The Magnetic Compass

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The Magnetic Compass

Overview

The magnetic compass was an important advance in navigation because it allowed mariners to determine their direction even if clouds obscured their usual astronomical cues such as the North Star. It uses a magnetic needle that can turn freely so that it always points to the north pole of the Earth's magnetic field. Knowing where north is allows the other directions to be determined as well. The compass was invented by the Chinese, and was widely used for navigation beginning in about the thirteenth century.

Background

The phenomenon of magnetism was known to the ancient Greeks, but the magnetic compass was invented by the Chinese. The thirteenth century explorer Marco Polo (1254-1324) is said to have brought a compass with him when he returned to Venice after his twenty years of service in the court of Kublai Khan (1215-1294). He may indeed have carried home such a souvenir. However, the knowledge that a piece of the naturally magnetic iron ore magnetite (Fe₃O₄), called a lodestone, would align itself from north to south if allowed to move freely, seems to have arisen at least a century before Marco Polo in Europe and the Arab world.

Scholars continue to debate whether this discovery was independent, or whether the new technology was spread westward from China through trade or other contact between civilizations. Some speculate that the Chinese may have used lodestones for navigation in voyages to the east coast of India in about 100 b.c. Chinese references to a "south pointer" are found in texts as early as the first century a.d. The south pointer was a spoon carved from lodestone, which was allowed to rotate on a smooth brass plate until its handle pointed south.

Magnets align themselves along the north-south axis because the Earth itself is a huge magnet. The poles of the Earth's magnetic field roughly correspond to the rotational axis of the globe. This means that the north magnetic pole is in the approximate direction of the north geographic pole, or true north. A light magnet that can move freely will align itself in the north-south direction. However, a heavy bar magnet lying on a tabletop will not move because gravity and friction counteract the magnetic force.

Before the magnetic compass, sailors navigated by the position of the stars. They knew, for example, that the North Star, Polaris, remained in a fixed northerly position in the sky while the other stars seemed to move around it. But this method was completely useless when the stars were obscured by clouds or fog. As a result, early mariners preferred to stay near the coast as they traveled from place to place. Even the daring Vikings, the first Europeans known to have reached the New World, did so in the northern latitudes where the open water distance was shorter, and used North Atlantic islands like Iceland and Greenland as stepping-stones.

Impact

The first magnetic compasses were needles or other bits of iron that were magnetized by being rubbed on a piece of lodestone, and then attached to straw or cork so that they would float in a bowl of water. Sometimes the needle was simply hung by a thread. Left free to spin, the splinter of iron would always align itself in a north-south direction. At first this phenomenon was associated with sorcerers more than with sailors. In China, the compass was valued for its contribution to feng shui, the protection of a site from harmful influences by adjusting the orientation of buildings and furnishings. In medieval Europe, lodestones were believed to have magical powers. As a result, the Church initially denounced the magnetic compass as an instrument of Satan.

The earliest records of compasses being used for navigation date from around the eleventh or twelfth century. The Chinese military text Wu Ching Tsung Yao, written about 1044 by Tseng Kung-Liang, describes a technique used by soldiers lost at night or in bad weather. They took a thin sliver of iron, heated and then rapidly cooled it to magnetize it, and floated it in a bowl of water. The earliest known mention of the compass in a European text was by the Englishman Alexander Neckam (1157-1217) in his 1180 textbook De Utensilibus (On instruments). By the mid-1200s, compasses were being used by the Vikings and Arab merchants. The ability to navigate no matter what the weather gave sailors the courage to venture farther from the sight of land.

Navigational aids were of particular importance to the English, whose territorial aspirations exceeded the bounds of their small island nation, and who grew to rely heavily upon their navy. English mariners and inventors developed several refinements to make the compass more useful. At first, compasses were only supplemental instruments, resorted to when neither the Sun nor the North Star could be seen. The devices could be frustrating, because the bobbing cork introduced uncertainty and made the compass hard to read, particularly aboard a moving vessel.

