358 



ELEMENTS OF ELECTRICAL ENGINEERING. 



the intensity near the middle of the rod is 60-2727 units, or 

 6 units. 



A clear idea of the significance of magnetizing force at a point 

 in iron, as above defined, may be obtained as follows : 



Figs. 5, 6 and 7 show an iron rod inside of a long coil of 

 wire. The magnetizing field at the point /, Fig. 5, is defined as 



Coil 



Iron rod 



Fig. 5. 



Coil 



the resultant field which would be produced at / by the combined 

 action of the coil and of the poles N and S of the rod. The 

 field which would be produced at / by the coil alone is repre- 

 sented by the arrow cV, the field which would be produced at p 

 by the poles N and 5 alone, is represented by the arrow df r t and 

 the magnetizing field at / is the resultant of W and eh". 



The fundamental definition of the intensity of a magnetic field 

 at a point as the force in dynes which would act upon a unit 

 test pole placed at the point, has no meaning unless it is possible 

 to place the test pole at the point, that is to say, the point must 

 be in air. But if a cavity is made around the point p as shown 

 by the dotted circle in Fig. 6, the walls of this cavity will be 



Coil 



"' 



Iron rod 



Coil 



Fig. 6. 



magnet poles as indicated by the letters , s, and the field in- 

 tensity in this cavity will not be that which is due merely to the 

 combined action of the coil and poles N and S. If, however, 



