MAGNETISM AND ELECTROMAGNETICS 



43 



The unit of pole strength is denned as follows: A magnetic 

 pole has unit strength if, when placed at a distance of one centi- 

 meter from a similar pole, it repels it with a force of one dyne. 



The force exerted on a unit pole at a distance of r cm. from a 

 pole of strength m is 



(70) 



FIG. 35. Magnet. 



FIG. 36. Magnet with armature on. 



32. Magnetic Field. The space surrounding a magnetic pole 

 or a current of electricity in which magnetic forces act is called 

 a magnetic field. The direction of the force at any point in the 

 field is the direction in which a unit north pole placed at the point 

 would tend to move and its intensity is the force in dynes exerted 

 on the unit pole. 



The magnetic field is represented by lines of magnetic induc- 

 tion or magnetic flux drawn in the direction of the force. 



Unit magnetic force or unit magnetizing force produces one 

 line of magnetic flux per square centimeter in air and /* lines 

 per square centimeter in a magnetic material of permeability p. 



The magnetic force at a point is expressed in dynes and is 

 represented by 3C; the magnetic flux density at a point is ex- 

 pressed in lines per square centimeter and is represented by 8. 



