A Medieval seaman's
code declares that any sailor caught tampering with the ships lodestone
".....shall, if his life be spared, be punished by having the hand
which he most uses, be cut and fastened by a dagger or knife thrust
through it, to the mast or principal timber of the ship....." In
the history of science, this strict code is vivid evidence of human's
early dependence upon magnetism. Lodestone was used for navigational
purposes in a compass and without a compass, captains could not find
Explain that a magnet has a north pole and a south pole. North and south
will attract, but "like" poles will repel each other.
Magnetism is created when electrons "spin" in the same
direction. In some substances this occurs naturally, but in most
substances it does not. Items can be magnetized by hitting them so that
all the electrons "spin" in the same direction.
The Earth acts as a huge
magnet. As a result, pieces of iron or steel which lie in a North -
South direction often became magnetized by "induction."
Induction means the "forcing" of an object. If such objects
are jolted while in this position, their molecules line up and they
become more easily magnetized. Thus, some materials can be magnetized by
tapping them with a hammer.
- Gently tap one end of an iron rod or bolt
about 20 times while holding it in a North - South position. Then
try to pick up some iron files. Best results are obtained if the rod
is located along the earth's magnetic lines of force. Since we are
in the Northern Hemisphere, tilt the north end of the rod downward.
It may be necessary to try various angles of inclination before the
rod is properly aligned with the Earth's magnetic line of force. If
you live near a railroad track, a discarded iron tie works great.
- Try this experiment with your students and
see if it works. Sometimes it sounds easier than it is and
conditions have to be ideal. Some students will get it to work,
others won't. Have the students try and figure out why it did or
didn't work. Have students experiment at home under supervision.
Make sure they hammer the piece of iron, and not their fingers!
- You may want students to present the
information they researched in the Post Lab.