8 4 



ADVANCED ELECTRICITY AND MAGNETISM. 



be produced at p by the combined action of the coil and of the 

 poles N and S of the rod. The field which would be produced 

 at p by the coil alone is represented by the arrow 3C', the field 

 which would be produced at p by the poles N and 5 alone 

 is represented by the arrow W , and the net magnetizing field 

 at p is the resultant of W and W" . 



Coil 



Iron rod 



Coil 



Fig. 61. 



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. 61, the walls of this cavity will be 



Coil 



ns N 



Iron rod 



Coil 



Fig. 62. 



magnet poles as indicated by the letters n and s, and the field 

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

 the combined action of the coil and poles N and 5 at the ends 

 of the rod. If, however, the cavity around p is very long and 

 narrow, as shown in Fig. 62, then the poles n and s at the ends 

 of this cavity are weak and far away from p, so that these poles 

 n and s do not produce at p any perceptible field, and therefore 

 the field intensity in the narrow longitudinal cavity shown in 

 Fig. 62 is the resultant of eV' and M" of Fig. 60. That is, the 



