What Is a Magnetic Compass?

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The magnetic compass is the most well-known of all instruments used in finding direction. It is the oldest navigational instrument and has been aiding sailors to cross the seas for many centuries. Mariners can use magnetic compasses to fix a ship’s position on a chart by using it to take bearings of visible objects as well as allowing them to steer a particular course.


The exact origins of the magnetic compass and the date that it was first used are unknown. However, it is certain the Ancient Greeks were aware of the attractive properties of magnetism, and the Chinese probably knew that iron bars acquired a directional north-south property when stroked with a lodestone up to 2,000 years ago. This idea reached Europe in the 10th century and was probably introduced by Arab traders who gained the information from China. Simple magnetic compasses were used in the Mediterranean in the 12th century, although they were often unreliable. In the Middle Ages, magnetic compasses were used widely, but little was known about how they worked.

How it Works

Earth’s two magnetic poles, which are located close to the North and South Poles, mean that the Earth is similar to a giant magnet, surrounded by a magnetic field. This magnetic field causes magnetized iron needles to swing into a north-south position when hung from a thread or put through a piece of wood floating in water. Originally, needles became magnetized by stroking them with a lump of magnetic rock known as a lodestone. As this effect was temporary, ships would carry lodestones in order to stroke the needle of the compass when its magnetism wore off.


Mariners, including Columbus, were aware that the magnetic compass needle did not point accurately to Earth’s true north as long ago as the 15th century. In fact, the needle makes an angle with true north, and this angle varies from one area of the globe to another. To correct this problem, needles began to be mounted under a card on a sharp pin and placed in a small box. These compass cards were originally marked with 32 points instead of degrees. The points matched the directions of winds that were familiar to mariners. The points that marked north, south, east and west were known as the cardinal points.


Even early compasses were mounted in square boxes with attachments featuring swiveling rings. This enabled the compass to hang in way that prevented it from swinging wildly with the movement of the ship on rough seas. Iron ships pose a problem for magnetic compasses as their own magnetic fields affect the reading. To counteract this problem, magnets and pieces of iron that have not been magnetized are placed near the compass as a method of neutralizing the ship’s magnetism. When a magnetic compass is used near the Earth’s magnetic poles, it is rendered useless. At these poles, the fields of force converge vertically on the region, with almost a 90-degree inclination and only a weak horizontal intensity. This tilts the compass needle up or down into the Earth, making it point only in the direction of where the compass is tilted.


About the Author

Born in Norfolk, United Kingdom, Hayley Ames' writing experience includes blog articles for a travel website. Ames was awarded a Bachelor of Arts in English language and literature from the University of Sheffield in the United Kingdom.

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