Purpose: Illustration of magnetic field lines
Supplies: Bar magnet, iron filings, compass
Background and Demonstration:
The magnetic field of the Earth is difficult for students to appreciate because magnetic field lines are invisible. A very common illustration of magnetic fields is possible using a bar magnet and iron filings. [I use iron filings encased in clear plastic filled with glycerine which can be placed on an overhead projector.] Iron filings sprinkled around the bar magnet will, after a bit of gentle tapping or shaking, align themselves with the magnetic field lines of the magnet. That is, they will form into curved lines which lead from one pole of the magnet to the other. To indicate that this is an invisible field, move a compass around the vicinity of the magnet. [I use a clear compass that can be seen on the overhead projector.] The compass needle will follow the lines defined by the iron filings, pointing "into" one of the poles of the magnet, and "out of" the other pole.
We can measure the Earth's magnetic field by measuring the direction a compass points from various points on the surface. Since a compass is free to rotate only in a horizontal direction, we are measuring the horizontal component of the Earth's magnetic field, and the direction of this component from the rotational north pole is the declination. If we had a compass that could rotate in a vertical direction, we would measure the direction of the Earth's magnetic field lines in a vertical plane (much like the compass demonstration above). This is the inclination.
An interesting side point: the needle of a compass is simply a magnet which is free to rotate in the horizontal direction. We define the north pole of a magnet as that pole which points toward north on a compass. Since opposite magnetic poles attract, the Earth's north magnetic pole is actually, by definition, a south pole of a magnet.
Jeffrey S. Barker (SUNY Binghamton) Demonstrations of Geophysical Principles Applicable to the Properties and Processes of the Earth's Interior, NE Section GSA Meeting, Binghamton, NY, March 28-30, 1994.
Questions or comments: jbarker@binghamton.edu
Last modified: March 18, 1996 (content), June 6, 2021 (reformatted and moved to Google sites)