PART II : MAGNETISM 



CHAPTER XIII 



GENERAL ACCOUNT OF MAGNETIC 

 ACTIONS 



Natural and artificial magnets PolesMechanical action of poles on 

 each other The two poles have opposite mechanical actions In a well- 

 magnetised bar the poles are near the ends The two poles always ac- 

 company each other in a magnet The two poles are equal in strength 

 The magnetic action takes place equally well through most media In- 

 duced magnetisation Magnetisation can take place without contact 

 Retentivity and permanent magnetism The earth is a great magnet 

 Magnetisation induced by the earth Methods of magnetisation Single 

 touch Divided touch Double touch The electro-magnetic method 

 Electro-magnets Ball-ended magnets Horseshoe magnets Compound 

 magnets Distribution of magnetisation Consequent poles Magnetisa- 

 tion chiefly near the surface Saturation and supersaturation Preserva- 

 tion of magnetisation Armatures Portative force Nickel and cobalt 

 magnets The magnetic field Lines of force Mapped by iron filings. 



Natural and Artificial magnets. Magnetite, a mineral found 

 in various parts of the world, having the composition Fe 3 O 4 , has been 

 known for ages to possess the property of attracting small pieces of 

 iron. It was found by the ancients near Magnesia, in Lydia, and from 

 the locality, probably, the Greeks gave it the name yuayyr??, whence 

 we derive the name magnet. A piece of the mineral exhibiting 

 the attractive property is now called a " natural magnet, 11 to 

 distinguish it from the steel magnets which are made by processes 

 described hereafter. 



If a natural magnet is dipped in iron filings it is usually found 

 that the filings cling in tufts, chiefly about two points at opposite 

 ends of the specimen, and if the magnet is suspended so that the 

 line joining these two points is horizontal, this line will in this 

 part of the world set nearly North and South, and from this 

 directive property the mineral acquired the name "lode-stone," 

 that is, way-stone. 



If a bar of steel is stroked in one direction with one end of a 

 natural magnet, the bar itself becomes a magnet, acquiring the 

 property of attracting iron filings, and the tendency to set North 



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