How Does a Magnetic Compass Tell Direction?
A magnetic compass is the most well known type of compass, which has become so popular that the term “compass” almost always refers a magnetic compass. While the design and construction of this type of compass has changed significantly over the centuries, the concept of how it works has remained the same.
Magnetic compasses consist of a magnetized needle that is allowed to rotate so it lines up with the Earth’s magnetic field. The ends point to what are known as magnetic north and magnetic south.
The magnetic compass relies on the Earth’s magnetic field to determine direction. The Earth’s magnetic force pulls the compass magnet, or needle, into a north-south position. Although the Earth’s magnetism makes one end of the compass needle point northward, the needle does not point straight to the North Pole.
It actually points to the north magnetic pole, which is located several hundred miles south of the actual North Pole. This discrepancy between magnetic north and true north is called variation (by mariners or pilots) or magnetic declination (by land navigators) and varies depending on location.
Variation is not significant when using magnetic compasses near the Equator, but closer to the North and South Poles, the difference is much greater and can lead someone many kilometers off-course. Charts help navigators correct their compass readings so that they can determine “true north.”
Other adaptations have been made to magnetic compasses over time, especially for their use in marine navigation. When ships evolved from being made of wood to being made of iron and steel, the magnetism of the ship affected compass readings. This difference is called deviation.
Adjustments such as placing soft iron balls (called Kelvin spheres) and bar magnets (called Flinders bars) near the compass helped increase the accuracy of the readings. Deviation must also be taken into account on aircraft using compasses, due to the metal in the construction of an airplane.
Magnetic compasses come in many forms. The most basic are portable compasses for use on casual hikes. Magnetic compasses can have additional features, such as magnifiers for use with maps, a prism or a mirror that allows you to see the landscape as you follow the compass reading, or markings in Braille for the visually impaired.
The most complicated compasses are complex devices on ships or planes that can calculate and adjust for motion, variation, and deviation. Eventually, as compass readings became more reliable and more explorers understood how to read them, the devices have become a critical navigational tool.