Wherever on earth we hold a compass, the needle points towards the magnetic North Pole. Compass is widely used in aircrafts, ships, military services, astronomical works and many more to locate directions.
Compasses are of many types. A magnetic compass has a magnet that interacts with the earth’s magnetic field and aligns itself to point to the magnetic poles. A gyro compass contains a rapidly spinning wheel whose rotation interacts dynamically with the earth’s rotation till its axis of rotation is parallel to that of earth’s axis. Whatever be the type, we see that the needle points in the direction of magnetic North Pole.
Our planet Earth has got some magnetic field generated deep down inside its core. You are aware of the structure of the earth. The earth has got an inner core, outer core, mantle and an outer crust. The Core is so very hot whose temperature is in the range of 6000 degrees F. Because of the immense pressure exerted on it, the inner core remains a solid mass. The outer core however consists of molten iron due to heat radiating from the core. The rotation of earth causes the movement of molten iron in a rotational pattern. This molten iron in the core acts like a huge bar magnet and the rotational force is supposed to be the cause for magnetic field on the earth’s surface. But this field is actually very weak as the surface of earth is very vast. The horizontal component of the earth’s magnetic field is in the direction of magnetic north. This horizontal component pulls a freely suspended magnetized compass towards the magnetic north pole.
In other words, a compass detects slight magnetic fields created by any object. To explain why the compass needle points to magnetic north, there is only an assumption theory. It is called the standard bar magnet assumption theory. As per this theory, imagine earth to be having a big bar magnet buried deep inside as shown in the picture. Now the bar magnet’s south end is considered to be at the earth’s North Pole and north end at earth’s South Pole. We know that like poles repel and unlike poles attract. Thus, a freely suspended compass needle points to the earth’s magnetic North Pole getting attracted to the south end of the ‘imaginary’ bar magnet.
Compass is also used in the laboratories to detect the small magnetic field produced by a wire carrying a current.