66 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



same direction, indeed, as would be taken by filaments of iron 

 filings at that place. Any region in which a compass needle 

 tends to point in a definite direction is called a magnetic field t and 



Fig. 30. 



the direction of the compass needle (arrow-head thought of as being 

 at the north-pointing end of the needle) is the direction of the field 

 at the place where the compass is located. 



The filaments of iron filings in a magnetic figure' shown in 

 Fig. 30 indicate the trend of what are called the lines of force of 

 the magnetic field. A line of force is at each point in the direc- 

 tion of the field at that point. 



Example. The fine parallel lines in Fig. 3 1 represent the 



lines of force of a magnetic 

 field in which a bar magnet 

 NS is placed, and the heavy 

 arrows /^represent the forces^ 

 with which the field acts on the 

 two poles of the magnet tend- 

 ing to turn it into the direc- 



Fig. 31. 



tion of the field. 



35. Intensity of a magnetic field at a point. A magnetic field 

 has been defined as a region in which a compass needle tends to 



