THE MAGNETISM OF IRON. 



79 



netic field which is due to AB, its intensity is equal to 

 throughout the whole region occupied by the pole A'B' ' , ac- 

 cording to equation (i), and therefore the force which is exerted 

 upon A'B' is equal to the product of the strength of A'B' 

 and the intensity of the field due to AB, whence we find : 



F= 



(24) 



in which F is the force in dynes with which the two poles in 

 Fig. 45 attract each other. It is noteworthy that this force is 

 independent of the distance d, provided 

 the distance d is small in comparison 

 with the length and breadth of the polar 

 areas AB and A'B'. 



To find the intensity of the field in the 

 region between the flat poles in Fig. 4.5. 

 The north pole AB, Fig. 45, tends to 

 produce in the region RR a uniform mag- 

 netic field directed towards the left, of 

 which the intensity is 2Trm/s, whereas 

 the south pole A' B' tends to produce in 

 the region RR a uniform magnetic field 

 directed towards the right, of which 

 the intensity is 27rm/s, and the net re- 

 sult is that the magnetic field intensity 

 in the region RR is zero, or, in other 

 words, no lines of force traverse the re- 

 gion RR. In a similar manner it can be shown that no . lines of 

 force traverse the region R'R 1 '. In the region between AB and 

 A'B' each magnet pole tends to produce a magnetic field towards 

 the right of which the intensity is 2'n-mfs, so that the actual in- 

 tensity H of the field between AB and A'B 1 is 



fdae view of two fiat poles 



Fig. 45. 



Jti = 



(25) 



SCIENCE. 



