ELECTROMAGNETS. MAGNETISM OF *IRON. 357 



The magnetizing force 3f at a given point in an iron rod is de- 

 fined as the resultant field intensity at the point due not only to 

 the external cause of the original field, but also to the newly cre- 

 ated magnetic poles of the rod. The given point must be as- 

 sumed to be in air as explained later. 



This influence of the poles of a magnetized rod is always such 

 as to reduce the intensity of the original field along the rod, and 

 it is of the nature of a reaction of the magnetized rod upon the 

 original field. 



When the iron rod is slim its poles are weak, and if the rod is 

 long its poles are at considerable distances from the middle por- 

 tions of the rod. In this case the poles of the rod do not pro- 

 duce any appreciable weakening of the original field along the 

 middle portions of the rod. 



When the iron rod is in the form of a ring which is magnetized 

 by a winding of wire distributed around the ring, then the rod 

 has no poles and consequently no demagnetizing action on 

 itself. That is to say, the magnetic field produced by the wind- 

 ing of wire is in this case wholly effective in magnetizing the 

 rod. 



Example. An iron rod 20 centimeters long is placed in a mag- 

 netic field which, but for the presence of the rod, would be a uni- 

 form magnetic field of 60 units intensity, the rod being parallel to 

 the direction of this field. Let us suppose that the rod becomes 

 magnetized to such a degree that the strengths of the poles at 

 the ends of the rod are -f 2,700 units and 2,700 units re- 

 spectively. In this case a point near the middle of the rod is ap- 

 proximately 10 centimeters distant from each of the poles, and 

 the actual field, c*f, near the middle of the rod may be thought of 

 as made up of three parts, namely, (a) the original field of 60 

 units intensity which is directed towards the N pole of the rod, 

 (b) the field of 2,7OO/io 2 units intensity due to the N pole of the 

 rod and directed away from the N pole, and (c) the field of 

 2,7OO/io 2 units intensity due to the 5 pole of the rod and directed 

 towards the 5 pole. Therefore the net result is a field of which 



