THE MAGNETIC FIELD 



127 



It is not necessary for the magnet to touch the nail in order that 

 the nail may become a magnet. If the two are simply brought near 

 each other, the nail takes on magnetic qualities. We say it is mag- 

 netized by induction. Before a bit of iron is drawn to a magnet, it is 

 magnetized by induction, and its N-seeking pole is attracted by the 

 S-seeking pole of the magnet, while its S-seeking pole is repelled by the 

 S-seeking pole of the magnet. 



The metals cobalt and nickel are also attracted by a magnet, but 

 less than iron. One oxide of iron, the magnetic oxide, is also attracted, 

 but rust is not. 



138. The Magnetic Field. That a magnet can affect 

 a magnetic needle, or a piece of nickel or iron, without 

 touching it, shows that a magnet affects surrounding 

 space; that it has about 

 it a " region of influence.' 7 

 This region is called the 

 magnetic field (Fig. 114). 

 It extends in all direc- 

 tions from the magnet, 

 but the strength of mag- 

 netic attraction, like that 

 of gravitation (cf. 20), 

 becomes less as the dis- 

 tance from the magnet 



increases. The magnetic needle may be enclosed in glass, 

 but this does not cut off the magnetic field. Air is not 

 necessary, for the magnetic influence is felt in a vacuum. 



FIG. 114. 



The filings arrange themselves parallel with the 

 magnet's "lines of force. " 



We can readily trace the magnetic field, in cross section (Fig. 114), 

 by placing the magnet under a sheet of glass or cardboard, and sprin- 

 kling iron filings upon the glass or cardboard cover. When the cover is 

 tapped gently, the filings arrange themselves end to end in the magnetic 



