148 ELECTRICITY 



action (that is, whether the magnet shall attract or repel 

 a body) varies according to the medium in which the 

 magnet is held ; a body which is attracted by a magnet, 

 while both are in the air, may be repelled by it when 

 they are in some other medium. 



171. Magnetic Field. The fact last stated (170) 

 shows that the magnetic force acts in the medium around 

 the magnet. Simple experiments with a magnet and a 

 compass needle will show that this force can act through 

 considerable distances from the magnet. The whole 

 space in which the magnetic force may be felt is called 

 the magnetic field. Any magnet, then, is surrounded by 

 a magnetic field, the different parts of which have vary- 

 ing intensities of force. 



In describing this field we commonly speak of it as 

 containing lines of magnetic force, or simply lines of 

 force. In that portion of the field where the force is 

 most intense the lines of force are most numerous; that 

 is, there the lines are most densely crowded together. 

 To get an idea of the arrangement of these so-called 

 " lines of force," lay a piece of cardboard upon a magnet 

 and sprinkle iron filings over the card. Try several dif- 

 ferent positions of the card upon the magnet. 



172. How Magnets are made. It is an easy matter 

 to make a magnet of a piece of iron or steel. A body 

 so treated is said to be magnetized. Two methods are 

 generally used. The first method is simply to place the 

 piece of iron or steel near a magnet, better in the more 

 intense part of its field. (Compare this, but do not con- 

 fuse it, with 159.) 



