Page:Elementary Text-book of Physics (Anthony, 1897).djvu/274

260 are most strongly manifested. These regions are called the poles of the magnet. The line joining two points in these regions, the location of which will hereafter be more closely defined, is called the magnetic axis. An imaginary plane drawn normal to the axis at its middle point is called the equatorial plane.

If the magnet be balanced so as to turn freely in a horizontal plane, the axis assumes a direction which is approximately north and south. The pole toward the north is usually called the north or positive pole; that toward the south, the south or negative pole.

If two magnets be brought near together, it is found that their like poles repel and unlike poles attract one another.

If the two poles of a magnet be successively placed at the same distance from a pole of another magnet, it is found that the forces exerted are equal in amount and oppositely directed.

The direction assumed by a freely suspended magnet shows that the earth acts as a magnet, and that its north magnetic pole is situated in the southern hemisphere.

If a bar magnet be broken, it is found that two new poles are formed, one on each side of the fracture, so that the two portions are each perfect magnets. This process of making new magnets by subdivision of the original one may be, so far as known, continued until the magnet is divided into its least parts, each of which will be a perfect magnet.

This last experiment enables us at once to adopt the view that the properties of a magnet are due to the resultant action of its constituent magnetic molecules.

239. Law of Magnetic Force.—By the help of the torsion balance, the principle of which is described in §5 109, 253, and by using very long, thin, and uniformly magnetized bars, in which the poles can be considered as situated at the extremities. Coulomb showed that the repulsion between two similar poles, and the attraction between two dissimilar poles, is inversely as the square of the distance between them.

A more exact proof of the same law was given by Gauss, who calculated the action of one magnet on another on the assumption