8o 



ELEMENTS OF ELECTRICITY AND MAGNETISM. 



The arrangement in Fig. 4.5 is equivalent to the arrangement 

 shown in Fig. 46. In the arrangement shown in Fig. 45 the 

 magnetic flux which crosses from AB to A' B' comes up 

 through the steel at C and goes down through the steel at D. 

 Figure 46 shows the flat ends of two massive steel or iron bars 

 which are magnetized so that the face of one bar is a north pole 



steel 



steel 



N 



S 



steel 



B B r 



Fig. 46. 



and the face of the other bar is a south pole as indicated. In this 

 case, the magnetic flux comes up to the polar areas through the 

 steel at C and D, Fig. 46. The two magnet poles in Fig. 46 

 act on each other in the same way as the two magnet poles in 

 Fig. 45, and equations (24) and (25) apply to both figures. 



Tension of the lines of force. The force attraction of the two 

 poles in Figs. 45 and 46 is due to the tension of the lines of force. 

 It is desirable to express this tension in terms of the field inten- 

 sity, and for this purpose the force of attraction of the two poles 

 must be expressed in terms of the field intensity between them, 

 instead of being expressed in terms of the strengths of the two 

 poles, as in equation (24). The strength of each pole may be 

 expressed in terms of the intensity of the field between the poles 

 by solving equation (25) for in. This value of m may then be 

 substituted in equation (24), giving 



F = 



sff* 

 STT 



