© Duane A. Cline 1999
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Navigation: The Mariner's Magnetic Compass By the late sixteenth century, the mariner's compass had evolved into an instrument not very different from the compass of
today. The case in which the compass itself is housed was made of wood or ivory in the early days. After that, brass came
into use since it does not affect the magnetic needle. The Pole Star (Polaris) served as the seaman's lodestar (or star that shows the way). Therefore the magnetic stone which
was used to magnetize the compass needle was called a lodestone. The magnetic, direction-finding property of the
lodestone had been discovered in China as early as the twelfth century. By the sixteenth century, the mariner's compass was made with a soft iron wire bent to a lozenge shape and attached to the
underside of a circular compass card, which was suspended at the center on an upright needle. Because the iron wire tended to lose its magnetism over a period of time, it was necessary for each ship to carry a good
lodestone to re-magnetize the wire when it weakened. There was one basic problem encountered in the use of the mariner's compass: The magnetized wire in the compass was
drawn by large land masses. This caused the compass to have variations in its readings. The mariners and mathematicians
of this early period were concerned about this problem, and a number of corrective measures were tried. However, at the
time the Mayflower sailed in 1620, the problem had not been satisfactorily solved. In order to familiarize the pupils with the principles of the lodestone or the magnetized needle, the teacher may wish to
conduct some classroom experiments. Using either a lodestone or a good, strong magnet, rub one end of a small, steel
sewing needle repeatedly -- moving it against the needle in the same direction with each stroke. After the needle is
magnetized, set it gently in a saucer or bowl of water and watch it align itself in the north-south position. Once the needle is functioning in its north-south alignment, try moving iron or steel objects around the bowl to show how
land masses would have caused a variation in the compass reading.
Last modified October 28, 1999
by
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