-213 7 A.E. Fanning, Steady A s She Goes, (Her Majesty's Stationery Office, London, 1986) 8 William Thomson, Archibald Smith, and the Magnetism of Ships, 'Obituary Notices' in the Proceedings of the Royal Society, 1874, reprinted in Matheniatical and Physical Papers, Lord Kelvin (University Press Cambridge 1911) Vol VI, pp 306-338 T !