94 



ELEMENTS OF ELECTRICITY AND MAGNETISM. 





A* S OFCOIL 



a long coil of wire is exactly equivalent to a magnet in so far as 

 its relation to surrounding objects is concerned, lines of magnetic 

 force flow out of one end of the coil through the surrounding 



region and into the other end 

 of the coil in the same way 

 that lines of force flow out 

 from the north pole of a mag- 

 /' net through the surrounding 

 region and in towards the 

 south pole of the magnet. 



The behavior of a magnet in 

 .., the neighborhood of a long 

 straight electric wire. The 

 small circles in Figs. 59 and 

 60 represent the section of a 

 long straight wire in which 

 current is flowing towards* 

 the reader. Figure 59 shows 

 the forces FFwith which the 

 magnetic field due to the wire 

 acts on the poles of a moder- 

 ately long magnet, and Fig. 60 shows the forces FF with which 

 the magnetic field of the wire acts upon the poles of a very short 

 magnet. Thus, a long magnet is drawn towards the wire, 

 although the forces acting on each pole are at right angles to the 

 dotted lines in Fig. 59, whereas a very short magnet is not per- 

 ceptibly attracted by the wire because the two forces FF in 

 Fig. 60 are very nearly opposite to each other in direction. The 

 north pole of a magnet tends to move around the wire in one 

 direction and the south pole of a magnet tends to move around 

 the wire in the opposite direction. Thus, the north pole tends to 



* In representing a flow of current towards the reader in the section of a wire, a 

 dot is used as if one were looking at the point of an arrow, and, when representing a 

 flow of current away from the reader, a cross is used as if one were looking at the 

 feathered end of an arrow ; thus, O represents a flow of current towards the reader 

 and represents a flow of current away from the reader. 



Fig. 58. 



