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Kelvin compass deflector
Kelvin compass deflector

Kelvin compass deflector

Manufacturer
Date1920s
Object number00038151
NameCompass deflector
MediumMetal, paint, timber box
DimensionsOverall: 85 x 190 x 75 mm, 0.34 kg
ClassificationsTools and equipment
Credit LineANMM Collection
DescriptionThis Kelvin compass deflector was designed to reduce the influence of variations in the Earth's magnetic field and of magnetic material on the ship's compass. By using the Kelvin compass deflector the adjuster could calculate the amount of deviation acting on the compass when the compass was deflected to the north, south, east and west cardinal points and adjust the compass accordingly. HistoryOver time navigational instruments have increased in complexity from relatively simple lodestones, sounding leads and log lines to the more complex back-staff, astrolabes, magnetic compass, quadrants, octants, chronometers, sextants, binnacles, sounding machines, patent logs, electronic logs and satellite navigation. The development of navigation mirrored the science of instrument making, with academic research into and the production of numerous thesis, mathematical tables and almanacs on 'the haven finding art'. The ship's compass, probably the single most important piece of navigational equipment on board, is a vessel's primary tool of navigation and allows the vessel to be steered and a course determined when the vessel is out of sight of land or other reference marks. Magnetic compasses, however, are influenced by variations in the Earth's magnetic field and by magnetic deviations caused by the presence of magnetic material in the vessel's hull or equipment. Magnetic variation is noted on marine charts and navigators take this variation into account when calculating course. Magnetic deviation varies from vessel to vessel and compass to compass and is usually calculated by a compass-adjuster. A compass which has not been adjusted, or a compass where the magnetic deviation is not known is virtually useless as a navigational instrument. The compass deflector was an instrument used to tentatively adjust a ship's compass when a more direct method, such as swinging a compass onto known geographical points was not possible due to inclement weather or sea conditions. The instrument consists of a hinged magnet with a screw adjustment. The lower part of the magnet creates the deflecting force on the compass being adjusted. The amount of magnetic force created is controlled by using a screw adjustment and the amount of magnetic force applied is calculated by using the built-in scale and micrometer. By using the compass deflector the adjuster could calculate the amount of deviation acting on the compass when the compass was deflected to the north, south, east and west cardinal points. The compass could then be adjusted, using the compensating iron balls, Flinders' Bars and the fore and aft magnets in the compass binnacle. As the manipulation of the compass deflector required considerable practice on the part of the adjuster and as the deflector could only assist in the compensation of a compass, even under the most favourable conditions, to within a few degrees it was not a very practical or a commonly used compass adjuster's instrument. This compass deflector was probably made by Kelvin, Bottomley and Baird, a company which originated in the highly successful relationship between William Thomson, later Lord Kelvin, (1824-1907), Professor of Natural Philosophy at Glasgow University from 1846-1899 and James White, a Glasgow optical maker. By 1854, White was already producing electrical instruments, electrometers and electrical balances from Thomson's designs. In 1870, White was largely responsible for equipping Thomson's laboratory in the new University premises at Gilmorehill. From 1876, he was producing accurate compasses for metal ships to Thomson's design, and this became an important part of his business in the last years of his life. He was also involved in the production of sophisticated marine sounding machinery that Thomson had designed to address problems encountered in laying cables at sea, helping to make possible the first transatlantic cable connection. In 1899, Lord Kelvin resigned from his University chair and became, in 1900, a director in the newly formed limited liability company, Kelvin & James White Ltd which acquired the business of James White. At the same time, Kelvin's nephew, James Thomson Bottomley (1845-1926), joined the firm. Kelvin & James White Ltd underwent a further change of name in 1913, becoming Kelvin, Bottomley & Baird Ltd.SignificanceThe ship's compass is arguably the single most important piece of navigational equipment on board a vessel. However magnetic compasses on ships were prone to the influence of other magnetic forces and therefore needed adjustment in order to be of any use as a navigational instrument. The Kelvin compass deflector is a fine and rare example of the Compass Adjuster's art and was essential for the safe navigation of a vessel at sea.