Marine Compass The Magnetic Compass The Magnetic compass was first used when people discovered that a piece of magnetised iron, if suspended or balanced, would point towards the north. In fact it does not point towards the North Pole, which would be True North, but does point towards the Magnetic Pole, in the Arctic regions of Canada, giving us Magnetic North. The difference between Magnetic North and True North is called the Magnetic Variation and changes by a small amount every year, as the location of the Magnetic Pole moves slightly all the time. Details of the local magnetic variation are printed on every nautical chart and Ordnance Survey map for the year they were published.
Most Sea Scouts will be familiar with the “Silva” Compass, used mainly on land for hiking or orienteering. This type of compass can also be used on small boats and canoes if taped down in front of the helmsman and placed right on the centre line of the boat with the arrow facing right ahead. To use a ‘Silva’ type compass which has been fitted to your boat, turn the bezel line up the course you want to steer with the index mark on the dial. Alter your course so that the red half of the magnetic needle lines up with the north pointing arrow. When they are lined up, the boat’s head is in the direction you want.
The essential part of a compass is a magnetic needle. Because of this you must not allow any other magnets or objects containing iron or steel near the compass as this will affect the accuracy of the compass. Interference caused by such metal objects or magnetic fields near the compass is called Compass Deviation.
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