Why Does a Compass Needle Point North to South?
When the magnetic needle of a compass is allowed to move freely it will automatically place itself in line with the earth’s magnetic field, one end pointing to the magnetic North Pole while the other indicates the South. The magnetic needle is mounted on a low-friction pivot point, in better compasses a jewel bearing, so it can turn easily. When the compass is held level, the needle turns until, after a few seconds to allow oscillations to die out, it settles into its equilibrium orientation.
Natural magnets, such
as loadstone or pieces of iron which have been touched by a loadstone, are to
be found the world over. It was the discovery that loadstone would always place
itself so as to lie in a magnetic north-to-south position that led to the
invention of the magnet.
During the 15th Century it was realized that the magnetic North Pole and the Geographic North Pole were not exactly in the same place. The small angle between the two is known by seamen as “the variation”. Some experts have claimed, however, that the Chinese were already aware of the existence of variation as early as the 11th Century. Again, in the 15th Century, it became apparent that the earth itself was a great magnet.
In navigation, directions on maps are usually expressed with reference to geographical or true north, the direction toward the Geographical North Pole, the rotation axis of the Earth. Depending on where the compass is located on the surface of the Earth the angle between true north and magnetic north, called magnetic declination can vary widely with geographic location. The local magnetic declination is given on most maps, to allow the map to be oriented with a compass parallel to true north.
The locations of the Earth’s magnetic poles slowly change with time, which is referred to as geomagnetic secular variation. The effect of this means a map with the latest declination information should be used. Some magnetic compasses include means to manually compensate for the magnetic declination, so that the compass shows true directions.