The Chinese Invent the Magnetic Compass

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The Chinese Invent the Magnetic Compass

Overview

Sometime before the fourth century b.c. the Chinese noticed that certain minerals, properly prepared, always pointed to the south. The mineral was magnetite, commonly called lodestone, and it was made into what the Chinese called a "south pointer." For many centuries, these were used primarily for mystical purposes. By the eleventh century a.d., the Chinese had turned their south pointer into a magnetic compass, and a century later this device spread to both Europe and the Islamic world. The magnetic compass would eventually find its most significant use as an aide to naval navigation during the late Middle Ages.

Background

The Earth has a planetary magnetic field, a fact known by every Boy Scout, hiker, and sailor. Like most planets, the geomagnetic field is roughly aligned with the planet's rotational axis with the result that magnetic north points towards the North Star, above the North Pole in the sky.

Due to the physics of magnetic fields, any magnetized objects are attracted to each other. Any magnetized object has what is called polarity; it has a north and a south magnetic pole and the magnetic field lines of force go through space between the two poles. Opposite poles are attracted to one another, so a south magnetic pole is attracted to a north magnetic pole and vice versa. Anyone playing with magnets witnesses this when they notice that, with one orientation the magnets stick together while, flipped, they repel one another.

Compasses are no different. The simplest compass is merely a magnet that is allowed to hang or float freely so that it can align itself with the Earth's magnetic field. The compass' south pole will try to connect with the Earth's north magnetic pole and so it will swing to the north if allowed, while the north pole of the compass will swing to the south. So, if we take a magnet shaped like a needle, the needle will align itself with the Earth's magnetic field and will point north and south.

Sometime before the fourth century b.c. an unknown Chinese person noticed that the mineral magnetite is naturally magnetic. How this was noticed may never be known, but it can be surmised that the magnetite was spontaneously attracted to another piece of magnetite or to an iron implement. In any event, this property is one of the defining characteristics of magnetite, and large pieces of pure magnetite are quite magnetic. Some time later (again, it is not certain when), it was noticed that pieces of magnetite would spontaneously line themselves up to point to north and south and that implements made of magnetite (such as needles) would do this, too. Special conditions were required; specifically, the magnetite had to be freely suspended on a silk thread or floating on a card in water. In some cases, the magnetite was hammered flat and floated on the water itself, staying afloat with surface tension. There is no doubt that, by the fourth century b.c. this phenomenon was so well know as to be taken quite for granted in surviving Chinese texts from that time.

At first, the Chinese formed the magnetite into shapes that could be used to point south (the Chinese preference). These shapes included needles, fish, turtles, and even spoons. In fact, one of the most common shapes was the spoon, resembling the Big Dipper, whose "pointer stars" also point to the north. Still later, the Chinese also discovered that magnetite could be used to magnetize steel and that steel needles could also be used in a compass.

Impact

For over 1,000 years the compass was used primarily to help align houses and other buildings in accordance with the Chinese practices thought to bring luck and positive energy. It was thought that lining buildings up in exact directions would allow energy to flow more easily into a house, bringing good fortune to those living and working within. It may have been thought that the forces acting to align a compass with north and south were the same the builders were trying to harness, but this is only speculation. However, it is certain that the first use to which magnetic compasses were put was geomancy.

These first devices were shaped like spoons, formed from pure magnetite with the handle being the north magnetic pole of the mineral. When set on a bronze platter marked with the compass directions, the spoon would slowly turn so the handle would point directly south, giving builders an accurate alignment for a new building.

Over time, the Chinese army took notice of this device and realized it could also be used to help soldiers maintain their bearings on cloudy days. In fact, this use of lodestone was not new. The first definite mention of a compass in world literature—written in the fourth century b.c., possibly by the philosopher Su Ch'in—notes, "When the people of Cheng go out to collect jade, they carry a south-pointer with them so as not to lose their way." However, this use seems to have escaped the notice of the military until several centuries later. The earliest mention of this use is in a book written in a.d. 1044 by Tseng Kung-Liang, but it is noted with such familiarity that it was almost certainly a well-known technique dating back to much earlier times. Similarly, the first use of a compass in marine navigation is noted in a book written in the tenth century a.d. but, again, this is noted as a common fact and not as a novelty. This also suggests a use dating back for many years rather than being a more recent innovation.

In these uses, the compass was not revolutionary, but was a huge improvement over existing tools. Its impact was not that it made navigation possible or even reliable, but that navigation was made more dependable under all conditions. No longer would a ship have to wonder through days of cloudy weather about its direction of travel; instead, the captain had simply to float a magnetized needle to determine his direction of travel. It must also be remembered that, at that time, ships rarely sailed out of sight of land, so most navigation consisted of simply looking for familiar landmarks. This is why the compass was more of a navigational aid rather than a revolutionary device. It was not until open-ocean sailing began in the fifteenth century that the magnetic compass really came into its own. When ships routinely sailed for days or weeks out of sight of land, the magnetic compass was indispensable and, in fact, such sailing likely would not have become routine without such devices.

The increasing use of compasses also had a somewhat wider, less obvious impact than its adoption for geomancy and navigation. In their quest to make better and more reliable compasses, the Chinese began experimenting in a number of areas. This led them to better steel-making techniques, and also to make some important observations about the physics of magnetized materials that were several centuries ahead of the rest of the world.

With magnetized metals in particular, the Chinese noticed that iron could be made magnetic by stroking it with a lodestone. This induced magnetism, however, faded rapidly with time and required renewal. Steel, however, stays magnetic longer, and the Chinese began experimenting with different "recipes" to try to find a steel that would stay magnetized as long as possible. What they found was that steels of higher purity and with a higher carbon content made the best magnets and, incidentally, were also harder and more durable than low-carbon steels.

In other experiments, the Chinese also noted that, when heated to red-hot, steels would lose their induced magnetism. This temperature and its significance is now called the Curie point, in honor of Pierre Curie (1859-1906), who explained it just about a century ago. It must be noted that the Chinese only observed this phenomenon and did not understand it. However, notice it they did, and they made use of their observation quite frequently.

At the same time, the Chinese noticed that steels would become spontaneously magnetized when they cooled down below the Curie point. This was shortly followed by noting that a steel needle could be aligned north-south during cooling and, by so doing, it turned into a permanently magnetized needle, suitable for use as a compass. What happens is that, when cooling through the Curie point, the needle picks up and "locks in" Earth's magnetic field. This becomes the needle's magnetic field, and lets it be used as a compass afterwards. Again, it must be emphasized that the Chinese did not understand the physics behind this phenomenon, and it is likely that they made no effort to do so. Even if they had, their understanding of science at that time simply was not up to the task and their explanations would likely have been more like magic or superstition than what we consider science. However, this is, to some degree, immaterial because they were the first to notice the phenomenon and to make use of it. Science usually proceeds by trying to explain an observation; without the initial observation there is nothing to try to explain, and science is not advanced. Therefore, the Chinese observations were as important to the progress of science as were later explanations of these effects.

In summary, it can be said with a fair degree of certainty that the Chinese were the first to notice the magnetic and "south-pointing" properties of magnetized materials. Although they first used these properties for geomancy, they also put them to use for direction-finding, which became the most significant use of such materials nearly 2,000 years later. The use of magnetic materials also spurred improvements in Chinese metallurgy and, later, caused Chinese scientists to make some interesting physical observations that further advanced the future manufacture of compasses and other magnetic devices.

P. ANDREW KARAM