199. Magnets.—Since the times of the early Greek philosophers men have known of certain stones that have the property of attracting to themselves objects of iron and steel. Such stones are called natural magnets. It is thought by many that the name magnet is derived from Magnesia in Asia Minor, where these stones are abundant, though this is but tradition.
It was also learned long ago that iron and steel objects when rubbed with natural magnets become magnetized, that is, acquire the properties of magnets. These are said to be artificial magnets.
Fig. 169.—A bar magnet.
Fig. 170.—A horseshoe magnet.
Some 800 years ago it was discovered that magnets, natural or artificial, when suspended so as to turn freely, always come to rest in a definite position pointing approximately north or south. This is especially noticeable when the magnet is long and narrow. Because of this property of indicating direction, natural magnets were given the name of lodestone (lode-leading).
Artificial magnets are made by rubbing steel bars with a magnet or by placing the steel bar in a coil of wire through which a current of electricity is flowing. The magnetized steel bars may have any form, usually they are either straight or bent into a "U" shape. These forms are known as bar and horseshoe magnets. (See Figs. 169 and 170.) Magnets retain their strength best when provided with soft-iron "keepers," as in Fig. 171.
Fig. 170.—A horseshoe magnet.Fig. 171.—Bar magnets with keepers.
200. Magnetic Poles.—If a magnet is placed in iron filings and removed, the filings will be found to cling strongly at places near the ends of the magnet, but for a portion of its length near the middle no attraction is found. (See Fig. 172.) These places of greatest attraction on a magnet are called poles. If a bar magnet is suspended so as to swing freely about a vertical axis the magnetic pole at the end pointing north is called the north-seeking pole; at the other end, is the south-seeking pole. In most places the needle does not point to the true north, but somewhat to the east or west of north. The direction taken by a magnetic needle is parallel to the magnetic meridian.
