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Marine Compass

  The magnetic compass first appeared in China and was probably first used as a fortune-teller\'s aid (with no purpose except dazzling the customer with what appeared to be magic). The earliest record of it being used at sea was in 1115 by Chinese mariners. It was not long before it was adopted by European and Islamic countries. Its adoption was to transform the economy of the Western world, as its use meant that sailing fleets could take to sea in any weather conditions at any time of year.

The major disadvantage of the magnetic compass is its magnetism which means that it may be affected by large masses of ferrous metal in its vicinity and also by electromagnetic machinery or magnetic rocks. The magnetic North Pole is not the true north of the world and the position of the magnetic pole changes with time, requiring corrections to its use on a regular basis.

A more sophisticated instrument is the gyroscopic compass. This depends on the high angular speed of a rotor driven by an electric motor. This will remain with its axis in a fixed direction unless considerable force is applied to it. The shaft is therefore mounted in gimbals which allow it to rotate about axes perpendicular to its axis of rotation. Thus as a ship moves at sea the gyroscope continues to lie with its axis in the same direction. It is this type of instrument which allows accurate navigation of submarines beneath the polar icecaps. AA

Further reading Duane Roller, The De Magnete of William Gilbert.



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