In the late 1200s, the magnetic needle was attached to a pivot standing on the bottom of the compass bowl, restricting it to circular motion. Soon mariners began mounting a compass card on the pivot directly beneath the needle, marked off with 32 points of direction. The points of direction consist of the four cardinal points (north, south, east, and west) and the intercardinal points between them. For example, the intercardinal points between north and east are called north by east, north-northeast, northeast by north, northeast, northeast by east, east-north-east, and east by north. Modern compasses generally include fewer intercardinal points (omitting "north by east," and so on) and are instead marked off with the 360 degrees of a circle.

In many compasses the bowl was still filled with water or oil even though the pivot actually held up the needle, since the fluid helped to dampen the effects of extraneous motion. However, fluid-filled compasses tended to leak and were difficult to repair, so dry-card compasses were also built. To keep the compass level despite the motion of the ship, the compass was hung on gimbals, or rings mounted on its side. This independent suspension allowed it to swing freely, rather than being placed upon a fixed surface that would tilt with the ship.

The technical improvements to the magnetic compass made it easier to read and less subject to being tilted and shaken, but did not eliminate its basic source of inaccuracy. The Earth's north magnetic pole does not correspond precisely to its north geographic pole. The resulting inaccuracy in the compass measurement is called the variation. The extent of the variation depends on where the compass is on the Earth's surface, and changes over time. Today, the north magnetic pole is about 800 miles (1290 km) south of the geographic pole. In the fourteenth century, the magnetic and geographic poles were about 1600 miles (2575 km) apart. Metal objects in the vicinity of the compass also affect the direction in which it points. The effect of these local influences is called deviation.

Despite its drawbacks, the magnetic compass became an essential navigational tool. Eventually navigators began to understand that the compass did not quite point to true north. At about the same time the use of the compass was spreading, the Moors who had conquered Spain introduced Europe to the astrolabe, an instrument for measuring the positions of stars. The English friar Nicholas of Lynn, on a mapping assignment for King Edward III (1312-1377) in the mid-1300s, discovered that the two instruments disagreed on the direction of north by about 15 degrees. Later a meridional compass was designed with its compass card adjusted so that the needle aligned with the north directional point at a particular spot off the coast of Cornwall.

The compass was key to the long-distance voyages undertaken by Europeans beginning in the fifteenth century. Prince Henry of Portugal (1394-1460), called Henry the Navigator, established an observatory and navigation school and encouraged the idea of ambitious voyages to faroff lands. The Portuguese mariner Gonzalo Cabral reached the Azores in 1427. Unbeknownst to the explorer and his crew, they had traveled about a third of the way to the New World.

In 1519, another Portuguese explorer, Ferdinand Magellan (c. 1480-1521), led a Spanish-financed expedition that sailed around the world for the first time. Magellan himself did not live to complete the three-year voyage, having been killed in a battle in the Philippines. However, his expedition, given the technology of his time, is still considered by scholars to be among the greatest navigational triumphs in history, and it was the magnetic compass that helped make it possible.

In 1600, William Gilbert (1544-1603), English scientist and physician to Queen Elizabeth I, in his book De Magnete, was the first to describe the Earth as a giant magnet. In terms of this model, he went on to demonstrate the expected behavior of the compass needle at various points on the Earth's surface. After hundreds of years of using the magnetic compass, sailors could finally understand why it worked.

The development of iron and steel ships in the late nineteenth century made magnetic compasses less useful in navigation. A metal hull affected the local magnetic field and reduced the accuracy of the compass. Large modern ships and aircraft use compasses mounted in pedestals called binnacles, which contain magnets and steel pieces to counteract the effects of the metal hull. They also depend on gyrocompasses, spinning devices which, once they are set to point north, continue to do so regardless of the magnetic field. However, ordinary magnetic compasses are still widely used in boating, hiking, surveying, and other activities.

SHERRI CHASIN CALVO