The magnetic force surrounding a magnet is not uniform. There exists a great concentration of force at each end of the magnet and a very weak force at the center. Proof of this fact can be obtained by dipping a magnet into iron filings (fig. 1-8). It is found that many filings will cling to the ends of the magnet while very few adhere to the center. The two ends, which are the regions of concentrated lines of force, are called the POLES of the magnet. Magnets have two magnetic poles and both poles have equal magnetic strength.
Law of Magnetic Poles
If a bar magnet is suspended freely on a string, as shown in figure 1-9, it will align itself in a north and south direction. When this experiment is repeated, it is found that the same pole of the magnet will always swing toward the north magnetic pole of the earth. Therefore, it is called the north-seeking pole or simply the NORTH POLE. The other pole of the magnet is the south-seeking pole or the SOUTH POLE.
A practical use of the directional characteristic of the magnet is the compass, a device in which a freely rotating magnetized needle indicator points toward the North Pole. The realization that the poles of a suspended magnet always move to a definite position gives an indication that the opposite poles of a magnet have opposite magnetic polarity.
The law previously stated regarding the attraction and repulsion of charged bodies may also be applied to magnetism if the pole is considered as a charge. The north pole of a magnet will always be attracted to the south pole of another magnet and will show a repulsion to a north pole. The law for magnetic poles is:
Like poles repel, unlike poles attract.
Reversal of Earth’ Magnetism
Considering that ships, planes and Boy Scouts steer by it, Earth’s magnetic field is less reliable than you’d think. Rocks in an ancient lava flow in Oregon suggest that for a brief erratic span about 16 million years ago magnetic north shifted as much as 6 degrees per day. After little more than a week, a compass needle would have pointed toward Mexico City.
The lava catches Earth’s magnetic field in the act of reversing itself. Magnetic north heads south, and — over about 1,000 years — the field does a complete flip-flop. While the Oregon data is controversial, Earth scientists agree that the geological evidence as a whole — the “paleomagnetic” record — proves such reversals happened many times over the past billion years.
“Some reversals occurred within a few 10,000 years of each other,” says Los Alamos scientist Gary Glatzmaier, “and there are other periods where no reversals occurred for tens of millions of years.” How do these flip-flops happen, and why at such irregular intervals? The geological data, invaluable to show what happened, registers only a mute shrug when it comes to the deeper questions.
For that matter, why is it that instead of quietly fading away, as magnetic fields do when left to their own devices, Earth’s magnetic field is still going strong after billions of years? Einstein is said to have considered it one of the most important unsolved problems in physics. With a year of computing on Pittsburgh’s CRAY C90, 2,000 hours of processing, Glatzmaier and collaborator Paul Roberts of UCLA took a big step toward some answers. Their numerical model of the electromagnetic, fluid dynamical processes of Earth’s interior reproduced key features of the magnetic field over more than 40,000 years of simulated time. To top it off, the computer-generated field reversed itself.
“We weren’t expecting it,” says Roberts, “and were delighted. This gives us confidence we’ve built a credible bridge between theory and the paleomagnetic data.” Their surprising results, reported as a cover story in Nature (Sept. 21, 1995), provide an inner-Earth view of geomagnetic phenomena that have not been observed or anticipated by theory. Furthermore, the Glatzmaier-Roberts model offers, for the first time, a coherent explanation of magnetic field reversal.
Roughly speaking, Earth is like a chocolate-covered cherry — layered, with liquid beneath the surface and a solid inner core. Beneath the planet’s relatively thin crust is a thick, solid layer called the mantle. Between the mantle and the inner core is a fluid layer, the outer core. According to generally accepted theory — the dynamo theory — interactions between the churning, twisting flow of molten material in the outer core and the magnetic field generate electrical current that, in turn, creates new magnetic energy that sustains the field. “The typical lifetime of a magnetic field like Earth’s,” says Glatzmaier, “is several tens of thousands of years. The fact that it’s existed for billions of years means something must be regenerating it all the time.”
How do we know if the dynamo theory is right? To the consternation of our desire to understand what’s happening inside the planet we live on, Jules Verne’s Journey to the Center of the Earth is still fiction. There’s no way to penetrate 4,000 miles to Earth’s center, nor to monitor fluid motions or magnetism in the outer core.
The Glatzmaier-Roberts computational model may be the next best thing to a guided tour of inner Earth. While other models have given good clues that theory is on track, they have been limited by a two-dimensional approach that required simplifying assumptions. Roberts and Glatzmaier set out to implement a fully three-dimensional model, based on a computer program Glatzmaier developed over many years, that would allow the complex feedbacks between fluid motion and the magnetic field to evolve on their own — in other words, to be solved “self consistently.”
Their objectives, in retrospect, were modest. “Mainly,” says Roberts, “we wanted to get a geomagnetic field that would maintain itself longer than the decay time. No one’s ever done that in a self-consistent manner.” After nearly a year running almost daily, as allocated computing time was about to expire, the model produced its Eureka moment.
By itself, the reversal is strong confirmation of the model, and other details — magnitude and structure of the field — also agree well with surface features of Earth’s field. The simulation also offers precious insight into the dynamics that sustain the magnetic field and generate reversals. Contrary to what anyone guessed till now, the model shows that in the inner core the magnetic field has an opposite polarity from the outer core, and this stabilizes the field against a tendency to reverse more frequently.
“No one even dreamed about this,” says Glatzmaier. “That’s the nice thing about a supercomputer. You can just let it do its thing, solve these equations over and over — a large set of variables affecting each other with nonlinear feedback, very hard to figure out. It’s a beautiful problem for a supercomputer, and it’s really exciting to see this structure and dynamics that no one imagined.”
Earth’s magnetic field evolving for about 9,000 years before, during and after the simulated reversal. The outer circle indicates the fluid outer core boundary; the inner circle, the solid inner core. The left hemisphere shows magnetic field contours directed clockwise (green) and counterclockwise (yellow). The right hemisphere shows contours directed westward (blue) and eastward (red), out of and into the plane of the paper.
The left hemisphere shows that the field penetrating the inner core is opposed in polarity to the outer core, a feature completely unanticipated by theory. “The outer core polarity,” explains Glatzmaier, “is continually trying to invade the inner core. Only when the whole field almost decays away, however [middle], does it finally have a chance to diffuse in. Once it does, the opposite polarity gets established. The inner core polarity is the stabilizing force, like an anchor, the slowest thing that can change.”
At about 36,000 years into the simulation the magnetic field reversed its dipole polarity over a period of only 1200 years. The image below is from an animated sequence showing how the field’s structure changed.