ELECTROMAGNETS. MAGNETISM] OF IRON. 83 



the form of a ring which is magnetized by a winding of wire 

 distributed around the ring, then the rod has no poles and con- 

 sequently no demagnetizing action on itself. In the case of a 

 closed iron ring and in the case of a very long slim rod, the 

 magnetic field produced by the winding of wire is the net actual 

 field which magnetizes the rod. 



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

 magnetic field, which, but for the presence of the rod, would be 

 a uniform 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 + 2,700 units, and 2,700 

 units respectively. In this case a point near the middle of the 

 rod is approximately 10 centimeters distant from each of the 

 poles, and the actual field &C 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 27OO/IO 2 units intensity due to the N 

 pole of the rod and directed towards the S pole, and (c) the field 

 of 2,7oo/io 2 units intensity due to the S pole of the rod and 

 directed towards the S pole.* Therefore the net result is a 

 field of which the intensity near the middle of the rod is 60 minus 

 27 minus 27 units, or 6 units. 



Coil 





Iron rod 



Coil 

 Fig. 60. 



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

 in iron, as above defined, may be obtained as follows: Figure 60 

 shows an iron rod inside of a long coil of wire. The magnetizing 

 field at the point p is defined as the resultant field which would 



* See Art. 9. 



