MAGNET AND MAGNETISM 



3595 



MAGNETO-ELECTRIC MACHINE 



Magnetic Induction. If the nail above re- 

 ferred to is held within a small fraction of an 

 inch of the magnet it will become magnetized, 

 though not as strongly as by actual contact. 

 Large bodies of iron affect the working of a 

 magnetic needle, even when some distance from 

 it; every compass on a ship has to be adjusted 

 so as to counteract the influence of the iron 

 and steel in the ship's framework. This in- 

 fluence is called magnetic induction, and the 

 space within which a magnet influences another 

 body is known as the magnetic field. 



The Earth a Magnet. Many experiments 

 have been made to test the magnetism of the 

 earth. Scientists discovered many centuries 

 ago that the magnetic needle points nearly 

 north and south because the earth is a great 

 magnet, with poles like other magnets. These 

 are called magnetic poles, but they are not lo- 

 cated at the geographic poles, the north mag- 

 netic pole being in Boothia Peninsula, north of 

 Hudson Bay, and the south magnetic, pole lying 

 almost directly south of Sydney, Australia, and 

 about 1,800 miles from the South Pole.. 



Because of the location of the magnetic poles 

 the magnetic needle points directly north and 

 south in only a few places, and the difference 

 between its direction and due north is known 

 as magnetic declination or variation. A- line 

 that passes through places where the needle 

 points due north is called a line of no variation. 

 One of these lines passes through the United 

 States, entering the country on the Atlantic 

 coast about fifty miles south of Charleston, 

 S. C., and passing northward through South 

 Carolina, North Carolina, Tennessee, Kentucky, 

 Ohio and Michigan and crossing the east end 

 of Lake Superior. According to the law of 

 poles the so-called north pole of a magnet is 

 the north-seeking pole, or in reality the south 

 pole, and the so-called south pole is the south- 

 seeking pole. 



History. Magnetism was discovered many 

 centuries ago in a variety of iron ore found 

 near Magnesia, Asia Minor, hence its name. 

 Pieces of this ore were found to attract iron, 

 and the name lodestone was applied to them. 

 A lodestone is a natural magnet, and is some- 

 thing of a curiosity because lodestones are sel- 

 dom found. Magnetic iron ore in which the 

 lodestone is found is widely distributed over 

 the earth, but its magnetic power is very sel- 

 dom strong enough to be detected. C.R.M. 



Consult Jeans' Mathematical Theory of Elec- 

 tricity and Magnetism; Jansky's Elementary 

 Magnetism and Electricity. 



Related Subjects. The reader is referred in 



this connection to the following articles in these 

 volumes : 



Compass Magnetite 



Electromagnet Magneto-Electric 



Electromagnetism Machine 

 Magnetic Needle 



MAGNET '1C NEEDLE, a thin, slender bar 

 magnet suspended by a thread or mounted on 

 a hard, almost frictionless needle-point, so ad- 

 justed that it is free to swing in any direction 

 in which it is influenced. When no magnetic 

 body is near it points approximately north and 

 south, that is, to the north and south magnetic 

 poles. See COMPASS; MAGNET AND MAGNETISM. 



MAG'NETITE. Some sand contains small 

 grains of a jet black color, commonly called 

 black sand. When a magnet is thrust into the 

 sand, the black grains stick to it and are sepa- 

 rated from the sand when the magnet is with- 

 drawn. These black grains are magnetic iron 

 ore, or magnetite. Magnetite is found in large 

 masses as well as in sand. Such masses occur 

 at the Cornwall mines in Pennsylvania, in the 

 Adirondack Mountains and in Sweden. It al- 

 ways has this black, glossy appearance and is 

 very hard. Magnetite is one of the most valu- 

 able ores of iron, for in addition to other ores 

 it greatly improves the quality of the metal 

 produced. 



MAG'NETO-ELECTRIC MACHINE, a de- 

 vice in which a magnet is used to generate an 

 electric current. The machine consists of a 

 powerful horseshoe magnet, in front of which a 

 pair of coils, called an armature, is caused to 

 rotate (see MAGNET AND MAGNETISM). The 

 armature contains a core of soft iron, which ac- 

 quires and loses magnetism as it draws near to, 

 and then recedes from, one of the poles of the 

 magnet. These changes of magnetic strength 

 set up alternating currents in the armature. 

 Their force depends upon the strength of the 

 magnet and the speed with which they are ro- 

 tated. In 1866 an inventor discovered that if 

 the current be passed through the coil of an 

 electromagnet, the force it produpes is far 

 greater than when the permanent magnet is 

 employed. By causing a still larger armature 

 to rotate in front of the electromagnet, the 

 inventor was able to increase the force of the 

 current still further. The industrial importance 

 of this discovery was very great, for it led to 

 the construction of powerful machines in which 

 the fixed magnets were electromagnets. Such 

 machines are called dynamos. Driven by 

 steam, they generate the electric currents that 

 light the streets of cities and furnish power for 



