By Robert M. Hazen, Ph.D., George Mason University
Perhaps the first researchers to conduct extensive studies on magnets were magnetic compass makers themselves. If one tries building a compass, one to understand how magnetism works, how to magnetize needles, etc, then a piece of unmagnetized iron, like a nail, for example, can be magnetized simply by stroking it with a magnet.
The Study of Compass
The concepts of mass, motion, energy and forces are the central attributes of the physical universe. One of the most important and mysterious forces in Newton’s day was magnetism, causing one end of an iron needle to accelerate in the direction of the Earth’s North Pole.
The study of magnetism was, of course, of extreme importance 500 years ago, because it was an age of ocean exploration, and the compass was the key tool in preventing sailors from getting lost, especially in days when the Sun and the stars were not visible. So the compass—a device that had a magnetically aligned needle that pointed north and south—this simple object played such a vital role in the discovery of the natural world.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Magnetite and the Phenomenon of Magnetism
Magnetic rocks, called magnetite or lodestone, were discovered by ancient Greeks. They were discovered in a region of Asia Minor called Magnesia. These rocks attracted pieces of iron, and this naturally occurring material began the study of magnetism. Magnetite is an abundant ore of iron that’s found all over the world and so it was widely available to the ancients, and the phenomenon of magnetism, therefore, was understood and observed quite early on.
A few basic features of magnets were well known to the ancients. A magnet suspended by a string, for example, can pivot, and a second magnet can apply forces to that. The forces acting as the magnet swing around under the influence of a magnetic field. The compass then is merely a magnet that’s suspended on a pivot point, and it can swing around in the Earth’s magnetic field.
When two magnets are brought together, the opposite poles attract each other, called north and south poles, but like poles—two north poles or two south poles—will repel each other.
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Robert Norman’s Research on the Magnetic Compass
Robert Norman, who lived from about 1550 to 1600, was an instrument maker, and a British sailor for many years. He became a maker of navigational instruments following his career as a sailor. Rather than simply duplicating the design of others, however, Norman attempted to improve the seagoing compass.
He then wrote some of his experiences in a book called The Newe Attractive, published in 1581. And this described the curious tendency of a compass needle to dip. The dip was somewhat of a nuisance to compass makers. This was the tendency of a compass needle not only to align itself north and south but also to adopt an angle dipping into the Earth.
Norman described his experience and frustration in trying to make a compass needle. This is why he was trying to correct a particularly beautiful and decorated needle. He created such a needle for a very elaborate compass, and the trouble was it kept dipping down touching the bottom of his compass, and so it didn’t work very well.
Robert Norman’s Solution
What Norman did was a very clever idea. He took the needle and put it through a cork, so it was precisely balanced. He then had it floating in the water, and the needle then would orient itself in water, it would actually orient itself up and down, so he was able to measure the magnetic needle by exactly balancing the weight of the needle in water.
Norman concluded by suggesting that the compass needle acquires a virtue in spherical form extending about the stone. The idea of this virtue extending about the stone was the first real suggestion that there was such a thing as a magnetic field—imaginary lines extending out from the magnetic object, which then interacted with the Earth’s magnetic field.
He also saw a very practical side to this magnetic dip that he was studying. He realized that since the dip varies with latitude on Earth, it might provide extra navigational aid. Sailors could actually use the dip of the compass needle to help locate where they were, not just north and south, but also how far north or south they were.
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William Gilbert’s Proposals about Magnets
Various ideas about magnets were synthesized in the research of the English physician and physicist, William Gilbert, who lived from 1544-1603. His great scientific work is De Magnete, published in 1600.
Gilbert cataloged the properties of magnets. He demonstrated, for example, that every magnet had two poles. He also showed that the broken magnet also had a north and south pole. Gilbert proposed that the Earth itself was a giant magnet with its own field, with its own north and south poles.
Common Questions about the Discovery of Magnet and Magnetic Compass
A magnetic compass is a device that helps sailors find their location. This device has a needle that is magnetically aligned so that it can point north and south.
At a time when exploring the oceans was key to discovering the world, understanding magnetism, and a tool such as the magnetic compass was crucial in helping sailors find their way in open seas.
Although the magnetic dip of the needle was a problem in the magnetic compass, Robert Norman was able to find its practical aspect and utilize it to discover location and distance, plus as an extra navigational assistance.
