Page:Popular Science Monthly Volume 34.djvu/303

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THE

POPULAR SCIENCE

MONTHLY.

 

JANUARY, 1889.


 

THE GUIDING-NEEDLE ON AN IRON SHIP.
By Lieutenant-Commander T. A. LYONS, U. S. Navy.

THERE is an agency that pervades the earth and is peculiarly resident in all its iron. It is magnetism. This force is akin to electricity, though not identical with it, and the manifestations of both are often similar.

The small steel wire, scarcely larger than a sewing-needle, which constitutes the mariner's compass—every iron vessel, even the huge steamship City of New York, and the earth itself—all have certain properties in common that warrant classing them as magnets; and, as the ship sails the earth and is guided by the compass, there is a very intimate though varying relationship between these three that should deeply interest those who traverse the ocean. To describe this relationship, its contentions, and the constant struggle of each member for mastery, rather than their amicable companionship, is the object of this article; and it will render our ideas of the subject clear if we begin by stating the properties of the ordinary bar-magnet. The needle, the ship, and the earth are but magnets of different size.

The Steel Bar-Magnet.—Fig. 1 represents a steel bar which has been magnetized. Its centers of power are located close to each extremity, while near the middle is a neutral ground over which the influence of neither end predominates. If fine iron filings be sprinkled around the magnet, they will form into curved lines emanating from each center, and eventually trending toward a union.

These centers are called poles. The magnetism in one is opposite in kind and equal in degree to that in the other; there is a mutual attraction between these opposite magnetisms, and this tendency to rush across the neutral ground, and, by combining,