Compass probes Earth’s interior |

Compass probes Earth’s interior

The compass needle does not point north. It points, rather, to a spot in northern Canada, about twelve degrees south of the geographic pole – backpackers have to allow for this difference, the “magnetic declination”. At 70 miles per degree of latitude, the “north magnetic pole” lies roughly 900 miles south of the true north pole.

Earth is a magnet. One might imagine a large chunk of naturally-magnetic lodestone, “direction stone,” lying in the center of the Earth, but there’s a problem with this picture.

Magnets are magnetic because their atoms, which are themselves tiny magnets, are arranged in regular patterns. Arranged randomly, the atomic magnets would cancel each other out.

Heat makes atoms bounce around, messing up their regular arrangement; heated hot enough, a substance melts, then vaporizes, and liquids and gasses are especially messy arrangements of atoms.

Even below its melting point, a magnetic substance has a “Curie temperature,” above which its atoms are so messed up it’s no longer magnetic.

Temperatures within the Earth are well above the Curie point of any known substance. A “permanent ” magnet cannot remain magnetized if it is buried deep in the Earth.

Rather than a permanent magnet, there is within the Earth another planet – a sphere the size of Mars, made up of iron and nickel (stainless steel!); so squeezed, it’s denser than lead; so hot, it’s molten, with the viscosity of water.

Sailors who survived the experience sometimes observed that a nearby lightning strike could mess up a compass.

In the winter of 1820, the connection between magnetism and electricity was demonstrated in the lab.

The demonstration is easy: Wrap a wire several times around a compass, parallel to the needle. Connect the ends of the wire to a flashlight battery. The current in the wire generates a magnetic field that twists the needle away from north.

Same trick backwards: Spin a conductor through a magnetic field and you create an electric current in the conductor, as the alternator in your car does to recharge the battery.

As the liquid iron core of the Earth cuts through the Earth’s magnetic field, electrical currents are generated within the metal. These currents generate the Earth’s magnetic field through which the metal spins.

Both electromagnet and electrical generator, Earth is a “self-exciting dynamo.”

Fluids are especially prone to turbulence. Sloshing and splashing of the Earth’s liquid core could explain why the Earth’s magnetic field is constantly changing.

Comparing ships’ logs from year to year, century to century, demonstrates that the compass does not point in the same direction for any great length of time; the magnetic pole drifts.

And the geomagnetic field also varies in strength. At times in the past, the field has actually disappeared.

Earth’s magnetic field may be disappearing right now. Since the mid-1800s, the strength of it has dropped at a rate that, should this continue, will bring it down to zero within 2,000 years.

When lava freezes to become solid rock, and when sediments fall to the floor of the sea, tiny crystals of magnetite (lodestone) align themselves with the geomagnetic field.

Studying the orientations of these microscopic magnets, scientists have found that, after the geomagnetic field dies away, it quickly re-builds. But there’s nothing that says it has to re-build in the same direction as it pointed before.

Should the Earth’s magnetic field fall to zero, it may very well re-build in the reverse direction. A compass needle would then not point even roughly toward the north; it would, instead, point south.

Alan Stahler trained as a biologist and is an amateur astronomer. He teaches enrichment classes for children and adults at Sierra Friends Center. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).

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