THE MAGNETISM OF IRON. 



77 



and when a magnetic field is reduced * in extent (volume) or in 

 intensity, a portion of its energy is transformed. ' 



A simple discussion of the tension and energy of the magnetic 

 field cannot be based on an arrangement like Fig. 43 because of 

 the non-uniformity of the field. Consider one end of a very 

 broad flat strip of magnetized steel, as shown in Fig. 44, and let 



Fig. 43. 



us assume that the total pole strength m is spread uniformly over 

 the end of the strip f as indicated by the shading in the figure. 



*When the two poles in Fig. 43 move nearer together the intensity of the inter- 

 vening field is increased in some parts and decreased in other parts. 



f The fundamental relations involved in the study of electricity and magnetism may 

 be established in a comparatively simple way by assuming simply geometrical forms and 

 distributions. Thus, the formula expressing the magnetic field intensity in the neigh- 

 borhood of a magnet pole is extremely complicated unless the pole be assumed to be 

 concentrated at a point, or to be spread uniformly over a certain length of a rod, or to 

 be spread uniformly over a certain plane area. The formula expressing the intensity 

 of a magnetic field in the neighborhood of a wire carrying an electric current is 

 extremely complicated unless the wire be simple in shape. Thus, the formula express- 

 ing the intensity of a magnetic field in the neighborhood of a long straight wire is very 



