magneticnavigation

Magnetic Navigation version 7

by Paul Doherty

Just before we left the last trees to step out onto the exposed north ridge of the Presidential Range of the White Mountains of New Hampshire we saw the sign: "Stop: The area ahead has the worst weather in America. Many have died there from exposure even in the summer. Turn back now if the weather is bad."

The late summer weather hadn't been bad then. But now we were wrapped in cloud; ice was forming on the talus blocks underfoot; it was windy; we were cold and getting colder. My two companions and I had to find shelter soon.

We weren't lost. Even though we were hiking off trail on a large flat field of broken rocks, we knew roughly where we were. The map showed that there was an emergency shelter to the north of us in Edmand's Col. But the cloud prevented us from seeing any landmarks. Which way was north?

I pulled out the compass that I always carry when hiking. The magnetized needle swung around on its pivot in its fluid-filled case and pointed. I had a sinking feeling in the pit of my stomach. I felt that I knew which way north was, and my guess was 90 degrees different from the direction of the compass needle. Should I follow the compass or my own internal sense of direction? We all risked hypothermia or frostbite if I made the wrong choice.

The magnetic compass has been the primary direction-finding tool for navigators for many centuries. However, some investigators have recently suggested that humans have a built-in magnetic sense of direction. There is strong evidence that other animals ranging in size from bacteria to whales have a magnetic sense. So which was more reliable my internal sense of direction or the compass? Let me tell you a little about compasses and a little about the magnetic sense of animals including humans, then I'll tell you what I did in New Hampshire.

The Compass

Written records referring to a "south pointer" indicate that the compass may have existed in China as early as the first century A.D. Jade carvers carved spoon shapes from lodestone, the naturally magnetic ore of the mineral magnetite (Fe3O4). When such a spoon was placed on a brass plate, it rotated on its smooth round bottom until the handle pointed south. (Photo of Rob's Chinese compass). Over the next thousand years, the Chinese refined their "south pointer" making more sensitive and accurate versions. In the 11th century, the Chinese astronomer Shen Gua wrote the following instructions for building a compass;"Rub the point of a needle with a lodestone, then it is able to point to the south....It is best to suspend it by a single cocoon fiber of new silk attached to the center of the needle with a piece of wax. Among such needles there are some which point to the north. I have needles of both kinds by me." When Shen Gua rubbed a needle with lodestone, the needle became a magnet. The needle magnet was then oriented by the earth's magnetic field. Depending on how he rubbed the needle, it might acquire a north pole at its point or a south pole, which is why some of his needles pointed north and some south. An updated version of this eleventh century compass-making recipe appears below in,"To Do and Notice: Making a compass from a nail."

Chinese navigators developed other types of compasses as well. In one, a magnetized iron needle was stuck through a piece of straw and floated on water. Another required no lodestone to magnetize a needle. A fish made of thin steel was shaped like a spoon so that it would float. The fish was held so that its nose pointed south and then heated to cherry red and cooled. When it was floated on water, the fish would point south.

Nowadays, scientists know that when you cool steel through a temperature known as its Curie point, the steel becomes magnetized in the direction of the applied magnetic field, in this case, the earth's magnetic field. (For iron the Curie point is a little over 1200 degrees Celcius.) Each of the iron atoms in a piece of steel is a magnet, with its own north and south pole. When the steel is heated above the Curie point temperature, the thermal jostling of the atoms keeps the iron atom magnets pointing in all different directions. The steel therefore has no net magnetism. At temperatures below the Curie point however, the jostling of the atomic magnets decreases. If the steel cools in an external magnetic field, the atoms will line up in that field. The steel will become magnetized.

In Europe, before the introduction of the compass, sailors used Portulan charts, accurate maps of coastline features marked with windrose lines.(Example see Connections p29) A windrose is a set of radial arrows: large arrows for the major winds( North, East, South and West), and smaller in-between arrows for the lesser winds, such as Northeast. At a given place and time of year in the Mediterranean, the winds blew reliably from one direction. So sailors watched the shore and judged directions by the sun, by the stars and "by the wind."

In the twelfth century, magnetic compasses became available in Europe and revolutionized navigation. The compass worked when the sky was cloudy and was much more reliable than even the steadiest wind. By the end of the thirteenth century, the magnetic compass was in widespread use throughout the Mediterranean. Sailors ventured out into new waters far from sight of land, then found their way back again using the compass.

The windrose evolved into a compass rose. A compass rose has 32 equally spaced direction arrows drawn on a card. A magnet is attached to the card. Then the card is floated on an oily fluid inside of a box so that it is free to rotate. This boxed compass is mounted to the boat. A line, called a lubbers line, is drawn across the center of the boxed compass parallel to the keel of the boat. The heading of the boat with respect to magnetic north can then be read by comparing the lubbers line to the direction lines of the compass rose.

European sailors and compass makers noticed what the Chinese had discovered before them: compasses didn't point to true north, i.e. to a known point in the sky near the north star. They pointed a few degrees to the side. Even worse, compasses pointed further from true north on the voyage to the New World. In the North Altantic Ocean compasses pointed more than 15° from true north. Sailors also noticed that as they sailed north the north end of their compasses dipped downward.

