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Edward J. Johnson and the early History of Ship Magnetism
Published online by Cambridge University Press: 23 November 2009
Extract
During the months of October and November 1835 a series of magnetic observations was made, under orders of the Lords Commissioners of the Admiralty, on board the iron steamship Garryowen by Commander Edward J. Johnson R.N. The results of Johnson's observations paved the way to Airy's solution to the crucial problem of compass management on board an iron ship.
Johnson related in a paper in which he described his experiments, that he proceeded to Ireland taking with him the necessary instruments for ascertaining the deviation of the magnetic needle produced by the local attraction of an iron steam-vessel, together with instruments for measuring dip and magnetic intensity. Every facility was afforded him by the owners of the Garryowen, the City of Dublin Steam Packet Company, and by the builders of the vessel, Messrs. Laird of Liverpool.
The Garryowen, built in 1833, was a paddle steamer of length 130 ft, breadth 21 ft 6 in and depth 11 ft. She was equipped with two engines each rated at 85 h.p. and her tonnage was 281 tons burden. She was fitted with four water-tight transverse bulkheads of wrought-iron ¼ in thick. Her hull and boilers were of malleable iron and the total weight of iron in her structure amounted to 180 tons.
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- Copyright © The Royal Institute of Navigation 1979
References
NOTES AND REFERENCES
1vide Cotter, C. H. (1976). George Biddell Airy and his Mechanical Correction of the Magnetic Compass. Annals of Science 33 (1976), 263–274.CrossRefGoogle Scholar
2Johnson, E. J. Report of Magnetic Experiments tried on board an Iron Steam-Vessel, by order of the Right Honourable the Lords Commissioners of the Admiralty. Philosophical Transactions of the Royal Society (1836), pp. 267–288.Google Scholar
3The term ‘local attraction’ was used to denote what nowadays is termed ‘compass deviation’. It is a measure of the angular disturbance of the compass needle due to the magnetic field of the ship.Google Scholar
4The terms ‘soft-iron’ and ‘hard-iron’ apply to ideal circumstances which, in practice, are never met. Soft-iron and hard-iron are essentially properties, rather than types of material. Material which would become instantaneously magnetized when subjected to a magnetic field, and which would instantaneously lose all of its magnetism when removed from the field is said to possess ‘soft-iron’ but no ‘hard-iron’ property. The hard-iron property of a material allows it to retain magnetism after having first been subjected to a magnetic field and then removed from it.Google Scholar
5vide Airy, G. B.Autobiography of Sir George Biddell Airy K.C.B. &c. Edited by Wilfrid, Airy (1896, Cambridge), p. 132.Google Scholar
6vide Airy, G. B.Autobiography of Sir George Biddell Airy K.C.B. &c. Edited by Wilfrid, Airy (1896, Cambridge), p. 134.Google Scholar
7vide Airy, G. B. Account of Experiments on Iron-built Ships, instituted for the purpose of discovering a correction for the deviation of the compass produced by the Iron of the Ship. Philosophical Transactions of the Royal Society, No. XIII (Vol. for 1839), p. 167.Google Scholar
8Johnson, E. J.Practical Illustrations of the Necessity for ascertaining the Deviations of the Compass. (1847, London).Google Scholar
9Many of the specimens to which Johnson referred in his book, together with many later specimens, form the remarkable collection of compasses now preserved at the National Maritime Museum at Greenwich.Google Scholar
10Johnson, E. J.Practical Illustrations of the Necessity for ascertaining the Deviations of the Compass. Second Edition. (1852, London), p. 113.Google Scholar
11Johnson, E. J.Practical Illustrations of the Necessity for ascertaining the Deviations of the Compass. Second Edition. (1852, London), p. 117.Google Scholar
12The original Admiralty Compass Committee was set up in 1837. The terms of reference for the committee required its members to consider the best form of compass for ships. From its investigations and examinations of a variety of compasses the committee made recommendations helpful in selecting a suitable position on board for the compass to ensure that error due to the ship's magnetism was kept to a minimum. The committee also designed a compass and gave detailed specifications of what became known as the Admiralty Standard Compass.Google Scholar