ELEMENTARY THEORY OF MAGNETISM. 



" north pole" of the needle points* in what is called the direction 



of the field. Thus the entire region surrounding the earth is a 



magnetic field; the region surrounding a magnet is a magnetic 



field ; and the region surround- 



ing a wire in which an electric 



current is flowing is a mag- 



netic field. Figure 4 shows a 



compass needle placed at a 



point p near a large magnet. 



The dotted arrow shows the 



direction of the magnetic field 



at p. 



rr^j . j f The dotted line shows the direction in 



The poles of a magnet are which the small magnet a t p points. 

 acted upon by equal and oppo- 



site forces when the magnet is placed in a magnetic field. (This 

 statement refers to what is called a uniform magnetic field, see 

 Art. n.) 



large magnet 



NJ 



- 4 - 



Fig. 5a. 



Figure 50 is a photograph of the figure obtained by dusting 

 iron filings on a pane of glass which is placed over two large 



* If the needle is perfectly balanced. 



