78 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



Magnetic lines of force emanate from both faces of the polar area 

 s as shown in the edge view in Fig. 44, and the magnetic field on 

 each side of the flat pole is a uniform field (except, of course, near 



polar area 



a_ square 



centimeters 



edge view 



flat view 



Fig. 44. 



the edges, but the polar area is assumed to be so large that the 

 edge complications may be ignored). Let H^ be the intensity 

 of this field. Then H^s* is the magnetic flux passing out from the 

 polar area on each side, zH^s is the total flux emanating from 

 the pole, and this must be equal to %jrm according to Art. 39, 

 so that we find : 



ii, = 



Consider two similar flat magnet poles AB and A'B' placed 

 side by side as shown in Fig. 45, one being a north pole and the 

 other a south pole, as indicated in the figure. Consider the mag- 

 simple. These simple modes of distribution of magnet pole, and long straight wires 

 carrying electric currents are never met with as actual facts, but they are possible and 

 therefore legitimate as starting points for the development of simple mathematical 

 theory. 



* This expression ignores the non-uniformity of the field near the edges of the flat 

 pole. 



