THE MAGNETIC COMPASS

The presence of the Earth’s magnetic field was noticed in ancient times. Certain rocks, called lodestones, when hung by strings, always orient themselves in a generally north-south direction. Long ago this was correctly attributed to the presence of a “force” in the air. It was some time before the reasons for this phenomenon were known, but the effect was put to use by seafarers and land explorers. Today, a magnetic compass can still be a valuable navigation aid, used by mariners, backpackers, and others who travel far from familiar landmarks. It can work when more sophisticated navigational devices fail. The geomagnetic field and the magnetic field around a compass needle interact so that a force is exerted on the little magnet inside the compass.
This force works not only in a horizontal plane (parallel to the Earth’s surface) but vertically, too, in most locations. The vertical component is zero at the geomagnetic equator, a line running around the globe equidistant from both geomagnetic poles. As the geomagnetic latitude increases toward either the north or the south geomagnetic pole, the magnetic force pulls up and down on the compass needle more and more. The extent of this vertical component at any particular location is called the inclination of the geomagnetic field at that location. You have noticed this when you hold a compass. One end of the needle seems to insist on touching the compass face, whereas the other end tilts up toward the glass.