MAGNETISM AND ELECTROMAGNETICS 69 



The product Al represents the volume of the magnetic field 

 and therefore the energy stored in the field per unit volume is 



or 



or 



co = - - ergs (102) 



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When the magnetic field is produced in air the field intensity 

 3C is equal to the flux density 3S, and the energy stored per cubic 

 centimeter is 



co = |^ergs, (103) 



or 



co = ergs (104) 



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48. Stress in the Magnetic Field. The force between two 

 magnetic poles is not exerted at a distance but is transmitted 

 through the medium separating them and the medium is stressed. 



There is a tension along the lines of force or induction tending 

 to shorten them and draw the boundary surfaces of the field 

 together and so decrease the magnetic energy stored in the field; 

 there is also a pressure at right angles to the lines tending to 

 spread them apart, and so decrease the flux density in the field 

 and, therefore, also the energy stored. Since the medium is in 

 equilibrium these two forces are equal in magnitude. 



The pull on the bounding surfaces is usually expressed hi dynes 

 per square centimeter and may be found as follows: 



The energy stored in a magnetic field in air was found to be 



- ergs in Art. 47. 



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Fig. 57 represents a horseshoe magnet with its armature re- 

 moved a distance x. If S is the flux density in the field between 

 the magnet poles and the armature and A sq. cm. is the area of 

 the two pole faces, the volume of the field is Ax cu. cm. and the 

 energy stored in it is 



W = 5 Ax ergs. 



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