ELEMENTARY ELECTRICITY AND MAGNETISM. 13 



When a piece of iron is placed in a magnetic field the original 

 trend of the lines of force in the field is greatly altered. Thus 

 Fig. 4 shows the effect of a small piece of iron upon the magnetic 

 field between two flat-ended magnet poles. In the absence of 

 the small piece of iron the field is as shown in Fig. 5. 



The effect of a piece of iron in a magnetic field is always such 

 as to suggest that " iron is a better carrier of lines of force than 



Fig. 5. 



air." The lines of force tend to converge into the piece of iron 

 and pass through it. 



9. The magnetic field due to an electric wire. A wire through 

 which an electric current is flowing is called an electric wire for 

 brevity. The lines of force of the magnetic field produced by an 

 electric wire encircle the wire. A north-pointing test pole tends 

 to move round the wire in one direction and a south-pointing test 

 pole tends to move round the wire in the opposite direction. 