To understand why compasses didn't point exactly to the north, people began experimenting. In 1269, Petrus Peregrinus de Maricourt, a French crusader, published his "Letter on the Magnet," reporting on a series of experiments. Maricourt placed iron slivers onto spheres made of magnetite. The slivers lined up along lines which radiated out from one point on the sphere and came together at a second point on the opposite side. These lines behaved like lines of longitude on the earth which radiate out from the poles. Peregrinus called the two points on his magnets the poles of the magnet. Later, the pole of a magnet that was attracted to the north pole of the earth became known as the "north seeking magnetic pole", which physicists have shortened to the "north pole". (See To do and notice: Seeing the field.)

In 1600, William Gilbert, physician to Queen Elizabeth, published a book on magnets. Like Peregrinus, Gilbert carved small spheres out of lodestone. He called his spheres terellae, or little worlds, and investigated the behavior of magnetized needles in the vicinity of these spheres. The magnetic needles surrounding his terellae pointed toward the magnetic poles. At the sphere's equator, the magnetic needles laid down flat. As a needle was moved toward a pole, the end nearer to the equator stood up more and more. Finally, at the pole itself, the needle became vertical. Since the needles on his terellae behaved like compasses on the earth, Gilbert decided that the earth itself must be a magnet.

The Earth's Magnetic Field

Compasses, are magnets, which are attracted to a bigger magnet &emdash; the earth. The needles on Gilbert's lodestone sphere and compass needles on the earth both trace out the lines defining the magnetic field.

The standing needles on Gilbert's sphere showed why compass needles dipped one end down as sailors approached the pole. A magnetized needle that is free to rotate up and down, called a dip needle, will be horizontal at the earth's magnetic equator and become more and more vertical as the pole is approached because it is aligned with the magnetic field of the earth.

The angle between the horizontal and the direction of the needle is known as the inclination of the magnetic field. The inclination can be used to find your magnetic latitude. When the Australian Antarctic Expedition went hunting for the south magnetic pole in 1913 they used a dip needle and searched for the place where the needle became vertical.

The next scientist to tackle compasses was Edmund Halley, of comet fame. He gathered reports about compass operation from sea captains and himself commanded a research ship for the Royal Navy. Halley verified that compasses did not point to geographic north, (also known as true north), which is the point where the rotation axis of the earth intersects the surface. He also found that the amount of error, now known as the magnetic declination, changed from place to place on the earth. In London, for example, compasses pointed 5 degrees east of true north.

Halley plotted the declinations measured by sea captains onto a map of the earth. Then he made a tremendous invention, an invention used today in many fields of science. Halley drew lines on the earth that connected places with the same declination. In so doing he invented the contour plot.

You see contour plots every day of your life. On weather maps, lines called isotherms connect places with the same temperature; lines called isobars connect places with the same air pressure. On contour maps, like the topographic maps published by the US geological survey, points of constant altitude are connected by contour lines.

Halley realized that the pattern of the magnetic declination, as well as the pattern of magnetic inclination or dip, could be partially explained if the magnetic poles of the earth were not located at the geographic poles. Today, the north magnetic pole is located in Northern Canada, more than 15 degrees of latitude from the geographic north pole. The south magnetic pole is located off the coast of Antarctica.( Map from Mcgraw Hill geomagnetism or Geo Encyclopedia)

Halley also noticed that the magnetic field of the earth was continuously changing. The magnetic declination at London had changed measurably even during Halley's lifetime. Over the 300 years since Halley's time it has continued to change. Compasses in London have swung back and forth over a total of more than 40 degrees of declination, pointing from 11 ° east of true north to 24° west and then back to 5°west today.

Halley's map of magnetic declination was a best seller for decades. A sea captain could use this map and a compass to make a crude estimate of his longitude. If a captain measured declination, then he could roughly locate his position along a line of constant declination on one of Halley's maps. (See To do and notice: Measuring your inclination and declination.)

Our Electromagnetic Earth

Halley thought that the earth was a permanent magnet like piece of lodestone. More recent discoveries show that the earth is an electromagnet. Natural permanent magnets (like lodestone) lose their magnetism when they are heated to their Curie point, a temperature less than 800 °C. The earth becomes hotter with depth, and this Curie-point temperature is reached at the shallow depth of 30 km. Since samples of this upper layer of the earth show that it is not magnetic, and since the interior of the planet is too hot to be permanently magnetized, we know the earth can not be a permanent magnet. So it must be an electromagnet &emdash; a magnet made by the flow of electrical current.

At the turn of the century, Richard Oldham of the Geological Survey of India recorded seismic waves which were generated by earthquakes on the other side of the earth and which had passed through the interior of the earth on their way to India. These seismic waves probed the material they passed through on their journey. Oldham's results have been refined by modern seismologists but are still basically correct. The earth has an outer crust of solid rock a few tens of miles thick. Beneath the crust is a 2,000 mile thick layer called the mantle, it is also made of rock. There is little electrical current in these layers since rock is an electrical insulator. Below the mantle is the core of the earth. Seismic waves suggest that the core is made mostly of iron, they show that the outer half of the radius of the core is liquid iron, while the inner half is solid. Liquid iron is a good conductor of electricity. The billion amps or so of electrical current which makes the earth an electromagnet flows through the liquid iron of the earth's core.

