THE COMPASS NEEDLE

Bay of Plenty Times, Volume LXVII, Issue 12456, 16 September 1938, Page 4 (Supplement)

THE COMPASS NEEDLE

WHY HOES IT POINT NORTH The places on a magnet where the attraction seems concentrated are called the poles of a magnet. la straight bars of magnetized steel, the poles are at the ends. If a bar magnet is suspended or pivoted so that it is free to turn horizontally, it will swing slowly around until it comes to rest in a north and south position. The pole of the magnet which points to the north is called the north, or north-seeking pole. The other pole is the south pole. If the south pole of a second bar magnet is brought near to the north pole of the pivoted magnet, the north pole of the latter will swing toward it. But if the second magnet is turned end for end, and its north pole brought near the north pole of the pivoted magnet, the north pole of the latter will swing away. Thus it is seen that unlike magnetic poles attract, and like poles repel. This globe on which we live is itself a huge magnet, and causes any magnetised bar or needle, which is free to turn, to swing around until it points roughly to the north. The needle points to the earth’s magnetic pole, and the magnetic and geographic poles do not coincide.

The north magnetic pole is located in the Boothia Peninsula, north of Hudson Bay and some distance from the geographic north pole; and the south magnetic pole lies almost due south from Sydney, Australia, and is about 1,800 miles from the geographic south pole. This difference results in what is known as the magnetic variation or declination of the compass. The declination, even for a given spot -on the earth’s surface, varies over a period of years, because the earth’s magnetic field is not stable. Its instability is due to the influence of our sun, which is

also a magnet and is many times more magnetically powerful than the earth.

Iron and other metals, and the presence of electricity in ships, and magnetic rocks on land (such as the approaches to Cape St. Francis, Labrador, and the coasts of Madagascar and Iceland) result in further compass disturbances, known as the deviation of the compass.

Both the declination and the deviation of the compass must he taken into consideration by mariners, with the result that the charting of a ship’s course across the trackless ocean is rather a complicated matter, even with the aid of a magnetic com pass.