Thermal magnetization and then some.


Objective: Help the students understand the formation of magnetic stripes on the ocean floor and the magnetic reversals in other cooled magmas.

Materials needed:

Demonstration:

Place petri dish on overhead and fill with water. Carefully place needle on water. The eye of the needle is easily recognizable on the screen. If the needle is at all magnetic, it will orient itself along the horizontal projection of the local magnetic field lines. It is important that the dish is made of a hydrophilic material, that is, that the meniscus laps onto the side of the dish. That pushes the needle toward the center of the dish. Using the magnet, the direction of the magnetization of the needle can be determined. Removing the needle, heating it and allowing it to cool while on the magnet can alter the direction of its magnetization. Re-floating it and repeating the procedure nicely demonstrates thermal re-magnetization.

Added benefit: Illustration of surface tension. Soap at the end of a toothpick when touched to the surface of the water reduces the surface tension and accelerates the needle away from the spot where the soap was introduced. This too can be done several times before the needle finally sinks.

Illustrated are the materials needed for this experiment, highlighted against a geology textbook diagram explaining magnetic polarities on Earth.