The earth makes its magnetic field with a magnetohydrodynamic dynamo. In simpler language, when the liquid metal conductor of the core of the earth is pushed by convection through a magnetic field, an electrical current is produced which makes its own magnetic field which is added to the original one. In some cases, the sum of these magnetic fields is larger than the original one. In these cases, any small initial magnetic field will be amplified. The initial magnetic field comes from the magnetic field of the sun. The energy for the amplification of the magnetic field comes from the rotation of the earth and the convection of the liquid metal. By the wonders of positive feedback the magnetic field of the earth, "lifts itself by its own bootstraps."

The earth's core metal boils with convection currents. Radioactive decay of elements in the earth heat the metal of the core, causing it to expand and to float upward. When the rising metal cools, it contracts, becomes denser, and sinks back down. These convection currents come and go over geologic time intervals of hundreds of thousands of years. So that the field on the surface of the earth changes with time. By studying changes in the magnetic field of the earth we are actually looking into the roiling molten metal at the center of our planet.

Magnetic minerals in surface rocks record the local magnetism when they cool through their Curie points. And here's the real surprise: the record of the earth's magnetism preserved in this natural rock magnetic recorder shows that the magnetic field of the earth completely reverses at irregular intervals, ranging from a hundred-thousand to ten million years. It is hard to imagine a permanent magnet reversing over and over again, but a self- generating electromagnet, powered by convection currents that come and go could reverse and indeed it does.

My magnetic sense

So the compass had been used for centuries and was well understood. What about my feelings of north? Do humans have a magnetic sense? The behavior of many animals including bacteria, honeybees, and homing pigeons is influenced by magnetic fields, which suggests that these animals have magnetic senses. Other studies show that bacteria, bees and pigeons contain magnetic particles surrounded by nerves which could be the source of their magnetic senses. What about humans?

Experienced human navigators are like pigeons in that they use many clues to navigate, including the sun, the wind and the stars. Tests performed by Robin Baker at the University of Manchester in the 1970s suggested that humans have a magnetic sense. However, when these tests were repeated by several other research groups, no magnetic sense was found. Baker is now contesting the results of these studies.

If teams of behaviorists and biologists can argue for a decade over whether humans have a magnetic sense or not, I wasn't going to figure it out while freezing to death on the top of a mountain. So, when it came time to decide which way to go, I followed the advice my father had given me as he taught me to fly an airplane. He said, "Trust your instruments." So I followed the compass.

Navigating to Safety

To find the direction to the shelter I had to line up the map with the mountains around me, so that the direction to the shelter on the map became the direction on the ground. Since I was inside a cloud and couldn't see the mountains, I aligned the map with the world with a compass. At the bottom of the map were two lines: one pointed true north; the other pointed 17 degrees to the west&emdash;to magnetic north. I placed the compass on my map so that the true north arrow pointed from the center of the compass to the north marker. I then rotated the map and compass together until the compass needle pointed along the magnetic north line. The map was then precisely aligned with the countryside around me. The direction toward the shelter on the map was then the direction toward the shelter on the ground.

The shelter was on a narrow ridge so that small navigation errors would be canceled as we were funnelled toward the shelter. However, a 90 degree navigation error like the one I would have made without the compass would have sent us down the wrong ridge.

We struggled across the broken and slippery talus blocks fighting through the wind. No trace of a trail appeared and I wondered whether I had made the right choice. Then, at last, a pile of stones taller than a man appeared in the mist ahead, it marked the trail which led to the emergency hut.

Before long we were huddled together on a wood floor inside a windowless steel culvert, the Edmand's Col shelter. To us, it was a mansion. A candle illuminated the clouds of mist that steamed from our wet clothes. Outside the wind raged; inside, thanks to a map a compass, and the roiling liquid metal of the earth's core, we were warming up and waiting for a break in the weather.

Sidebar on unfortunate nomenclature.

The names of the magnetic poles of the earth are wrapped in confusion. To physicists, magnets have north poles and south poles. The north pole of a magnet is the one that points toward the north geographic pole of the earth, which is located in the arctic. So the north-seeking end of a compass needle is a north magnetic pole. But, magnetic poles which are unlike attract each other while like magnetic poles repel. This means that the north pole of the compass magnet, the one that points north, must point toward the south pole of the magnet that is the earth. This means that the south pole of the magnet that is the earth is located near the north geographic pole. To make all of this even more confusing, geographers refer to this magnetic south pole of the earth as the north magnetic pole.

notes

Pat, my date for the Chinese compass came from the library's book summarizing Needham's research. It is a very vague date that scholars are still arguing about. The Chinese certainly had magnetic compasses by 1200 AD, they had "south pointers" as early as 100 AD but whether the south pointers were magnetic compasses? maybe.