ELEMENTARY THEORY OF MAGNETISM. 7 



Uniform and non-uniform fields. A magnetic field is said to 

 be uniform when it has everywhere the same direction and the 

 same intensity, otherwise the field is said to be non-uniform. 

 The earth's magnetic field is sensibly uniform throughout a room. 

 The magnetic field surrounding a magnet is non-uniform. The 

 magnetic field surrounding an electric wire is non-uniform. 



9. Direction and intensity of the magnetic field surrounding 

 an " isolated " magnet pole of strength M. By an "isolated" 

 magnet pole is meant one pole of a very long slim magnet the 

 other pole being so far away as to be negligible in its action. 



Fig. 6. 



Fig. 7. 



The magnetic field in the neighborhood of an isolated north 

 pole is everywhere directed away from the pole as shown by 

 the radiating straight lines (lines of force, as they are called) in 

 Fig. 6. The magnetic field in the neighborhood of an isolated 

 south pole is everywhere directed towards the pole as indicated 

 in Fig. 7. 



Consider two magnet poles M and m which are at a distance 

 of r centimeters apart as shown in Fig. 8. The force F with 



which M repels m is equal to , according to Art. 7 ; but 



the force exerted on m can also be expressed as equal to mH 

 where H is the intensity at m of the magnetic field which is due 



